A reduced risk of breast cancer is associated with pregnancies lasting 34 weeks or longer, according to an observational study in Nature Communications.
Both the number and the timings of pregnancies have long been associated with changes in breast cancer risk; in particular early age (below 30 years old) full-term pregnancies and a higher number of childbirths have been associated with a lowered breast cancer risk. However, how these factors influence breast cancer development is still unclear.
Mads Melbye and colleagues used the Danish national registries on childbirths and cancer to establish a cohort of 2.3 million women and assessed the association between the duration of a pregnancy and the long term risk of breast cancer. They found that a lower risk of breast cancer was associated with pregnancies that last 34 weeks or more, whereas a pregnancy length of 33 weeks or less did not confer a reduction in risk. They also found that other factors including the number of childbirths and socioeconomic status did not explain the association. Additionally, this analysis was replicated in an equivalent Norwegian cohort of 1.6 million women. The authors suggest that a distinct biological effect might be introduced around week 34 of pregnancy and these findings may enable investigation of the causal factor behind this effect.