Birth weight for gestational age of Mexican American infants born in the United States

Objective: To develop a reference for birth weight for gestational age to identify Mexican American infants born in the United States who are small or large for gestational age. Methods: Reference percentiles were developed for Mexican American and non-Hispanic white births, using national vital statistics from 1992-1994 for Mexican Americans (n = 1,197,916) and 1994 for non-Hispanic whites (n = 2,238,457). Birth weights and gestation from the last menstrual period were taken from birth certificates. Smoothed curves were fit, using unweighted fourth-degree polynomial equations, for the tenth, 50th, and 90th percentiles by gender and parity. Results: Mexican American infants were heavier than non-Hispanic white infants between 30 and 37 weeks' gestation for all parities and both genders. However, at term there was consistent crossover. Non-Hispanic white infants were heavier at or after 37 through 42 weeks' gestation, whereas the growth of Mexican American infants appeared to slow. Beginning at 37 weeks, the differences in weights of infants of primiparas increased to more than 100 g by 40 weeks; the differences were only slightly less for infants of multiparas. Conclusion: Given differences in distribution of birth weights for gestational age between Mexican Americans and non-Hispanic whites, the ability to recognize fetal growth restriction (FGR) or excessive growth is questionable. These data provide a reference for Mexican Americans for clinical use and for future studies in identifying infants at risk for FGR or overgrowth. (Obstet Gynecol 1999;93:943-7.)