Induction of Labor: A Review
Corresponding Author: Shravani Devarasetty, Department of Obstetrics and Gynaecology, Mahatma Gandhi Medical College and Research Institute, Sri Balaji Vidypeeth, Puducherry, India, Phone: +91 8056693978, e-mail: firstname.lastname@example.org
How to cite this article Devarasetty S, Habeebullah S. Induction of Labor: A Review. J Basic Clin Appl Health Sci 2019;2(4):128–133.
Source of support: Nil
Conflict of interest: None
Induction of labor is a common procedure in modern obstetrics and accounts for 20% of all deliveries. The goal of induction of labor is to preempt the natural process of labor by initiating its onset artificially by stimulating cervical ripening and uterine contractions before these occur spontaneously. The favorability of the cervix assessed by Bishop score has a substantial impact on the potential success of any labor induction. There is a consensus that the success of induced labor is directly related to the status of the cervix, with higher cesarean section rates in those with an unfavorable cervix. The review overviews about various methods, indications and contraindications, and predictors of induction of labor outcome.
Keywords: Bishop score, Cesarean section, Induction of labor, Labor.
Induction of labor is a common procedure in day-to-day practice of obstetrics and accounts for 20% of all deliveries.1 In the United States, the rate of labor induction has increased over the years from 9.5 to 19.4%.2 The reasons could be widespread availability of cervical ripening agents, pressure from patients, convenience for physicians, logistic factors, psychosocial reasons, and fear of litigation.3
Labor involves painful uterine contractions leading to dilatation of cervix and descent of the fetus through birth canal resulting in delivery.4 Prior to this, there are extensive changes in the form of remodeling of cervix, increased responsiveness of myometrium, ripening, effacement, and loss of integrity of cervix. The most important changes in the cervix begin a few weeks before the uterine contractions. The composition of proteoglycans, glycosaminoglycans, and collagen undergoes changes. The cervical stroma is also invaded by inflammatory cells which produce mediators resulting in the softening of cervix. This is called cervical ripening.
Induction of labor is defined as artificial termination of pregnancy after the period of viability but before the spontaneous onset of labor with or without ruptured membranes.5 The need for induction arises when it is likely to be associated with better outcome compared to the process being left alone. Centuries ago labor was induced mainly to deliver a dead fetus or to avoid cephalopelvic disproportion. Initially mechanical methods and artificial rupture of membranes were practiced but since the middle of 20th century oxytocin and subsequently prostaglandins have been extensively used along with other previously mentioned methods.
Rates of Induction of Labor
The rates of induction of labor have increased during the last few decades. The rate of induction depends on the availability of resources. Now many agents are available for cervical ripening. The rates of induction are higher in developed countries compared to developing countries.6 Rate of induction of labor was 20% in the United States and United Kingdom.7,8 In India, the rates are widely variable depending on private/government facility and nonteaching and teaching/tertiary care hospitals. In Pune, it ranged from 6.59 to 23.9%.10
Indications for Induction of Labor
The indications may vary from country to country and institution to institution. Common maternal indications include postterm pregnancy, prelabor rupture of membranes (PROMs), gestational hypertension, preeclampsia, eclampsia, hemolysis, elevated liver enzymes and low platelets (HELLP) syndrome, fetal demise, chorioamnionitis, placental abruption, diabetes mellitus, chronic hypertension, renal disease, chronic pulmonary disease, antiphospholipid syndrome, etc. Fetal indications include severe fetal growth restriction, Rh isoimmunization, oligohydramnios.7
The common indications for induction are discussed below.
