Background: Data on birth outcome and offspring health after the appearance of breast cancer are limited. The aim of this study was to assess the risk of adverse birth outcomes in women previously treated for invasive breast cancer compared with the general population of mothers. Methods and Findings: Of all 2,870,932 singleton births registered in the Swedish Medical Birth Registry during 1973-2002, 331 first births following breast cancer surgery - with a mean time to pregnancy of 37 mo (range 7-163) - were identified using linkage with the Swedish Cancer Registry. Logistic regression analysis was used. The estimates were adjusted for maternal age, parity, and year of delivery. Odds ratios (ORs) and 95% confidence intervals (CIs) were used to estimate infant health and mortality, delivery complications, the risk of preterm birth, and the rates of instrumental delivery and cesarean section. The large majority of births from women previously treated for breast cancer had no adverse events. However, births by women exposed to breast cancer were associated with an increased risk of delivery complications (OR 1.5, 95% CI 1.2-1.9), cesarean section (OR 1.3, 95% CI 1.0-1.7), very preterm birth (<32 wk) (OR 3.2, 95% CI 1.7-6.0), and low birth weight (<1500 g) (OR 2.9, 95% CI 1.4-5.8). A tendency towards an increased risk of malformations among the infants was seen especially in the later time period (1988-2002) (OR 2.1, 95% CI 1.2-3.7). Conclusions: It is reassuring that births overall were without adverse events, but our findings indicate that pregnancies in previously treated breast cancer patients should possibly be regarded as higher risk pregnancies, with consequences for their surveillance and management.