Cnattingius S et al. (2005) Pregnancy characteristics and maternal risk of breast cancer. JAMA 294: 2474–2480

Breast carcinogenesis is known to be influenced by hormones that are markedly elevated in pregnancy. Most such hormones are produced in the placenta. Previous studies had found that indications of placental impairment were associated with reductions in maternal risk of developing breast cancer; Cnattingius et al. report similar results, and suggest that placental weight could be an indirect marker of maternal breast-cancer risk.

Data were obtained from four Swedish population-based registers. The cohort included 314,019 primiparous women who gave birth between 1982 and 1989, and for whom prospective follow-up data through to 2001 were available. In all, 2,216 women developed breast cancer; almost all were diagnosed before age 50. A subset of 121,285 women had a second single birth during the study period.

The risk of maternal breast cancer increased with increasing placental weight. There was a 38% increase in risk with placental weight >700 g, compared with <500 g, over the whole cohort (P = 0.001). Among women aged 30 years or older at first delivery, the increase in risk was even greater (70%; P <0.001). Breast-cancer risk doubled for women whose placental weight was >700 g in two successive pregnancies (compared with women with two successive placental weights <500 g). Placental weight was found to be a better indicator of maternal breast-cancer risk than birth weight.

These findings are only applicable to women with premenopausal breast cancer, say the authors; however, the results support the hypothesis that pregnancy hormones have an effect on maternal risk of breast cancer.