Ethnicity modifies seasonal variations in birth weight and weight gain of infants.

Early growth is considered critical for the development of obesity and adulthood cardiovascular diseases. Because season and environmental temperature at birth have also been associated with obesity, it is important to examine whether birth weight and postnatal weight gain vary by season. Data from the National Collaborative Perinatal Project were used to assess the influence of season on birth weight and weight gain during the first 4 mo of life. The sample included 11,091 whites, 11,477 blacks, 1536 Puerto Ricans and 221 subjects of other ethnic groups born at full-term gestation. Black infants born in the fall had a significantly lower birth weight (3.12 +/- 0.42 kg) than those born in the winter (3.16 +/- 0.43 kg, P = 0.002). This difference was not found in the other ethnic groups. Additionally, weight gain (g/mo) for black and Puerto Rican infants during the first 4 mo of life was significantly lower for those born during the fall (black: 816 +/- 186; Puerto Rican: 820 +/- 181) compared to those born in the spring (black: 844 +/- 194, P < 0.001) and summer (Puerto Rican: 861 +/- 185, P < 0.04). Birth weight and early infancy weight gain varied by season and were modified by ethnicity. The potential importance of seasonal variations in pre- and postnatal growth was evaluated in this study.