Does a Discrepancy Between Gestational Age Determined by Biparietal Diameter and Last Menstrual Period Sometimes Signify Early Intrauterine Growth Retardation?

Objective: To assess the association between gestational age estimated from the last menstrual period (GA(LMP)) or from the biparietal diameter (GA(BPD)), and the subsequent birthweight for gestational age.

Design: Population-based follow up study.

Setting: Of 21,936 pregnancies contained in the ultrasound database, 16,387 singleton pregnancies with a reliable last menstrual period date and an ultrasound examination between 12 and 22 weeks of gestation were included.

Main Outcome Measures: Correlation between: 1) birthweight deviation (birthweight-expected weight for gestation); 2) birthweight; and 3) pregnancy length and (GA(LMP)-GA(BPD)). Relative risk of birthweight < 2,500 g and low birthweight for gestation (> 22% below normal weight) related to five levels of discrepancy between (GA(LMP)-GA(BPD)).

Results: (GA(LMP)-GA(BPD)) was not associated with deviation of birthweight related to GA(BPD). However the risk of low birthweight (< 2,500 g) and low birthweight for gestational age was significantly increased when (GA(LMP)-GA(BPD)) was > 7 days.

Conclusion: A biparietal diameter smaller than expected from the last menstrual period date is mainly a problem of an error related to estimated time of ovulation. At the same time the relative risk of a low birthweight infant is slightly increased.