Transperineal ultrasound helps predict preterm delivery in threatened preterm labour

Cervical length as measured by transperineal ultrasound can predict preterm delivery in women with threatened preterm labour, according to results of a retrospective observational study in 60 Hong Kong women.

Preterm birth is defined as delivery before 37 completed weeks of gestation. [Acta Obstet Gynecol Scand 1977;56:247-253] It is important to identify women at risk of preterm labour and birth, to ensure timely implementation of measures such as use of antenatal corticosteroids to reduce the risk of respiratory distress syndrome and neonatal death as well as use of tocolysis and magnesium sulphate for neuroprotection. [Royal College of Obstetricians and Gynaecologists (RCOG) Green-top Guideline 74; National Institute for Health and Care Excellence (NICE) Guideline 25]

The NICE guideline on preterm labour suggests measuring cervical length by transvaginal ultrasound to determine the likelihood of birth within 48 hours. [NICE Guideline 25] “Transvaginal ultrasound is the gold standard, but it is invasive and may not be readily accepted by women. Transabdominal ultrasound causes least discomfort, but it tends to overestimate cervical length, leading to underdiagnosis in ≤50 percent of cases. Transperineal ultrasound is reasonably accurate and offers a sensible alternative in cases of premature preterm rupture of membranes [PPROM] to reduce the risk of infection and in women reluctant to undergo vaginal examination,” wrote the researchers. [J Matern Fetal Neonatal Med 2006;19:755-762; J Matern Fetal Neonatal Med 2012;25:1682-1689]

To evaluate utility of cervical length as measured by transperineal ultrasound for prediction of preterm delivery and establish cervical length cut-off value, the researchers retrospectively reviewed medical records of 60 women (mean age, 32 years) with singleton pregnancy and no PPROM admitted to Kwong Wah Hospital in Hong Kong between 1 January 2019 and 31 December 2021 for threatened preterm labour at a gestational age between 24 weeks and 33+6 weeks (mean, 31 weeks). Among these women, 12 delivered at early preterm (<34 weeks) and nine delivered at late preterm (34 to 36+6 weeks). Ten women delivered within 7 days of admission. “Cervical length as measured by transperineal ultrasound on admission was positively correlated with the admission-to-delivery interval [r=0.61; p<0.001],” noted the researchers. [Hong Kong J Gynaecol Obstet Midwifery 2023;24]

While previous preterm delivery, maternal age, history of antepartum haemorrhage, and cervical length were identified as risk factors in univariate analysis, in multivariate analysis, only cervical length remained significantly associated with preterm delivery. According to receiver operating characteristic curve analysis, the cut-off cervical length of 2.5 cm was the most sensitive (90.0 percent) and specific (86.0 percent) for predicting delivery within 7 days of admission, with a positive predictive value of 63.5 percent and a negative predictive value of 97.7 percent.

“Nonetheless, different cut-off values should be used at different gestational ages, due to natural shortening of the cervix as gestation advances,” advised the researchers. “The false-positive rate is higher if the cut-off value of 2.5 cm is used after 32 weeks of gestation; therefore, a cut-off value of 1.5 cm should be used after 32 weeks to improve sensitivity and positive predictive value. Our sample was too small to stratify according to gestational age.” [Ultrasound Obstet Gynecol 2007;29:421-426]