When the expected date of delivery is exceeded by 2 weeks or more11 it is called postterm pregnancy. Postterm pregnancies need to be terminated because they are associated with increased risk of uteroplacental insufficiency which leads to fetomaternal complications.12 According to The American College of Obstetricians and Gynecologists (ACOG), Royal College of Obstetricians and Gynaecologists (RCOG), and World Health Organization guidelines, induction of labor in postterm pregnancy is done at 41 weeks of gestation.11,13 Induction of labor at 41 weeks of gestation in uncomplicated pregnancies is acceptable because cesarean delivery rates are less in these women; and after 41 weeks, the chances of meconium-stained liquor and uteroplacental insufficiency are high.14–16 Perinatal morbidities that are associated if the pregnancy extends beyond 41 weeks include meconium aspiration, low Apgar score, neonatal acidemia, and birth injuries.17,18 It is also associated with risk of neonatal encephalopathy and death in the first year of life.19 According to some studies, the perinatal mortality rate is twice at 42 weeks of gestation compared to 41 weeks and it increases 4-fold at 43 weeks and 5- to 7-fold at 44 weeks.19,20
Prelabor Rupture of Membranes
It is defined as spontaneous rupture of amniotic sac before the onset of labor. If the rupture occurs after 37 completed weeks of gestation, then it is called PROM; and if rupture occurs before 37 weeks, it is called preterm prelabor rupture of membranes (PPROM). It occurs in 4–18% of pregnancies, mostly after 37 completed weeks of gestation.21 Even though the etiology of PROM is not known exactly, risk factors that lead to PROM are urinary tract infections, genital tract infections, polyhydramnios, multiple pregnancies, cervical incompetence, placenta previa, abruptio placentae, and amniocentesis. According to some authors, PROM can happen in women with nutritional deficiencies such as deficiency in zinc, copper, and vitamins C and E.22,23 Prelabor rupture of membrane occurs due to the weakening or thinning of epithelium of the membranes which can be due to altered collagen metabolism or increase in intra-amniotic pressure.22 There is a high chance for maternal and fetal complications if it involves prolonged rupture of membranes. If the latency period exceeds 24 hours, then it is called prolonged rupture of membranes.24 The major maternal complication is sepsis and it can develop in postpartum period as endometritis. The major fetal complication includes fetal and neonatal sepsis which is more common if there is prolonged rupture of membranes. Other fetal complications include fetal distress, meningitis, pneumonia, pyoderma, umbilical sepsis, and conjunctivitis.25 Since there is increase in perinatal morbidity if the period of latency is long after the rupture of membranes, early action such as induction of labor should be taken.26 Induction of labor should be done when the women does not go into labor even after 4–6 hours after rupture of membranes. In a study done by Kappy et al., it was found that cesarean delivery rate was high in women who had labor stimulation for preterm rupture of membranes compared with women who are under expectant observation.27
It is defined as amniotic fluid index of less than 8 cm. It is also sonographically defined as absence of fluid pocket 2–3 cm in depth.28 Amniotic fluid is mainly produced by fetal urine. Absence or reduction in production of fetal urine results in oligohydramnios. It causes fetal abdomen compression leading to pulmonary hypoplasia due to limited movement of diaphragm. The incidence of oligohydramnios ranges from 0.5 to 5%.29,30 Oligohydramnios is associated with increased risk of perinatal morbidity because of umbilical cord compression leading to uteroplacental insufficiency and meconium-stained liquor. According to Schwartz et al., oligohydramnios in term pregnancy is considered as a solid indication for induction of labor.31 It is also associated with increased risk of cesarean delivery because of fetal distress.32
Hypertensive Disorders in Pregnancy
In India, hypertensive disorders are the third most common cause of maternal mortality. They can lead to perinatal mortality and morbidity. They can lead to fetal complications such as intrauterine growth restriction due to uteroplacental insufficiency, low-birth-weight babies, and intrauterine fetal death. Maternal and perinatal complications are more with preeclampsia and eclampsia. So in such patients continuation of pregnancy beyond 37 weeks is not advisable. Accordingly it is considered to induce hypertensive patients after they cross 37 weeks of gestation.33
Gestational Diabetes Mellitus
It is one of the most common obstetric complications and it affects about 3–5% of all pregnancies.34,35 The increased incidence is due to the risk factors such as obesity, hypertension, and elderly pregnancies (women over 35 years of age). In cases of uncontrolled diabetes and fetal macrosomia, time for induction is 38 completed weeks of gestation. Boulvain et al. opined that induction of labor at 37–39 weeks of gestation has positive outcomes in women with gestational diabetes mellitus.36
Contraindications for Induction
These include pelvic structural deformities and gross cephalopelvic disproportion (CPD), abnormal fetal lie or presentation (e.g., transverse lie or footling breech), placenta previa, vasa previa or cord presentation, previous rupture uterus, prior classical or inverted T uterine incision, significant prior uterine surgery (e.g. full thickness myomectomy), active genital herpes, invasive cervical cancer, etc.
