Birth weights of South African babies. III. Seasonal variation in birth weight.

Reports on seasonal variation in birth weight are conflicting and inconclusive. No variation was observed by Brenton (1922) or by Bakwin and Bakwin (1929); babies born in the summer were said by Murray (1924), Abels (1926), Toverud (1933), Bivings (1934), Li (1936), and Donald (1939) to be heavier than those born in the winter; Adersen (1899) reported that babies born in the winter were heavier. We have previously examined the birth weights of South African babies in relation to racial group, sex, and birth rank. (Salber and Bradshaw, 1951 a, b). The data were obtained from records of 3,165 European, 1,058 Colouredt, 2,190 Bantu, and 1,403 Indian births in Durban, Capetown, and Pietermaritzburg. Using the same data, we here explore the possibility of seasonal variation in birth weight. It has been shown that distribution of births by birth rank is very different in the four racial groups; for example over 20 per cent. of the Indian babies are of birth rank "6 and over" as compared with less than 1 per cent. of the European babies (Salber and Bradshaw, 1951b). Since it is also known that mean birth weight is related to birth order (Pearson, 1914; Martin, 1931), we have separated our data, as far as numbers permit, according to birth rank, and for each of the four racial groups the mean weight of babies born during the South African summer (October to March) is compared with the mean birth weight of winter-born babies of the same sex and birth rank. The number of European babies of birth rank "6 and over" (eighteen males and twelve females) was too small to permit subdivision by season of birth; they have therefore been omitted from the Table.