A prospective study of the independent effects of parity and age at first birth on breast cancer incidence in England and Wales

Abstract

The independent effects of parity and age at first birth on breast cancer incidence are investigated in a 1% sample of women aged 16 to 59 from the 1971 Census of England and Wales. Over the period 1971-81, 1,003 breast cancer cases occurred in the cohort of 113,263 women who were either married, widowed or divorced at the time of Census. Age at first birth was positively related to breast cancer risk, women giving birth to their first child after 35 years being at greater risk than nulliparous women. This effect remained, after adjustment for number of live-born children. Breast cancer risk showed a statistically significant decline with increasing parity even after adjustment for age at first birth. These results are consistent with other published evidence which suggests that other births subsequent to the first have an independent effect on breast cancer risk.

Risk Factors for Breast Cancer among Japanese Women in Tokyo : A Case-Control Study

“…Several recent studies have noted that frequent full-term pregnancy or birth has a protective effect against breast cancer, independently of age at first birth/full-term pregnancy2, 16,19,23,24,26) In our study also, a large number of livebirths was independently associated with a decreased risk, particularly in premenopausal women. This finding may essentially indicate that a recently increasing breast cancer incidence in Japan is partly related to the rapidly declined birth rate50) Ever-lactated women were not at a significantly smaller risk of breast cancer, when the covariates were adjusted.…”

Reproductive Factors in Breast Cancer Epidemiology

“…Fifteen studies with more than 21 000 cases have shown independent associations with parity and age at first birth (9,14,15,17,33,34,45,(56)(57)(58)(59)(60)(61)(62)(63)(64). Five out of a total of six cohort-studies (33,34,58,62,63,64) belong to this group. Six additional studies (8,(65)(66)(67)(68)(69) with more than 3 000 cases, 94% from population-based case-control studies, showed an inverse association with parity, but no significant association with ' Cases and controls from screening clinics.…”

Effects of Reproduction on Spontaneous Development of Endometrial Adenocarcinomas and Mammary Tumors in Donryu Rats

“…[3][4][5][6][7][8][9][10][11] The risk of endometrial cancer is remarkably decreased in women experiencing 4 or more pregnancies/deliveries. 5) While the precise reasons for the association of multiple reproductions with decreased risk of endometrial cancer are still uncertain, they are presumed to be due to alteration in the hormonal milieu.…”

“…In humans, it is reported that the age of reproductive experience is one of the principal factors influencing this tumor, and reproductive frequency probably has an independent effect on breast cancer risk. [8][9][10][11] It is proposed that decrease of breast cancers in multiparous women is probably related either to long-lasting hormonal alterations or to differentiative changes that render the mammary tissue less susceptible to carcinogenic agents. 20) With regard to the hormonal aspect, certain hormonal changes may last for at least several years after pregnancy/delivery and lactation.…”

“…[3][4][5][6] With breast cancer, on the other hand, it is reported that the age of reproductive experience is one of the principal factors influencing this tumor and reproductive frequency probably has an independent effect on breast cancer risk. [7][8][9][10][11] However, there is no report concerning the effects of reproduction on development of these tumors in experimental animals. Previously, we described spontaneous tumors in Donryu rats.…”