Risks/complications of Induction
Failure to achieve active labor, tachysystole, fetal heart rate abnormalities, cord prolapse, chorioamnionitis, increased vaginal operative delivery, and increased cesarean section rate.37 Rarely, uterine rupture (especially of scarred uterus) can also occur.
Predictors of Successful Induction of Labor
Many factors have been identified to be associated with the successful outcome of induction of labor. They include maternal age, parity, gestational age at induction, body mass index (BMI), and cervical status. Some of the factors are discussed below.
Many studies have shown that maternal age is associated with outcome of induction of labor. The cesarean section rates increase with increase in maternal age. Rayamajhi et al. observed that there is failure rate with maternal age >30 years.38 One study showed vaginal delivery rate of 51.32% and the mean age was 22.3 years.39 Yet another study concluded that there is higher cesarean section rate with advanced maternal age.40
Bueno et al. concluded that the success of induction of labor depends on parity of the women.41 Cesarean section rates are higher in nulliparous women as was shown in a few studies.42–44 Admani et al.45 observed that vaginal delivery rates are higher in multiparous women than in primiparous women.
Park found that the cesarean section rates were higher in earlier gestational age.46,47 But a few studies have observed that cesarean section rates are higher in greater gestational age.48–50 So gestational age is a strong predictor of successful outcome of induction of labor.
Body Mass Index
Obesity is associated with increased risk of failed induction and cesarean section. Crane et al. concluded that higher BMI women are associated with increased risk of failed induction and cesarean section rates.48 Similarly, recently others showed that women with lower BMI had successful outcome of induction of labor.39,40,51
Then the most important factor for predicting the success of induction of labor is assessment of cervix. The status of cervix plays an important role in the outcome of induction of labor.
Cervical Assessment Methods
In 1964, Bishop introduced a scoring to predict whether induction will be successful.49 Initially it had five components, i.e., position of cervix, consistency, effacement, dilatation of cervix, and station of presenting part, each with a score of 0 to 2 or 0 to 3. Subsequently in modified Bishop scoring, effacement was assessed more objectively by cervical length in centimeters, the total score being 13. This is now followed extensively. Transvaginal ultrasound for cervical length measurement was also used for prediction of successful induction of labor.52
Modified Bishop Score53
|Cervical feature||Modified Bishop score|
|Length of cervix (cm)||>4||2–4||1–2||<1|
Methods of Induction
Induction of labor can be achieved through several methods. Some commonly used methods are discussed below.
Stripping of Membranes
It is the oldest method for cervical ripening.50 It is done by introducing index finger through internal cervical os and sweeping the membranes 360° to separate the membranes from lower uterine segment. This process causes release of prostaglandins, results in spontaneous onset of labor, and thus reduces the chances of pharmacological methods of induction. Complications of this procedure are infection, accidental rupture of membranes, and bleeding in case of placenta previa or low lying placenta. This procedure cannot be done if cervix does not allow one finger.
