To our knowledge, this is the first large population-based investigation to demonstrate the exact prevalence of umbilical cord prolapse in association with the use of balloons for cervical ripening. In the present study, of all 369 cases, one-fourth of the present subjects (93 cases) experienced umbilical cord prolapse during and after the use of a balloon. The prevalence of fore-lying or prolapse of the umbilical cord was only 0.005% in cases not associated with the use of a balloon for cervical ripening, compared to 0.064% (OR 13.67) in the cases associated with the use of a balloon.
A previous study suggested that the use of a trans-cervical balloon catheter with 180–250 ml of saline increases the risk of cord presentation [14]. Similar to previous study, in particular, the use of a ball-type balloon filled with large amount of water (130.9 ± 60.3 ml) was associated with a remarkably high risk (OR 25.83). This odds ratio is highest among those of known risk factors in previous reports [3,4,7,12,15]. However, it is supposed that the increased risk of umbilical cord prolapse in cases involving the use of an intra-cervix balloon filled with approximately 40 ml of water was limited, as the incidence of umbilical cord prolapse after balloon removal did not differ between the patients treated with and without an intra-cervix balloon.
According to the answers to questions in which prolapse of the umbilical cord occurring during labor associated with the use of balloons for cervical ripening, umbilical cord prolapse occurred after a while balloon removal in more than half of the cases (53%). Even when umbilical cord prolapse did not occur during the use of a balloon or at removal, it may be possible to preserve the elevating fetal presenting part out of the pelvis and induce the wrong rotation of the fetal head, resulting in umbilical cord prolapse. Furthermore, the use of a balloon may involve occult umbilical cord prolapse during the procedure, after which umbilical cord prolapse is detected due to the identification of the descending fetal presenting part or rupture of the membranes. Unfortunately, only 57% of doctors participated in the present study answered that umbilical cord presentation was routinely confirmed using ultrasound scans during the use of a balloon for cervical ripening (data are not shown). Thus, the ultrasound confirmations of the umbilical cord presentation to diagnose fore-lying and occult prolapse of the umbilical cord before balloon placement, after and prior to removal might improve perinatal outcomes.
Questionnaire surveys in large population to obtain enough examples of a rare occurrence have limitations. Compared to western countries, there are a lot of small private hospitals that provide maternity services across Japan. Doctors worked such small hospitals did not retrospectively obtain detail obstetric information and did not answer to this questionnaire survey, because they were unlikely to have computerized database. Therefore, although we believe that the quality of obtained answers was good, this survey was limited by number of response.
Alternatively, since cases whose umbilical cord prolapse was found at just before delivery resulting in delivery without any neonatal complications might not be reported, prevalence of the umbilical cord prolapse might be underestimated. Besides, since the purpose of the present study was to clarify adverse effect of cervical balloon itself such as cord prolapse, the subjects were collected only singleton vertex cases complicated with fore-lying or prolapse of the cord during labor. Therefore, prevalence of fore-lying or prolapse of the cord in the present study was lower (0.005-0.064%) than in some former studies reported a prevalence of them ranging from 0.1 to 0.6% [2-6].