Estrogen profiles of primiparous and nulliparous women in Athens, Greece.

Urine estrogens of 144 primiparous women and 122 nulliparous women of similar age were measured. The primiparae were at least 6 months post pregnancy. The ratio of the estriol concentration to the sum of the concentrations of estrone and estradiol (the estriol ratio) was higher among primiparous women under 25 years of age than among nulliparous women of the same age. The difference was greater in specimens collected in the luteal phase of the menstrual cycle than in those collected in the follicular phase and was statistically significant (P less than 0.05, one-tailed t-test) only in the luteal phase. The high estriol ratios of the primiparous women did not decline with increasing interval after pregnancy, at least up to 24 months. Among the younger women, significantly lower levels of pregnanediol, suggestive of a higher frequency of anovular menstrual cycles, were also seen in the nulliparous women. The findings were consistent with the hypothesis that the protective effect against breast cancer of a full-term pregnancy at an early age operates either through a change in the pattern of estrogen metabolism induced by the pregnancy or through a change in the frequency of ovular cycles.