Early age at first birth (i.e., before age 20) reduces the risk of breast cancer by about 50% relative to nulliparous women. Full-term pregnancies at later ages add smaller increments of protection, and women who have a first pregnancy over the age of 30 are actually at higher risk of breast cancer than nulliparous women.24 This paradoxical effect of a later first full-term pregnancy has been repeatedly confirmed in epi-demiological studies. Furthermore, a recent full-term pregnancy also increases risk.25 Among women giving birth during the previous 3 years, breast cancer risk is nearly three times higher than that of women of the same age, parity, and age at first birth whose most recent birth occurred at least 10 years earlier. First-trimester abortions, whether spontaneous or induced and occurring before the first full-term preg-nancy,26,27 have also been associated in some studies with increased breast cancer risk, although the interpretation of these studies has been questioned.28
Based on these results, it appears that two contradictory effects of pregnancy on risk of breast cancer are particularly notable during a first pregnancy: a short-term increase in risk, fol lowed in the long term by a substantial reduction in risk.24 This apparent paradox has a physiological explanation based on patterns of estrogen and prolactin secretion and metabolism during pregnancy. During the first trimester of pregnancy, the level of free estradiol rises rapidly. However, as a pregnancy progresses, prolactin and free estradiol levels lower and SHBG levels rise, yielding a net overall benefit with respect to the endogenous estrogen profile. Perhaps more importantly, the effect of a first pregnancy may be to cause some premalignant cells to terminally differentiate, thereby losing their malignant potential.
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