Maternal and fetal characteristics affect discrepancies between pregnancy-dating methods: a population-based cross-sectional register study.

INTRODUCTION: Gestational age is estimated by ultrasound using fetal size as a proxy for age, although variance in early growth affects reliability. The aim of this study was to identify characteristics associated with discrepancies between last menstrual period-based (EDD-LMP) and ultrasound-based (EDD-US) estimated delivery dates.

MATERIAL AND METHODS: We identified all singleton births (n = 1 201 679) recorded in the Swedish Medical Birth Register in 1995-2010, to assess the association between maternal/fetal characteristics and large negative and large positive discrepancies (EDD-LMP earlier than EDD-US and 10th percentile in the discrepancy distribution vs. EDD-LMP later than EDD-US and 90th percentile). Analyses were adjusted for age, parity, height, body mass index, smoking, and employment status.

RESULTS: Women with a body mass index >40 kg/m(2) had the highest odds for large negative discrepancies (-9 to -20 days) [odds ratio (OR) 2.16, 95% CI 2.01-2.33]. Other factors associated with large negative discrepancies were: diabetes, young maternal age, multiparity, body mass index between 30 and 39.9 kg/m(2) or <18.5 kg/m(2) , a history of gestational diabetes, female fetus, shorter stature (<-1 SD), a history of preeclampsia, smoking or snuff use, and unemployment. Large positive discrepancies (+4 to +20 days) were associated with male fetus (OR 1.80, 95% CI 1.77-1.83), age ≥30 years, multiparity, not living with a partner, taller stature (>+1 SD), and unemployment.

CONCLUSIONS: Several maternal and fetal characteristics were associated with discrepancies between dating methods. Systematic associations of discrepancies with maternal height, fetal sex, and partly obesity, may reflect an influence on the precision of the ultrasound estimate due to variance in early growth.