The results are compatible with the hypothesis that the increase in breast cancer risk with increasing BMI among postmenopausal women is largely the result of the associated increase in estrogens, particularly bioavailable estradiol.

Summary The associations between serum concentrations of oestradiol, progesterone, testosterone and sex hormone-binding globulin (SHBG) and risk of breast cancer in premenopausal women were investigated in a prospective study of breast cancer on the island of Guernsey. Sixty-two women diagnosed with breast cancer an average of 8 years subsequent to blood collection were matched for day of menstrual cycle, age and year of blood collection with 182 control subjects. Cases had a 12% higher mean oestradiol concentration over the whole menstrual cycle (P = 0.17) with a large difference at mid-cycle (75% higher, P = 0.04). Differences between cases and control subjects in progesterone (luteal phase), testosterone and SHBG were small and not statistically significant: luteal phase progesterone 9% lower in cases, P = 0.64; testosterone 4% higher, P = 0.57; SHBG 8% higher, P = 0.24. The small difference in oestradiol concentration could be aetiologically important, but larger prospective studies are needed.

(SHBG) and risk of breast cancer in post-menopausal women were investigated in a prospective study on the island of Guernsey. Sixty-one women who developed breast cancer an average of 7.8 years after blood collection were matched for age, year of blood collection and number of years post-menopausal with 179 control subjects. Women using exogenous hormones at the time of blood collection were excluded from the study. Women who subsequently developed breast cancer had a 29% higher geometric mean oestradiol concentration than control women (P = 0.004). The odds ratio for breast cancer in the top third compared with the lowest third of the oestradiol concentration distribution was 5.03 (95% confidence interval 2.02-12.49, Pfor trend < 0.001). Adjusting for testosterone and SHBG concentrations did not substantially alter the odds ratio for oestradiol. Although testosterone and SHBG concentrations were associated with breast cancer risk, the concentrations of these hormones were correlated with those of oestradiol; the associations were not statistically significant after adjusting for oestradiol concentration. These data provide evidence that serum oestradiol concentrations in post-menopausal women may have a substantial effect on breast cancer risk.

This study examined the hormonal differences between premenopausal mothers of twins and other premenopausal parous women during and after pregnancy. Serum concentrations of oestradiol and testosterone between 6 and 20 weeks of gestation were measured for 11 mothers of twins and 115 mothers of singletons selected from the controls in a case-control study of cryptorchidism. Serum concentrations of oestradiol, progesterone, testosterone, follicle stimulating hormone and sex hormone-binding globulin during the menstrual cycle were measured for 25 mothers of twins and 38 mothers of singletons recruited as a part of a prospective study of breast cancer risk. During pregnancy, women carrying twins had a 58% higher geometric mean oestradiol concentration (p = 0.02) and a 50% higher testosterone concentration (p = 0.03) than women carrying singletons. Women who had previously had twins demonstrated a 49% higher mean concentration of follicle stimulating hormone (p = 0.02) and a 42% higher concentration of sex hormone-binding globulin (p = 0.03) than women who had singletons only, but no significant differences in oestradiol, progesterone and testosterone concentrations. The increased concentrations of follicle stimulating hormone during the menstrual cycle of mothers of twins, which has also been reported in two previous studies suggests that follicle stimulating hormone level may be an important determinant of dizygotic twinning.

The biological, social and reproductive characteristics of women who have had twins were compared with those of other parous women using questionnaire data gathered for a prospective cohort study of women aged 35 and over on the island of Guernsey. Data for 97 mothers of twins and 4026 other parous women were available for analysis. The two groups were similar in height, first degree family history of breast cancer, use of hormone replacement therapy or other hormones, age at menarche, length of menstrual cycle and age at first and last birth. The mothers of twins were slightly heavier, more likely to smoke and consume more caffeine, less likely to have used oral contraceptives in the past, slightly younger when reaching the menopause and had a larger number of pregnancies. Adjustment for age did not alter these results. This study does not provide evidence that mothers of twins differ markedly from other parous women across a range of characteristics that might be associated with twinning or associated with breast cancer risk.