Induction of labour (IOL) is a common obstetric procedure performed when the benefits of early delivery outweigh the risks of continuing the pregnancy¹. It is indicated in situations such as post-term gestation, preeclampsia, gestational diabetes, intrauterine growth restriction, premature rupture of membranes, and other maternal or fetal complications². Globally, the rate of labour induction varies between 10% and 30%, depending on institutional policies, regional practices, and maternal characteristics³. With rising cesarean section rates worldwide, optimizing the success of IOL has become a critical focus of obstetric care. The success of induction largely depends on cervical readiness, fetal position, parity, and the engagement of the fetal head in the maternal pelvis⁴. Traditionally, Bishop’s scoring system has been employed to assess cervical favourability and predict the likelihood of vaginal delivery⁵. While widely used, the Bishop score has several limitations: it is subjective, shows significant interobserver variability, and may not accurately predict induction success, especially in nulliparous women⁶. To overcome the limitations of clinical assessment, ultrasound evaluation of labour has emerged as a reliable, objective, and non-invasive method to assess fetal head engagement and descent⁷. Transperineal ultrasound (TPUS), in particular, allows real-time visualization of the fetal head in relation to maternal bony landmarks and the perineum⁸. Key TPUS parameters include the angle of progression (AoP), head–symphysis distance (HSD), and fetal head–perineum distance (FHPD)⁹. Among these, FHPD—the shortest distance from the fetal skull to the maternal perineum—has been recognized as a simple, reproducible, and clinically useful measurement. Evidence suggests that a shorter FHPD is associated with a higher probability of successful vaginal delivery and a reduced need for operative interventions¹⁰. Eggebo et al. reported that an FHPD of less than 40 mm was significantly correlated with successful vaginal delivery following induction¹¹. Similarly, Torkildsen et al. demonstrated that FHPD could independently predict labour outcome, even after adjusting for parity and Bishop score¹². These findings highlight the potential utility of FHPD as an adjunct to conventional methods in predicting induction success. In addition to improving prediction of vaginal delivery, TPUS and FHPD measurement offer several practical advantages: they are non-invasive, well-tolerated by patients, and reproducible across examiners, providing dynamic assessment during the labour process¹³. Real-time evaluation of fetal head descent allows obstetricians to make informed decisions about timing of interventions, including augmentation of labour or consideration of cesarean delivery, thereby optimizing maternal and fetal outcomes¹⁴. Despite increasing evidence, the routine use of FHPD in clinical practice remains limited, and data on its predictive accuracy across diverse populations are still evolving. Given its simplicity, reproducibility, and objective nature, integrating FHPD measurement into labour management may enhance decision-making, reduce induction failures, and improve maternal and neonatal outcome es¹⁵. Therefore, this study aims to evaluate the influence of fetal head–perineum distance on the outcome of induction of labour and to determine its role as a reliable, adjunct parameter for predicting successful vaginal delivery. Need for Study: Successful induction of labour (IOL) is a key component of obstetric care, yet predicting its outcome remains challenging. Traditional assessment tools, such as the Bishop score, are subjective and have interobserver variability. Failed inductions are associated with prolonged labour, increased maternal morbidity, higher cesarean section rates, and adverse neonatal outcomes. Transperineal ultrasound (TPUS) offers an objective assessment of fetal head descent, with the fetal head–perineum distance (FHPD) emerging as a promising predictor of successful vaginal delivery. Despite evidence supporting its clinical utility, FHPD is not routinely incorporated into labour management, and population-specific cut-off values are lacking. Assessing FHPD before induction may allow for better prediction of induction success, enable timely clinical decision-making, reduce unnecessary cesarean deliveries, and improve both maternal and neonatal outcomes. Therefore, this study aims to evaluate the role of fetal head–perineum distance in predicting the outcome of induction of labour at term. Methodology: This observational study was conducted November 2023 to February 2024. A total of 59 pregnant women undergoing induction of labour were included

Source of Data: Department of Obstetrics and Gynaecology, Subbaiah Institute of Medical Sciences, Shivamogga. Study Design: Observational prospective study Study Period: November 2023 to February 2024 Study Population: Pregnant women undergoing induction of labour at ≥37 weeks gestation with singleton pregnancies in cephalic presentation with Reassuring CTG with no contraindication for vaginal delivery were included Transperineal ultrasound (TPUS) was performed in the supine lithotomy position. The fetal head–perineum distance (FHPD) was measured as the shortest distance between the outermost bony part of the fetal skull and the maternal perineum.

