Accurate assessment of the difference in birthweight between first and second live-births to the same woman having excluded the effects of physiological factors known to affect birthweight. Retrospective longitudinal observational study. Three large obstetric units in the East Midlands of the United Kingdom. Women in whom data were recorded for their first two pregnancies on the UK East Midlands Obstetric database which resulted in the delivery of a liveborn, singleton and congenitally normal baby. Six thousand five hundred and thirty such cases were identified, of which 3457 had complete datasets and delivered both babies at term (259 to 300 days). An analysis was performed of changes between the paired pregnancies of physiological factors known to affect birthweight. Regression analyses were used to enable prediction of the second birthweight with the knowledge of the first birthweight. The mean crude birthweight difference between first and second pregnancies was an increase of 138 g. Significant differences between the paired pregnancies were found in maternal booking visit weight, blood pressure, maternal age and gestation at delivery. Independent factors affecting difference in birthweight were gestation at delivery, maternal booking weight and baby's sex. Regression towards the mean was demonstrated which meant that a woman delivering a first baby weighing more than 3720 g could expect a lighter baby for her second delivery provided that all other factors remained constant. In general terms a woman is more likely to deliver a heavier baby in her second pregnancy than in her first pregnancy. However, maternal physiological factors differ in the two pregnancies and these differences have additional effects on birthweight. The effects of both these observations are tempered by regression towards the mean which has a profound influence in predicting the likely change in birthweight between first and second pregnancies. Clinical decisions should not be based on the assumption that a second baby will inevitably be heavier than the first baby.