Very low second-trimester maternal serum alpha-fetoprotein: Association with high birth weight

OBJECTIVE: To investigate the relationship between very low maternal serum alpha-fetoprotein levels (MSAFP), neonatal size, and possible associations with obstetric complications. METHODS: This is a retrospective case-control study in a population managed prospectively by a standardized protocol. Perinatal outcomes were compared between patients with unexplained very low MSAFP (less than or equal to 0.25 multiples of the median) and control pregnancies with normal MSAFP, matched by precise gestational age, parity, maternal age within 1 year, and gender of the newborn. RESULTS: Of the 84,909 women screened, 464 (0.55%) met the definition of very low MSAFP. On tertiary evaluation, 226 had dates reassigned by ultrasound. After exclusion of overt diabetics, patients who were not pregnant, invalidated MSAFP, and 17 patients lost to follow-up, 178 women (0.21% of the total) had true very low MSAFP. True very low MSAFP was associated with subsequent miscarriage in 67 women and with fetal aneuploidy and/or serious abnormalities in 12 patients, leaving a population of 97 women (1.14 per 1000 women screened) with unexplained very low MSAFP. Without obvious demographic or obstetric factors, these women had heavier babies, more babies above the 90th percentile, more delivery complications caused by large birth weight (41 versus 16, χ², P < .001) compared with gestational-age matched controls from the same screened population who had normal MSAFP. CONCLUSION: Very low MSAFP predicts an unusually high rate of large birth weight infants, with increased fetal, intrapartum, and neonatal consequences. Maternal medical conditions or obvious demographic factors do not explain these consequences. These findings suggest a role for close fetal surveillance in the third trimester and extended efforts to assess maternal and neonatal glucose status.