1)A total of 59 women undergoing induction of labor were included in the study. The distribution of patients according to parity and their mode of delivery is shown in Table 1. Table 1: Mode of Delivery According to Parity Parity Number of cases Vaginal delivery Cesarean section Primigravida 35 19 16 Multigravida 24 19 5 Vaginal delivery was observed in 38 out of 59 women (64.4%), while cesarean section was required in 21 women (35.6%). The likelihood of vaginal delivery increased with higher parity, with multiparous women showing higher rates of successful vaginal delivery compared to primigravidas. 2)The relationship between fetal head–perineum distance (FHPD) measured by transperineal ultrasound and mode of delivery is summarized in Table 2. Table 2: Mode of Delivery According to Fetal Head–Perineum Distance FHPD Number of cases Vaginal delivery Cesarean section 2-4 cm 2 2 --- 4-5 cm 11 9 2 5-6 cm 17 16 1 6-7 cm 12 8 4 7-8 cm 12 3 9 8-9 cm 5 --- 5 FHPD Vaginal delivery Cesarean section ≤6cm 27 (55.2%) 3 (14.2%) >6cm 11 (28.9%) 18 (85.7%) Vaginal delivery was most frequent when the FHPD was between ≤6cm, with 16 out of 17 women (94.1%) achieving vaginal birth. Conversely, cesarean delivery rates increased significantly at FHPD >6 cm, with 18 out of 29 women (62.06%) requiring cesarean section. These findings suggest that a shorter FHPD is associated with a higher likelihood of successful vaginal delivery.

In the study conducted by Torkildsen et al. (2011), an FHPD ≤ 4 cm was strongly predictive of spontaneous vaginal delivery, whereas an FHPD ≥ 5 cm was associated with a higher risk of cesarean section due to failure to progress. In our study, vaginal delivery was most frequent when the fetal head–perineum distance (FHPD) measured between ≤6cm, with 94.1% of women achieving vaginal birth. Conversely, when the FHPD exceeded 6 cm, the cesarean delivery rate increased to 62.06%, indicating that a shorter distance between the fetal head and perineum favors successful vaginal delivery. Eggebo et al. reported that an FHPD of less than 40 mm was significantly correlated with successful vaginal delivery following induction,when compared to our study where FHPD ≤6cm was associated with successful vaginal delivery. In our study, an FHPD ≤ 6 cm was significantly associated with a higher rate of vaginal delivery, while values >6 cm correlated with an increased likelihood of cesarean section. Similar findings were seen by Álvarez-Colomo and Gobernado-Tejedor (2016), who noted mean FHPD values of 4.47 cm in the vaginal group and 5.13 cm in the cesarean group. All these results are in agreement with our findings, supporting that a shorter FHPD is associated with advanced fetal descent and favorable labor progress. The slight variation in the cutoff values between the two studies may be attributed to differences in sample size, parity distribution, and timing of ultrasound measurement during labor. Multiparity further favored successful induction, reflecting the physiological advantages of previous vaginal deliveries, including more efficient cervical ripening and uterine contractility. Transperineal ultrasonography provides a simple, non-invasive, and reproducible method for assessing FHPD, allowing clinicians to anticipate labor outcomes and tailor management strategies accordingly. Despite its strengths, the study’s single-center design and limited sample size may restrict generalizability. Future multicenter studies with larger cohorts are needed to validate these thresholds and integrate FHPD assessment into standardized induction protocols. In summary, FHPD is a practical and clinically meaningful parameter, with ≤6 cm indicating a higher likelihood of vaginal delivery. Incorporation of this measure into routine practice could improve prediction of induction outcomes and optimize patient counseling. Author Contributions All authors have reviewed the final version to be published and agreed to be accountable for all aspects of the work. Concept and design: Dr Shivamurthy H M, Dr Ashwini Pai, Dr Veena, Dr Ankitha BN Acquisition, analysis, or interpretation of data: Dr Shivamurthy H M, Dr Ashwini Pai, Dr Veena, Dr Ankitha B N Drafting of the manuscript: Dr Husena, Dr Soujanya, Dr Arshiya, Mr Naveen K S, Mr Manju Naik Critical review of the manuscript for important intellectual content: Dr Shivamurthy HM, Dr Ashwini Pai, Dr Veena, Dr Ankitha BN Supervision: Dr Shivamurthy H M Disclosures Human subjects: Consent for treatment and open access publication was obtained or waived by all participants in this study. The Institutional Ethical Committee of Subbaiah Institute of Medical Sciences (IEC-SUIMS) issued approval IEC-SUIMS/12/2024-25. Animal subjects: All authors have confirmed that this study did not involve animal subjects or tissue. Conflicts of interest: In compliance with the ICMJE uniform disclosure form, all authors declare the following: Payment/services info: All authors have declared that no financial support was received from any organization for the submitted work. Financial relationships: All authors have declared that they have no financial relationships at present or within the previous three years with any organizations that might have an interest in the submitted work. Other relationships: All authors have declared that there are no other relationships or activities that could appear to have influenced the submitted work. Acknowledgements We extend our gratitude to the medical director, Dr. Nagendra S; the executive director, Dr. Latha R. Telang; the dean, Dr. Vinayaka G; and the principal, Dr. Siddalingappa CM, of Subbaiah Institute of Medical Sciences

perineum distance (FHPD) measured by transperineal ultrasound is a reliable predictor of successful induction of labour. An FHPD ≤6 cm was significantly associated with a higher likelihood of vaginal delivery. Multiparity also correlated with favorable induction outcomes. Conversely, an FHPD above 6 cm was associated with an increased risk of cesarean delivery. These findings support the use of pre-induction FHPD assessment as an objective, reproducible tool to guide clinical decision-making, optimize labour management, and reduce unnecessary surgical interventions. Incorporation of FHPD measurement into routine obstetric practice may enhance prediction of induction success.

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