Objective: To analyse the association between fetal size at time of dating ultrasound and risk for preterm delivery and small-for-gestational-age (SGA) birth and to evaluate if timing of ultrasound, that is before 14 weeks of gestation or after 16 weeks affects this association.
Design: Retrospective cohort study.
Setting: Ultrasound departments of Ultragyn, Stockholm, Sweden.
Population: A total of 28,776 singleton pregnancies dated between 1998 and 2004.
Methods: Obstetric outcome was assessed through linkage of the cohort to the Swedish Medical Birth Register.
Main outcome measures: Risks of preterm delivery, low birthweight for gestational age, pre-eclampsia, asphyxia, respiratory distress, instrumental delivery, caesarean section, and postterm birth were calculated for the groups dated early and late.
Results: When the expected date of delivery was postponed after ultrasound dating by 7 days or more, there was an increased risk for preterm delivery and pre-eclampsia in the late dating group (OR 1.49, 95% CI 1.27-1.73 and OR 1.27, 95% CI 1.02-1.60, respectively) but not in the early dating group. In both dating groups, there was an increased risk for SGA birth (OR 1.77, 95% CI 1.13-2.78 and OR 2.09, 95% CI 1.59-2.73, respectively) There was no increased risk for any of the other diagnoses.
Conclusion: Our study gives further support to the notion that intrauterine growth restriction may be present as early as the first trimester. Accordingly, our study also suggests that surveillance of pregnancies with postponed estimated date of delivery may provide means for increased detection of fetal growth restriction.