Epidemiological evidence suggesting that subjects with lower birthweight have an increased risk of adult cardiovascular disease has led to increased interest in factors influencing birthweight. We have documented large changes in mean birthweight over a relatively short historical period from 1857 to 1883. Mean birthweight declined progressively from 7.9 lb (3.6 kg) in the period 1857–63 to 6.9 lb (3.1 kg) in 1874–78, then rose to 7.5 lb (3.4 kg) for the period 1879–83. We found the expected relationships between infant birthweight and maternal age, parity and marital status, and identified an association between birthweight and maternal country of birth. However, neither temporal changes in recorded maternal characteristics nor external economic indicators for the colony explained the trends in birthweight. From historical information, we believe that the explanation for our findings lies primarily with the increasing poverty, disease, mental illness, alcohol abuse and criminal activity among women admitted to the hospital between 1860 and the mid-1870s, and improvements from around 1880, when old housing was replaced and prostitutes were moved out of the area. Our findings highlight the difficulty of capturing lifestyle factors using routinely collected data, and the need for historical expertise when examining historical data.