Single-balloon and double-balloon catheters are available for cervical ripening. The mechanism of cervical ripening by the balloon catheters is by mechanical stretching of cervix and lower uterine segment. It stimulates the production of endogenous prostaglandins by separation of chorionic membrane and deciduas.54,55 It also helps in production of hyaluronic acid thus causing cervical swelling and softening.56 The myometrial stretching caused by balloon catheter results in increased activation of cyclooxygenase 2, thus increasing the production of prostaglandins.57 It also stimulates the secretion of inflammatory cytokines such as interleukins and matrix metalloproteins.58
Insertion of a balloon catheter may cause some discomfort for the women who are nulliparous with poor Bishop score.59 Few studies have shown that digital insertion is better than instrumental insertion.60 Cervical bleeding can occur while inserting the catheter in 2–6% of women.59,61 Increase in cervical dilatation with increase in balloon volume was observed but with no difference in the mode of delivery.62,63 Very few limitations such as maternal infection and bleeding from cervix are observed. These catheters cannot be used in cases of ruptured membranes because of the increased risk of infection.
It is artificial rupture of membranes and is an iatrogenic rupture of membranes done by Kocher’s clamp. It is usually performed in those women with favorable cervix (good Bishop score). To avoid the risk of cord prolapse, amniotomy should be done when cervix is well applied to vertex. Fetal heart rate should be checked before and after the procedure. The color and character of amniotic fluid should be checked and are documented.
The use of prostaglandin analogs as intravenous infusion was first reported in 1968. But later prostaglandin E2 (PEG2) was reported as an agent for cervical ripening, and a few studies also showed that small dose of PGE2 applied locally resulted in improvement of Bishop score and with very few side effects.50 Now two prostaglandin analogs are available for cervical ripening: dinoprostone (PGE2) and misoprostol [prostaglandin E1 (PEG1)]. Both the prostaglandin analogs have good efficacy. But improvement of obstetric outcome using these agents is unclear.64–66
It is an agent for induction but usually not for cervical ripening. It is synthesized in the paraventricular neurons and supraoptic nuclei of hypothalamus and is stored in neurohypophysis. Its half-life is 3–10 minutes. Its mechanism of action involves increasing the activity of phosphodiesterases in the myometrial cells. This decreases cyclic AMP concentration and causes the release of calcium from endoplasmic reticulum, thus increasing the level of intracellular calcium leading to contraction of myometrial cells. Oxytocin also forms gap junctions for propagation and synchronization of uterine contractions.
It should be given as a continuous intravenous drip to deliver small amounts of oxytocin. It is administered in cases of good Bishop score (favorable cervix). It causes periodic uterine contractions and the responsiveness to oxytocin increases with increase in gestational age. The disadvantage of oxytocin is that it causes hyponatremia and hypotension if administered in large quantities. It can cause uterine hyperstimulation if coadministered with prostaglandin analogs. And in such cases it can cause fetal asphyxia. The risk of infection is also increased in women with prolonged duration of induction to delivery interval.
Cochrane review (2009) of randomized controlled trials (RCTs) showed that there is not much difference in the cesarean section rates in women undergoing induction of labor and women who underwent expectant management. Cesarean section rates were more in women with unfavorable cervix and with prolonged pregnancy.
Operative Vaginal Delivery
In the RCTs (Cochrane, 2009), much statistical significance is not observed in women who underwent instrumental delivery in the induction group.
Length of Labor
According to Greenberg et al., the length of labor varies with ethinicity.67 A number of authors found that the length of labor can vary with maternal weight and BMI and gestational age and maternal age.68–70
In 1996, Hannah et al. found that the maternal infection rates were more in women in vaginal PGE2 group compared to the oxytocin infusion group.71 This may be due to repeated vaginal examination which can be avoided. Induction is associated with increased risk of chorioamnionitis.
A Cochrane review conducted in 2009 showed no significant difference between induction group and expectant group. But postpartum hemorrhage is common in multiparous women. Women in prolonged latent phase and women on prolonged oxytocin infusion were associated with PPH.
This condition is seen in women induced with prostaglandins and can be managed by giving tocolytics. It can also occur in women with oxytocin infusion. In such cases, oxytocin infusion can be stopped. It can be avoided by monitoring the women and uterine contractions and by starting low-dose oxytocin infusion.
This is common in women with postdated pregnancy and in women in prolonged latent phase of labor. It can also occur in women with uterine hyperstimulation. This condition can be avoided by carefully monitoring the contractions.
