In a large International Premenopausal Breast Cancer study which included close to 900,000 women, compared with nulliparous women, parous women had an increased risk for breast cancer that peaked about 5 years after childbirth and then gradually decreased about 24 years after childbirth.The increase in breast cancer risk after childbirth may be due to proliferation of breast cells during pregnancy which could promote accelerated development of latent initiated tumor cells. Childbirth also brings about maternal changes beyond breast tissue including altered immune function and microbiota, increased stress, and accelerated aging processes. Health Care Professionals should take these factors into account when considering individual risk profiles for breast cancer in premenopausal women.

SUMMARY: Breast cancer is the most common cancer among women in the US and about 1 in 8 women (12%) will develop invasive breast cancer during their lifetime. Approximately 268,600 new cases of female breast cancer will be diagnosed in 2019 and about 41,760 women will die of the disease. Breast cancer is the most common cancer type in reproductive-aged women. Women with biological children (parous women) are at a lower risk for developing breast cancer compared to nulliparous women. However, parity as a protective factor largely applies to breast cancer developing after age 60 years and may not apply for younger premenopausal women. Evidence from national registry linkage studies in Scandinavian countries suggested that recent childbirth confers a short-term increase in breast cancer risk which may last for 10 years or more, and this risk may be further increased in women who are older at first childbirth. Studies published thus far have not shown consistent findings and have had limited ability to account for factors influencing breast cancer risk such as breastfeeding and family history of breast cancer.

One biological explanation for an increase in breast cancer risk after childbirth may be due to proliferation of breast cells during pregnancy which could promote accelerated development of latent initiated tumor cells. This may also explain the higher breast cancer risk, conferred by older age at first childbirth, as a result of higher proportion of latent initiated tumor cells at older ages. Further, the postpartum breast microenvironment, characterized by lactational involution may also facilitate cancer cell migration and metastasis. Childbirth also brings about maternal changes beyond breast tissue including altered immune function and microbiota, increased stress, and accelerated aging processes.

The authors in this study used data from the International Premenopausal Breast Cancer Collaborative Group and conducted a pooled analysis of individual-level data from 15 prospective cohort studies. In this analysis, the researchers included women younger than 55 years and evaluated the risk of breast cancer in relation to recent childbirth, while taking into account other factors that relate to breast cancer risk such as breastfeeding, numbers of pregnancies and births and family history of breast cancer. It is felt that understanding these risk patterns may have implications for identifying risk-reducing strategies among vulnerable subgroups. A total of 889, 944 women, were available for analysis after excluding women who reported a first birth before age 13 years, women who were 50 years or older at study entry and at most recent birth, or reached parity greater than 10 births before enrollment. All these events were considered to have greater potential for data errors. The mean age at study entry was 42 years.

The researchers noted that compared with nulliparous women, parous women had an increased risk for breast cancer that peaked about 5 years after childbirth and then gradually decreased about 24 years after childbirth. These findings however were not noted among women who had only 1 child, or had their first child before age 25 years. The risk was highest in women who were older at the time of first childbirth, multiparous women and those who had a family history of breast cancer. Among those women with a family history of breast cancer, the risk was the greatest for Estrogen Receptor negative breast cancer. Breast feeding did not influence breast cancer risk patterns.

It was concluded that compared with nulliparous women, parous women have an increased risk for breast cancer after childbirth that is highest the first 5 years but decreases over the following 20 years. Health Care Professionals should take these factors into account when considering individual risk profiles for breast cancer in premenopausal women. Breast Cancer Risk After Recent Childbirth: A Pooled Analysis of 15 Prospective Studies. Nichols HB, Schoemaker MJ, Cai J, et al. Ann Intern Med. 2019;170:22-30