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Introduction: A short cervix (cervical length < 25 mm) in the mid-trimester (18 to 24 weeks) of pregnancy is a powerful predictor of spontaneous preterm delivery (gestational age at delivery < 37 weeks). Although the biological mechanisms of cervical remodeling have been the subject of extensive investigation, very little is known about the rate of change in cervical length over the course of a pregnancy, or the extent to which rapid cervical shortening increases maternal risk for spontaneous preterm delivery.
Methods: A cohort of 5,160 unique women carrying 5,971 singleton pregnancies provided two or more measurements of cervical length during pregnancy. Cervical length was measured in millimeters using a transvaginal 12-3 MHz ultrasound endocavity probe (SuperSonic Imagine). Maternal characteristics, including relevant medical history and birth outcome data, were collected for each participant. Gestational age at delivery was measured from the first day of each woman’s last menstrual period and confirmed by ultrasound. Repeated measurements of cervical length during pregnancy were modeled as a longitudinal, multilevel growth curve in MPlus. A three-level variance structure was used to account for non-independence of repeated measurements clustered within pregnancies, which are clustered within participants.
Results: The average number of cervical length measurements per pregnancy is 6. Shorter mid-trimester cervical lengths and accelerated rates of cervical shortening are associated with shorter gestational duration. A smaller initial cervical length (p < 1*10-4) and a faster rate of change in cervical change length during pregnancy (p < 1*10-4) are significantly associated with an earlier gestational age at delivery. A higher pre-pregnancy body mass index (BMI) is associated with shorter initial cervical length in early pregnancy (p < 1*10-4), while maternal age is associated with a more rapid rate of change in cervical length (p < 1*10-4). Parameters describing cervical length and its rate change during pregnancy (i.e., intercept, linear slope, and quadratic slope parameters) explained 59% more variance in gestational age at delivery than a single mid-rimester cervical length measurement, which is the current gold standard in clinical practice. However, a significant amount of residual variance in individual estimates of cervical length growth parameters remains (p < 1*10-4), which could be accounted for, in part, by common variation in the population.
Conclusion: We have developed longitudinal models of cervical length that describe individual and group level trajectories of cervical change across pregnancy. Extensions of this model incorporating genomic data, can be used to estimate the heritability of cervical length and its role in mediating the timing of birth.
Maternal and Child Health | Obstetrics and Gynecology | Statistical Models
Timothy P. York, Ph.D.
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VCU Graduate Research Posters