An increased risk of breast cancer after delayed first parity

As delayed childbirth increases for socioeconomic and fertility reasons, its impact on breast cancer risk needs definition. From 1975 to 1981, 1,307 women with childbirth at ≥40 years of age were identified. They were divided into four groups by estimated first birth median ages (EFBMA): 23, 34, 38, and 41 years, corresponding to previous parity of more than 3, 2 or 3, 1, and zero, respectively. Cancer Registry cross-referencing identified those diagnosed with breast cancer. Breast cancer developed in 39 women. The EFBMA of 41 years carried a relative risk of 3.7, (95%CI: 1.30 to 10.5) compared with age 23. Odds ratio of breast cancer was 1.08 (95%CI: 1.02 to 1.14) with each year older at first birth and 0.79 (95% CI: 0.67 to 0.93) for each additional previous birth. Increased breast cancer risk with advancing maternal age at first childbirth is supported by 3.7 relative risk in women with an EFBMA of 41 years compared with those with an EFBMA of 23 years.