There is growing evidence that perinatal factors associated with altered gestational hormones may influence subsequent breast cancer risk in the mother. Events occurring during the first pregnancy may be particularly important. In this matched case‐control study, we investigated the relation between characteristics of a woman's first pregnancy and her later breast cancer risk using linked records from the New York State birth and tumor registries. Cases were 2,522 women aged 22 to 55 diagnosed with breast cancer between 1978 and 1995 and who had also completed a first pregnancy in New York State (NY) at least 1 year prior to diagnosis. Controls were 10,052 primiparous women not diagnosed with breast or endometrial cancer in NY and matched to cases on county of residence and date of delivery. Information on factors characterizing the woman's first pregnancy was obtained from the pregnancy record of each subject. The association of these factors to breast cancer risk was assessed using conditional logistic regression. Extreme prematurity (< 32 weeks gestational age) was associated with elevated maternal breast cancer risk [adjusted odds ratio (OR)=2.1, 95% confidence interval (CI) 1.2,3.9], as were abruptio placentae (OR = 1.8, CI 1.1,3.0) and multifetal gestation (OR=1.8, CI 1.1,3.0). Preeclampsia was associated with a marked reduction in breast cancer risk among women who bore their first child after age 30 (OR=0.3, CI 0.2,0.7) and in the first 3 years after delivery (OR=0.2 (0.1–0.9). These findings suggest that certain perinatal factors influence maternal breast cancer risk and offer indirect support for a role of gestational hormones, and particularly gestational estrogens, in the etiology of breast cancer. © 2004 Wiley‐Liss, Inc.