The association between maternal serum alpha-fetoprotein and preterm birth, small for gestational age infants, preeclampsia, and placental complications

OBJECTIVE: To determine whether high levels of serum alpha-fetoprotein (AFP) predict increased risk of adverse pregnancy outcomes, including preterm birth (before 37 weeks), preterm birth occurring at or before 28 weeks, small for gestational age (SGA) infant, preeclampsia, and placental abnormalities, and to determine whether low levels of serum AFP predict increased or decreased risk of these outcomes. METHODS: Using the mother's first name, last name, and zip code, we linked the records of 51,008 women who participated in the California Alpha-Fetoprotein Screening Program between June 15, 1986, and October 31, 1987, with California birth certificates for singleton infants born in 1987. The accuracy of the data linkage was confirmed by manually examining complete names, mother's ethnicity, and mother's age for a sample of 500 of the mother-infant linkages. Blood samples were obtained at 15-19 weeks. RESULTS: A strong gradient of increasing risk of preterm birth with increasing levels of serum AFP was observed (test for trend, P < .01). Among women with high levels of serum AFP (at least 2.5 multiples of the median MoM), 24.3% had preterm births, compared with 3.8% of women with low levels of serum AFP (0.81 MoM or less), odds ratio 8.7, 95% confidence interval 7.1-10.7). This gradient persisted when preterm infants of 28 weeks or less were examined separately. Similar gradients were observed for the risk of preeclampsia and placental abnormalities. There was a weaker U-shaped relation between serum AFP level and the risk of an SGA infant. CONCLUSION: Low levels of second-trimester maternal serum AFP are associated with a very low risk of preterm birth, preeclampsia, and placental complications. High levels of serum AFP are strongly associated with preterm birth, preeclampsia, and placental abnormalities. There is a modest association between AFP levels (both low and high) and SGA birth.