Association between cervical length and gestational age at birth in singleton pregnancies: a multicentric prospective cohort study in the Brazilian population

BACKGROUND: Short cervical length measured during the second trimester of pregnancy is an important risk factor for spontaneous preterm birth (sPTB). The aim of this study is to identify the association between mid-pregnancy cervical length (CL) and gestational age at birth in asymptomatic singleton pregnant women. METHODS: This is a prospective cohort study involving singleton pregnant women who participated in the screening phase of a Brazilian multicenter randomized controlled trial (P5 trial) between July 2015 and March 2019. Transvaginal ultrasound to measure CL was performed from 18 to 22 + 6 weeks. Women with CL ≤ 30 mm received vaginal progesterone (200 mg/day) until 36 weeks’ gestation. For this analysis we considered all women with CL ≤ 30 mm receiving progesterone and a random selection of women with CL > 30 mm, keeping the populational distribution of CL. We obtained prognostic effectiveness data (area under receive operating characteristic curve (AUC), sensitivity and specificity and estimated Kaplan–Meier curves for preterm birth using different CL cutoff points. RESULTS: We report on 3139 women and identified a negative association between cervical length and sPTB. CL ≤ 25 mm was associated with sPTB < 28, sPTB < 34 and sPTB < 37 weeks, whereas a CL 25–30 mm was directly associated with late sPTB. CL by transvaginal ultrasound presented an AUC of 0.82 to predict sPTB < 28 weeks and 0.67 for sPTB < 34 weeks. Almost half of the sPTB occurred in nulliparous women and CL ≤ 30 mm was associated with sPTB at < 37 weeks (OR = 7.84; 95%CI = 5.5–11.1). The number needed to screen to detect one sPTB < 34 weeks in women with CL ≤ 25 mm is 121 and we estimated that 248 screening tests are necessary to prevent one sPTB < 34 weeks using progesterone prophylaxis. CONCLUSIONS: CL measured by transvaginal ultrasound should be used to predict sPTB < 34 weeks. Women with CL ≤ 30 mm are at increased risk for late sPTB. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1186/s12978-022-01557-w.