Height and weight differences among South African Urban schoolchildren born in various months of the year.

Among 1,165 subjects aged 6-18 years who were examined once cross-sectionally, a significant variation in body height and weight according to the month of birth of the subjects was found. All subjects belonged to the so-called Cape Coloured community, were born locally, and their parents belonged to the most affluent sector of the community. Individuals born in February to July were shorter and weighed less (by 13 to 17% of the standard deviation) than those born in August to January. These results are similar to those obtained for Canadian children, who were measured on their birthdays. Since our individuals were measured only once, at the same time of the year, observed differences cannot be ascribed to seasonal fluctuations of growth rates prior to measurement but must constitute a persistent effect of seasonal fluctuation in earlier growth. Immediate environmental conditions (food, temperature, rainfall, and insolation) in the Southern Hemisphere fluctuate seasonally 6 months out of phase from those in the Northern Hemisphere. Thus the similarity of our findings to those from the Northern Hemisphere suggests a factor common for the entire earth, possibly related to the ellipsoid shape of the orbit of the planet.