Adverse obstetric outcome in fetuses that are smaller than expected at second trimester routine ultrasound examination

Background.  Adverse obstetric outcome in fetuses that are smaller than expected at second trimester routine ultrasound examination. Aim.  To assess the obstetric outcome when the interval between the estimated date of delivery by last menstrual period and the estimated date of delivery by second trimester ultrasound exceeds 14 days. Materials and methods.  Population‐based prospective cohort study. Included were all women ( n  = 16 302) with a singleton pregnancy, in one Norwegian county from 1989 to 1999, with a reliable menstrual history and whose second trimester ultrasound examination was conclusive with a 35–60‐mm biparietal diameter. Results.  Adverse outcome was defined as preterm birth (< 37 weeks), perinatal death, low birthweight (< 2500 g) and small‐for‐gestational age (< 2 standard deviation). The risk for perinatal death (odds ratio = 2.3), preterm birth (odds ratio = 1.7), low birth weight (odds ratio = 1.5), and small‐for‐gestational age (odds ratio = 1.5) was highly significantly increased in deliveries where the discrepancy between the two terms was more than 14 days. The increased risk is not explained by differences in parity, maternal age, smoking, fetal malformations, or use of oral contraceptives before pregnancy. Conclusion.  Fetuses that are smaller than expected at the mid‐second trimester ultrasound corresponding to a discrepancy of more than 14 days have an increased risk for adverse obstetric outcome.