This can occur in women with meconium-stained liquor and those with uterine hyperstimulation. No statistical significance is observed between women with induction and women with expectant management.
No significance of neonatal jaundice is observed in women undergoing induction of labor.
Risks Associated with Induction of Labor
Maternal risks include maternal discomfort and increased need for analgesia, failure of induction and need for operative delivery, water retention and hyponatremia with prolonged use of oxytocin, uterine rupture, abruption, and postpartum hemorrhage. Fetal risks include iatrogenic prematurity, fetal compromise due to uterine hyperstimulation, cord prolapse, chorioamnionitis, and neonatal jaundice with prolonged use of oxytocin.
Induction of labor is the safest alternative to spontaneous onset of labor. The obstetrician and the patient must understand the need for induction, risk of induction, methods of induction, and the risk associated with the chosen method. The need for other modes of delivery in case of failed induction should be emphasized. The goal of induction of labor is always to achieve the best maternal and fetal outcome.
5. Induction of Labour at Term - Journal SOGC [Internet].[cited 2018 Oct 21]. Available from: https://www.jogc.com/article/S0849-5831(16)31465-3/abstract.
6. Lydon-Rochelle MT, Cárdenas V, Nelson JC, Holt VL, Gardella C, Easterling TR. Induction of labor in the absence of standard medical indications: incidence and correlates. Med Care 2007;45(6):505–512. DOI: 10.1097/MLR.0b013e3180330e26.
7. ACOG Committee on Practice Bulletins – Obstetrics. ACOG Practice bulletin no. 107: induction of labor. Obstet Gynecol 2009;114(2 Pt 1):386–397.
8. Bukola F, Idi N, M’Mimunya M, Jean-Jose W-M, Kidza M, Isilda N, et al. Unmet need for induction of labor in Africa: secondary analysis from the 2004–2005 WHO global maternal and perinatal health survey (a cross-sectional survey). BMC Public Health 2012;12(1):722. DOI: 10.1186/1471-2458-12-722.
9. Kaur S, Kawthalkar A. Induction of labour-the current trend in a tertiary care hospital. panacea. J Med Sci 2015;5(2):85–86.
10. Chawla, Induction of labor: Our experience [Internet].[cited 2018 Oct 12]. Available from: http://www.marinemedicalsociety.in/article.asp?issn=0975-3605;year=2017;volume=19;issue=2;spage=96;epage=98;aulast=Chawla.
11. WHO Recommendations for Induction of Labour [Internet]. Geneva: World Health Organization; 2011[cited 2018 Oct 21]. (WHO Guidelines Approved by the Guidelines Review Committee). Available from: http://www.ncbi.nlm.nih.gov/books/NBK131963/.
12. Hilder L, Costeloe K, Thilaganathan B. Prolonged pregnancy: evaluating gestation-specific risks of fetal and infant mortality. BJOG Int J Obstet Gynaecol 1998;105(2):169–173. DOI: 10.1111/j.1471-0528.1998.tb10047.x.
14. Olesen AW, Westergaard JG, Olsen J. Perinatal and maternal complications related to postterm delivery: a national register-based study, 1978-1993. Am J Obstet Gynecol 2003;189(1):222–227. DOI: 10.1067/mob.2003.446.
15. Sanchez-Ramos L, Olivier F, Delke I, Kaunitz AM. Labor induction versus expectant management for postterm pregnancies: a systematic review with meta-analysis. Obstet Gynecol 2003;101(6):1312–1318. DOI: 10.1097/00006250-200306000-00029.
16. Mishanina E, Rogozinska E, Thatthi T, Uddin-Khan R, Khan KS, Meads C. Use of labour induction and risk of cesarean delivery: a systematic review and meta-analysis. CMAJ Can Med Assoc J Assoc Medicale Can 2014;186(9):665–673. DOI: 10.1503/cmaj.130925.
