Birth weight has implications for multiple psychological and physical conditions throughout life. Birth weight is known to be affected by many factors, such as gestational age, maternal nutrition, cigarette smoking, hormones, and maternal height, weight, age, and parity. An additional intriguing correlate of birth weight is season-of-birth. Existing research suggests that seasonality in birth weight from equatorial countries depends on food availability and maternal physical labor. North of the Tropic of Cancer (23.5N), higher birth weights occur in the late winter and spring, and the most prevalent explanatory hypotheses concern the influences of temperature or day length. The seasonal pattern would be expected to be offset by six months in the Southern Hemisphere, but only one study was found from south of the Tropic of Capricorn (23.5S). Thus, in this study, time series analyses of a 20-year, million-birth New Zealand sample tested the hypothesis that heavier birth weights will occur in the spring. Theresults support hypotheses that seasonally varying factors, such as temperature and day length, affect fetal growth during the gestational period, resulting in seasonal birth weight differences.