Objective
We hypothesized that change in cervical length (CL) on serial ultrasounds is associated with spontaneous preterm birth (SPTB) <36 weeks for women with a short cervix (CL <25 mm).
Study Design
This was a secondary analysis of a multicenter prospective observational study designed to study predictors of preterm birth. Women from the general obstetric population had serial CL ultrasounds between 20-33 weeks’ gestation and were followed up until delivery.
Results
Two thousand six hundred ninety five women had sonographic CL measurements. Change in CL was associated with SPTB for women with CL <25 mm (odds ratio, 0.97; 95% confidence interval, 0.96–0.98). Among women with a short cervix, for every 1 mm of cervical shortening between ultrasounds, there was a 3% increase in odds of SPTB. The association between change in CL and SPTB remained significant after controlling for age, race, body mass index, tobacco use, and fetal fibronectin test status.
Conclusion
Among women with a sonographically short cervix, the rate of change in CL is associated with SPTB, independent of fetal fibronectin test and other important risk factors for SPTB.
Preterm birth remains the leading cause of mortality and morbidity among newborns in developed countries. Decades of research have focused on identifying women at risk for preterm birth and exploring strategies for prevention. Several modifiable and nonmodifiable maternal factors are associated with spontaneous preterm birth (SPTB), however the ability to predict SPTB based on these factors alone is poor.
Sonographic cervical length (CL) has been the most promising tool to date to aid in the prediction of SPTB. The process of cervical effacement was initially revealed by serial sonography that demonstrated progression of effacement from the internal to external cervical os, and ultrasound remains the most accurate method for assessing CL. Delivery outcomes are associated with both CL and gestational age at diagnosis of a short cervix. Various definitions of short CL have been proposed, but CL <25 mm, which represents the 10th centile for CL in the general population, has been widely accepted as a threshold for abnormal CL at early gestational ages. At any point in gestation <34 weeks, shorter CL confers increased risk for SPTB. Likewise, earlier gestational age at diagnosis of short CL increases the likelihood of SPTB.
Identifying short CL by ultrasound improves ability to predict preterm birth; however, the rate of false-positive results remains high. Most women with a CL <25 mm will deliver at term. For women who deliver preterm, there is a period of latency between detection of short CL and delivery of at least 2 weeks, whereas other authors have reported a much longer interval to delivery. It is unknown whether change in CL is associated with SPTB. The purpose of this study was to determine if change in sonographic CL over time is associated with risk for SPTB.
Materials and Methods
This was a secondary analysis of the Preterm Prediction Study conducted by the National Institute of Child Health and Human Development Maternal-Fetal Medicine Units Network. The Preterm Prediction Study was a multicenter prospective observational cohort study designed to study predictors of preterm birth. Subjects (n = 3073) were enrolled at 10 centers from 1992 through 1994. Subjects were followed up from enrollment at <24 weeks’ gestation through delivery.
Women with a singleton gestation from the general obstetrical population with well-dated pregnancies <24 weeks’ gestation were eligible for enrollment. Women with a multiple gestation, placenta previa, cerclage, and fetal anomalies were excluded. Demographic data, obstetric history, blood assays, cultures, and fetal fibronectin samples were collected. CL ultrasounds were performed at 24 weeks (range, 21–28 weeks) and at 28 weeks (range, 25–33 weeks) of gestation. Only CL ultrasounds performed at these visits were included in the present analysis.
The technique for sonographic measurement of CL and the quality assurance protocol used for this cohort have been described previously. Three transvaginal CL measurements were taken at each ultrasound visit. The shortest measurement on a high-quality image was recorded. Measurements were made without applying fundal pressure. When a cervical funnel was present, the remaining CL below the funnel was recorded. Physicians of study patients were not informed of sonogram results unless fetal death, advanced cervical dilation, prolapsed membranes, oligohydramnios or polyhydramnios, or regular uterine contractions were detected.
The difference between length at 24 weeks and 28 weeks (change in CL) was our primary exposure variable of interest. Loss of CL between 24-28 weeks was represented as a negative value for change in CL while gain in CL between visits resulted in a positive value for change in CL. Rate of change was also calculated by dividing the difference in CL by the number of days between measurements. The relationships between change in CL and average daily change in CL and SPTB were evaluated using logistic regression. Statistical analyses were performed using software (Stata 10; StataCorp, College Station, TX).
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