The risk of spontaneous preterm birth in asymptomatic women with a short cervix (≤25 mm) at 23-28 weeks' gestation

Abstract

Background: Asymptomatic short cervical length is an independent risk factor for spontaneous preterm birth. However, most studies have focused on the associated risk of a short cervical length when encountered between 16 and 23 weeks' gestation. The relationship between cervical length and risk of spontaneous preterm birth after 23 weeks is not well known.

Objective: To evaluate the risk of spontaneous preterm birth in asymptomatic women with a short cervix (≤25 mm) at 23-28 weeks' gestation.

Materials and methods: A retrospective cohort study of women with asymptomatic short cervix (cervical length ≤25 mm) at extreme prematurity, defined as 23-28 weeks' gestation, was performed at a single center from January 2015 to March 2018. Women with symptoms of preterm labor, multiple gestations, fetal or uterine anomalies, cervical cerclage, or those with incomplete data were excluded from the study. Demographic information as well as data on risk factors for spontaneous preterm birth were collected. Patients were divided into 4 groups based on the cervical length measurement (≤10 mm, 11-15 mm, 16-20 mm, and 21-25 mm). The primary outcome was time interval from enrollment to delivery. Secondary outcomes included delivery within 1 and 2 weeks of enrollment, gestational age at delivery, and delivery prior to 32, 34, and 37 weeks, respectively. Continuous variables were compared using Kruskal-Wallis test, whereas categorical variables were compared using the χ² or Fisher exact test as appropriate. The Wilcoxon test for difference in survival time was used to compare gestational age at delivery among the 4 cervical length groups, with data stratified based on gestational age at enrollment.

Results: Of the 126 pregnancies that met inclusion criteria, 22 (17.4%) had a cervical length of ≤10 mm, 23 (18.3%) had a cervical length of 11-15 mm, 37 (29.4%) had a cervical length of 16-20 mm, and 44 (34.9%) had a cervical length of 21-25 mm. Baseline characteristics were similar among all 4 groups. The shorter cervical length group was associated with a shorter time interval from enrollment to delivery (cervical length ≤10 mm, 10 weeks; cervical length 11-15 mm, 12.7 weeks; cervical length of 16-20 mm, 13 weeks; cervical length of 21-25 mm, 13.2 weeks; P = .006). Regardless of the cervical length measurement, delivery within 2 weeks was extremely uncommon (1 patient; 0.8%). The prevalence of spontaneous preterm birth at <32 weeks or <34 weeks was higher in women with a cervical length of ≤10 mm compared to those with a longer cervical length (P < .001).

Conclusions: The risk of spontaneous preterm birth in asymptomatic women with a sonographic short cervix increases as cervical length decreases. The risk is substantially higher in women with a cervical length of ≤10 mm. Women with a cervical length of ≤10 mm also had the shortest time interval to delivery. Nevertheless, delivery within 1 or 2 weeks is highly unlikely, regardless of the cervical length at the time of enrollment. Therefore, based on our data, we suggest that management decisions such as timing of administration of antenatal corticosteroids in asymptomatic patients with a cervical length of ≤25 mm at 23-28 weeks' gestation may be delayed until additional indications are present.

Keywords: antenatal corticosteroids; anticipated delivery; cervical length; periviability; prematurity; preterm birth; short cervix; transvaginal ultrasound.