Determinants and consequences of discrepancies in menstrual and ultrasonographic gestational age estimates

Objectives To assess the association between maternal and fetal characteristics and discrepancy between last normal menstrual period and early (<20 weeks) ultrasound-based gestational age and the association between discrepancies and pregnancy outcomes. Design Hospital-based cohort study. Setting Montreal, Canada. Sample A total of 46,514 women with both menstrual- and early ultrasound-based gestational age estimates. Main outcome measures Positive (last normal menstrual period > early ultrasound, i.e. menstrual-based gestational age is higher than early ultrasound-based gestational age, so that the expected date of delivery is earlier with the menstrual-based gestational age) discrepancies greater than or equal to+7 days, mean birthweight, low birthweight, stillbirth and in-hospital neonatal death. Results Multiparous mothers and those with diabetes, small stature or high pre-pregnancy body mass index were more likely to have positive discrepancies. The proportion of women with discrepancies greater than or equal to+7 days was significantly higher among chromosomally malformed and female fetuses. The mean birthweight declined with increasingly positive differences. The risk of low birthweight was significantly higher for positive differences. Associations with fetal growth measures were more plausible with early ultrasound estimates. Conclusions Although most discrepancies between last normal menstrual period- and early ultrasound-based gestational age are attributable to errors in menstrual dating, our results suggest that some positive differences reflect early growth restriction.