17. Caughey AB, Stotland NE, Washington AE, Escobar GJ. Maternal and obstetric complications of pregnancy are associated with increasing gestational age at term. Am J Obstet Gynecol 2007;196(2):155.e1–155.e6. DOI: 10.1016/j.ajog.2006.08.040.
18. Arias F. Predictability of complications associated with prolongation of pregnancy. Obstet Gynecol 1987;70(1):101–106.
19. Divon MY, Haglund B, Nisell H, Otterblad PO, Westgren M. Fetal and neonatal mortality in the postterm pregnancy: the impact of gestational age and fetal growth restriction. Am J Obstet Gynecol 1998;178(4):726–731. DOI: 10.1016/s0002-9378(98)70482-x.
21. Arulkumaran. The Management of Labour. Orient Blackswan; 2004. p. 458.
24. Mati JK, Aggarwal VP, Sanghvi HC, Lucas S, Corkhill R. The Nairobi birth survey. II. antenatal care in Nairobi. J Obstet Gynaecol East Cent Afr 1983;2(1):1–11.
25. Practical Guide to High Risk Pregnancy and Delivery - 3rd Edition [Internet]. [cited 2018 Oct 21].Available from: https://www.elsevier.com/books/practical-guide-to-high-risk-pregnancy-and-delivery/arias/978-81-312-3163-0.
26. Mozurkewich E, Chilimigras J, Koepke E, Keeton K, King VJ. Indications for induction of labour: a best-evidence review. MCN Am J Matern Nurs [Internet] 2009;34(6):Available from: https://ohsu.pure.elsevier.com/en/publications/indications-for-induction-of-labour-a-best-evidence-review-4.
27. Kappy KA, Cetrulo CL, Knuppel RA, Ingardia CJ, Sbarra AJ, Scerbo JC, et al. Premature rupture of the membranes: a conservative approach. Am J Obstet Gynecol 1979;134(6):655–661. DOI: 10.1016/0002-9378(79)90647-1.
28. Phelan JP, Ahn MO, Smith CV, Rutherford SE, Anderson E. Amniotic fluid index measurements during pregnancy. J Reprod Med 1987;32(8):601–604.
29. Hill LM, Breckle R, Wolfgram KR, O’Brien PC. Oligohydramnios: ultrasonically detected incidence and subsequent fetal outcome. Am J Obstet Gynecol 1983;147(4):407–410. DOI: 10.1016/s0002-9378(16)32235-9.
31. Schwartz N, Sweeting R, Young BK. Practice patterns in the management of isolated oligohydramnios: a survey of perinatologists. J Matern-Fetal Neonatal Med Off J Eur Assoc Perinat Med Fed Asia Ocean Perinat Soc Int Soc Perinat Obstet 2009;22(4):357–361. DOI: 10.1080/14767050802559103.
32. Rossi AC, Prefumo F. Perinatal outcomes of isolated oligohydramnios at term and Post-term pregnancy: a systematic review of literature with meta-analysis. Eur J Obstet Gynecol Reprod Biol 2013;169(2):149–154. DOI: 10.1016/j.ejogrb.2013.03.011.
33. Edmonds DK, Dewhurst JSir, Edmonds DK, Dewhurst CJ. Dewhurst’s textbook of obstetrics and gynaecology for postgraduates [Internet].Malden, Mass.: Blackwell Science; 1999.Available from: https://trove.nla.gov.au/version/45458325.
34. DeSisto CL, Kim SY, Sharma AJ. Prevalence estimates of gestational diabetes mellitus in the United States, pregnancy risk assessment monitoring system (PRAMS), 2007-2010. Prev Chronic Dis 2014;11:E104. DOI: 10.5888/pcd11.130415.
36. Boulvain M, Senat M-V, Perrotin F, Winer N, Beucher G, Subtil D, et al. Induction of labour versus expectant management for large-for-date fetuses: a randomised controlled trial. Lancet Lond Engl 2015;385(9987):2600–2605. DOI: 10.1016/S0140-6736(14)61904-8.
