Maternal obesity has been positively associated with risk of adverse pregnancy outcomes, but evidence of a causal relation is scarce. Causality would be lent support if temporal changes in weight affected risk of adverse pregnancy outcomes. We examined the associations between change in prepregnancy body-mass index (BMI) from the first to the second pregnancies, and the risk of adverse outcomes during the second pregnancy in a nationwide Swedish study of 151 025 women who had their first two consecutive singleton births between 1992 and 2001. Compared with women whose BMI changed between -1.0 and 0.9 units, the adjusted odds ratios for adverse pregnancy outcomes for those who gained 3 or more units during an average 2 years were: pre-eclampsia, 1.78 (95% CI 1.52-2.08); gestational hypertension 1.76 (1.39-2.23); gestational diabetes 2.09 (1.68-2.61); caesarean delivery 1.32 (1.22-1.44); stillbirth 1.63 (1.20-2.21); and large-for-gestational-age birth 1.87 (1.72-2.04). The associations were linearly related to the amount of weight change and were also noted in women who had a healthy prepregnancy BMI for both pregnancies. These findings lend support to a causal relation between being overweight or obese and risks of adverse pregnancy outcomes. Additionally they suggest that modest increases in BMI before pregnancy could result in perinatal complications, even if a woman does not become overweight. Our results provide robust epidemiological evidence for advocating weight loss in overweight and obese women who are planning to become pregnant and, to prevent weight gain before pregnancy in women with healthy BMIs.