P13.06: The effect of second‐trimester fetal morphometry on duration of pregnancy

Abstract

Mean length of pregnancy is calculated to be 280–282 days from the first day of the last menstruation (LMP). Second-trimester ultrasound biometry seems to determine day of confinement more precisely than certain LMP. We hypothesized that the reason could be that small fetuses have a longer duration of pregnancy compensating for the underestimated age at time of scanning. The aim of this study was therefore to determine the relationship between second-trimester biometry and duration of pregnancy. This was a prospective cross-sectional study of 541 low-risk singleton pregnancies with regular certain LMP and spontaneous delivery. Head circumference (HC), abdominal circumference (AC) and femur length (FL) were measured at 10–24 weeks of gestation. Gestational age was calculated from LMP and birth weight noted. ANOVA and parametrical tests were used for the analysis. Results: Small fetuses (< 10 thpercenthile) had significantly (p < 0.005) longer duration of pregnancy than large fetuses (> 90 thpercenthile). Smallness of HC, AC and FL was associated with pregnancy duration of 288.4, 286.8, and 286.8 days, respectively. Large HC, AC and FL resulted in pregnancy duration of 280.8, 277, and 277 days, respectively. Interestingly, small HC and AC resulted also in significantly (p < 0.005) lower birth weight (3570 and 3418 gram, respectively) compared to those with large biometry (3943 and 3837 gram, respectively). Small fetuses measured at the second-trimester scan tend to have lower birth weight and longer pregnancies compared to large fetuses (at the most 9.8 days). An extended duration of pregnancy did not compensate reduced birth weight.