37. Leduc D, Biringer A, Lee L, Dy J. Clinical Practice obstetrics committee, special contributors. induction of labour. J Obstet Gynaecol Can JOGC J Obstet Gynecol Can JOGC 2013;35(9):840–857. DOI: 10.1016/S1701-2163(15)30842-2.
39. Marroquin GA, Tudorica N, Salafia CM, Hecht R, Mikhail M. Induction of labor at 41 weeks of pregnancy among primiparas with an unfavorable bishop score. Arch Gynecol Obstet 2013;288(5):989–993. DOI: 10.1007/s00404-013-3006-6.
40. Lee HR, Kim M-N, You JY, Choi S-J, Oh S-Y, Roh C-R, et al. Risk of cesarean section after induced versus spontaneous labor at term gestation. Obstet Gynecol Sci 2015;58(5):346–352. DOI: 10.5468/ogs.2015.58.5.346.
41. Bueno B, San-Frutos L, Pérez-Medina T, Barbancho C, Troyano J, Bajo J. The labor induction: integrated clinical and sonographic variables that predict the outcome. J Perinatol Off J Calif Perinat Assoc 2007;27(1):4–8. DOI: 10.1038/sj.jp.7211619.
42. Khan NB, Ahmed I, Malik A, Sheikh L. Factors associated with failed induction of labour in a secondary care hospital. JPMA J Pak Med Assoc 2012;62(1):6–10.
43. Hurissa BF, Tefera Belachew MG. Prevalence of failed induction of labor and associated factors among women delivered in hawassa public health facilities, ethiopia, 2015. J Womens Health Care [Internet] 2015;04(05):Available from: https://www.omicsgroup.org/journals/prevalence-of-failed-induction-of-labor-and-associated-factors-among-women-delivered-in-hawassa-public-health-facilities-ethiopia-2015-2167-0420-1000253.php?aid=59686.
44. Zandvakili F, Shahoei R, Roshani D, Nasab LH. Labor induction and the risk of cesarean delivery among term pregnancies. 2016; 6.
45. Admani R, Wanyoike GJ, Odawa FXO. Predictors of successful induction of labour in post-term pregnancies at the Kenyatta National Hospital, Nairobi. J Obstet Gynaecol East Cent Afr 2014;26(2):45–50.
46. Park KH. Transvaginal ultrasonographic cervical measurement in predicting failed labor induction and cesarean delivery for failure to progress in nulliparous women. J Korean Med Sci 2007;22(4):722–727. DOI: 10.3346/jkms.2007.22.4.722.
47. Bello AF, Akinyotu OO. Predictors of successful induction of labor at a tertiary obstetric service in Southwest Nigeria. Trop J obstet gynaecol 2016;33(2):143–148.
49. Bishop EH. Pelvic scoring for elective induction. Obstet Gynecol 1964;24:266–268.
51. Tolcher MC, Holbert MR, Weaver AL, McGree ME, Olson JE, El-Nashar SA, et al. Predicting cesarean delivery after induction of labor among nulliparous women at term. Obstet Gynecol 2015;126(5):1059–1068. DOI: 10.1097/AOG.0000000000001083.
52. Yang SH, Roh CR, Kim JH. Transvaginal ultrasonography for cervical assessment before induction of labor. J Ultrasound Med Off J Am Inst Ultrasound Med 2004;23(3):375–382. DOI: 10.7863/jum.2004.23.3.375,quiz 384–5.
54. Manabe Y, Manabe A, Takahashi A. F prostaglandin levels in amniotic fluid during balloon-induced cervical softening and labor at term. Prostaglandins 1982;23(2):247–256. DOI: 10.1016/0090-6980(82)90052-1.
55. Keirse MJ, Thiery M, Parewijck W, Mitchell MD. Chronic stimulation of uterine prostaglandin synthesis during cervical ripening before the onset of labor. Prostaglandins 1983;25(5):671–682. DOI: 10.1016/0090-6980(83)90121-1.
