Study design and setting
A prospective observational study was carried out at an obstetric unit, North Colombo Teaching Hospital (NCTH), Ragama, Sri Lanka. Eligible pregnant women were selected after hospital admission for the elective induction of labour. All the consecutive inductions performed over an 8 weeks period were assessed for eligibility.
Inclusion and exclusion criteria
They were singleton pregnancies with vertex presentation at term. Women with normal booking body mass index (BMI), normal weight gain during pregnancy, and those with an unfavourable cervix (Bishop score less than 6) at the time of hospital admission were included. These women were induced electively after 37 weeks of gestation. Indications such as past delivery date, pregnancy-induced hypertension without evidence of fetal compromise, and well-controlled gestational diabetes mellitus with adequate fetal growth were included in the study.
Women with previous uterine scar including caesarean delivery or myomectomy, multiple pregnancies, malpresentation, pre-labour rupture of membranes, evidence of fetal compromise, history of previous cervical surgeries, Bishop score more than 6 at the time of hospital admission and women with established labour were excluded from the study.
Sonographic cervical volume assessment
The pre-induction cervical assessment was done using both the Bishop score and transvaginal ultrasonography. Sonographic assessment in all cases included was done by the first author (SRA). The procedure was standardized according to the recommendations for cervical length assessment published by the International Society of Ultrasound in Obstetrics and Gynecology (ISUOG) . Transvaginal ultrasound (TVS) was performed with a sector phased array of 7 MHz probes (Samsung Medison Co.Ltd-Korea), according to the ISUOG practice advice . The appropriate 1-day training was obtained prior to the study from an accredited sonographer in this field. Women were asked to empty the bladder before transvaginal ultrasound examination. Digital cervical assessment using Bishop score was performed by the treating physicians in the unit. The sonographer was blinded to the findings of the digital cervical assessment.
Three measurements were taken and the shortest measurement was taken as the final cervical length. Measurement of the anteroposterior diameter of the cervix was obtained at the midpoint of the cervix, right-angled to the endocervical canal. Cervical volume was calculated assuming the cervix as a cylinder in geometric view (V = πr2h).
Protocol for induction of labour
Induction of labour was performed according to the local unit protocol which is based on the National Institute of Clinical Excellence (NICE) clinical guideline for induction of labour and the guideline prepared by the Sri Lanka College of Obstetricians Gynaecologists [13, 14]. After a morning dose of Dinoprostone 3 mg (prostaglandin E2 vaginal tablet), women were assessed after 6 h. Depending on the findings, insertion of another Dinoprostone 3 mg tablet (maximum of two tablets giving rise to one cycle) was placed if the cervix was unfavourable (Bishop score less than 6). If the cervix was favourable (Bishop score more than 6), women were scheduled for amniotomy on the next day morning. Oxytocin augmentation was started if uterine contractions were inadequate after 2 h of observation after amniotomy in the labour ward.
Induced women were allowed to progress if labour started before the scheduled amniotomy. Women who proceeded into labour before the scheduled amniotomy after prostaglandins were augmented with oxytocin, only if their progress was poor in due course of labour. All women were offered intrapartum continuous CTG monitoring. If induction has not been successful with one cycle of Dinoprostone, women were managed according to the local unit protocol which consisted of three options: another cycle of induction with prostaglandins, insertion of transcervical Foley catheter for a maximum of 48 h and caesarean delivery.
Independent variables (test variables) were cervical volume, cervical length, and Bishop score. Dependent variables (outcomes) were frequency of delivery within 24 h and induction to delivery interval. Baseline variables (possible confounders) were parity and maternal age. The primary outcome measure was the frequency of delivery within 24 h.
Means and standard deviations were used to summarise numerical data where relevant. Correlation between induction to delivery interval and Bishop score, cervical length, and cervical volume were calculated using Pearson correlation coefficient. Cut-off levels for the Bishop score, cervical length, and cervical volume to predict the likelihood of delivery within 24 h of induction were calculated using a Receiver Operating Characteristics (ROC) curve and Youden index that maximizes the vertical distance from the line of equality (reference line) to the left upper point of the respective curve. Logistic regression analysis was used to correlate those who delivered vaginally within 24 h with cervical volume, cervical length, and Bishop score while controlling for confounders such as BMI, parity, maternal age, oxytocin augmentation, repeat prostaglandin use, and gestational age at induction. P-value < 0.05 was taken as the significant level. Ethical approval was obtained from the Ethics Review Committee, Faculty of Medicine, University of Kelaniya, Sri Lanka.