56. Takemura M, Itoh H, Sagawa N, Yura S, Korita D, Kakui K, et al. Cyclic mechanical stretch augments hyaluronan production in cultured human uterine cervical fibroblast cells. MHR Basic Sci Reprod Med 2005;11(9):659–665. DOI: 10.1093/molehr/gah229.
57. Mohan AR, Sooranna SR, Lindstrom TM, Johnson MR, Bennett PR. The effect of mechanical stretch on cyclooxygenase type 2 expression and activator protein-1 and nuclear factor-kappaB activity in human amnion cells. Endocrinology 2007;148(4):1850–1857. DOI: 10.1210/en.2006-1289.
60. Jonsson M, Hellgren C, Wiberg-Itzel E, Akerud H. Assessment of pain in women randomly allocated to speculum or digital insertion of the foley catheter for induction of labor. Acta Obstet Gynecol Scand 2011;90(9):997–1004. DOI: 10.1111/j.1600-0412.2011.01197.x.
61. Panelius E, Heikinheimo O, Rahkonen L. [Foley catheter versus intravaginal Misoprostol for labour induction]. Duodecim Laaketieteellinen Aikakauskirja 2012;128(20):2093–2102.
62. Delaney S, Shaffer BL, Cheng YW, Vargas J, Sparks TN, Paul K, et al. Labor induction with a foley balloon inflated to 30 mL compared with 60 mL: a randomized controlled trial. Obstet Gynecol 2010;115(6):1239–1245. DOI: 10.1097/AOG.0b013e3181dec6d0.
63. Levy R, Kanengiser B, Furman B, Ben Arie A, Brown D, Hagay ZJ. A randomized trial comparing a 30-mL and an 80-mL foley catheter balloon for preinduction cervical ripening. Am J Obstet Gynecol 2004;191(5):1632–1636. DOI: 10.1016/j.ajog.2004.03.033.
64. Wing DA, Ortiz-Omphroy G, Paul RH. A comparison of intermittent vaginal administration of misoprostol with continuous dinoprostone for cervical ripening and labor induction. Am J Obstet Gynecol 1997;177(3):612–618. DOI: 10.1016/S0002-9378(97)70154-6.
65. Smith CV, Miller A, Livezey GT. Double-blind comparison of 2.5 and 5.0 mg of prostaglandin E2 gel for preinduction cervical ripening. J Reprod Med 1996;41(10):745–748.
66. Smith CV, Rayburn WF, Miller AM. Intravaginal prostaglandin E2 for cervical ripening and initiation of labor. comparison of a multidose gel and single, controlled-release pessary. J Reprod Med 1994;39(5):381–384.
67. Greenberg MB, Cheng YW, Hopkins LM, Stotland NE, Bryant AS, Caughey AB. Are there ethnic differences in the length of labor? Am J Obstet Gynecol 2006;195(3):743–748. DOI: 10.1016/j.ajog.2006.06.016.
68. Vahratian A, Zhang J, Troendle JF, Savitz DA, Siega-Riz AM. Maternal prepregnancy overweight and obesity and the pattern of labor progression in term nulliparous women. Obstet Gynecol 2004;104(5 Pt 1):943–951. DOI: 10.1097/01.AOG.0000142713.53197.91.
69. Bergholt T, Lim LK, Jørgensen JS, Robson MS. Maternal body mass index in the first trimester and risk of cesarean delivery in nulliparous women in spontaneous labor. Am J Obstet Gynecol 2007;196(2):163.e1-5. DOI: 10.1016/j.ajog.2006.09.026.
71. Hannah ME, Hodnett ED, Willan A, Foster GA, Di Cecco R, Helewa M. Prelabor rupture of the membranes at term: expectant management at home or in hospital? the TermPROM study group. Obstet Gynecol 2000;96(4):533–538. DOI: 10.1097/00006250-200010000-00010.
© The Author(s). 2019 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (https://creativecommons.org/licenses/by-nc/4.0/), which permits unrestricted use, distribution, and non-commercial reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated.