Multiple births and risk of breast cancer

A linked-registry study of gestational factors and subsequent breast cancer risk in the mother

BACKGROUND

Women who were younger at their first live birth have a reduced breast cancer risk. Other pregnancy characteristics, including complications, also may affect risk but because they are rare, require large datasets to study.

METHODS

The association of pregnancy history and breast cancer risk was assessed in a population-based study including 22,646 cases diagnosed in Washington State 1974 to 2009, and 224,721 controls, frequency matched on parity, age, calendar year of delivery, and race/ethnicity. Information on prediagnosis pregnancies derived from linked birth certificate and hospital discharge databases. Adjusted odd ratios (ORs) and 95% confidence intervals (CI) were calculated.

RESULTS

Multiple gestation pregnancies were associated with decreased breast cancer risk (OR, 0.65; 95% CI, 0.57-0.74) as was prepregnancy obesity (OR, 0.76; 95% CI, 0.65-0.90). Infant birth weight was positively associated (6% per 1,000 g; 95% CI, 3%-9%). The ORs for first trimester bleeding (OR, 3.35; 95% CI, 1.48-7.55) and placental abnormality/insufficiency (OR, 2.24; 95% CI, 1.08-4.67) were increased in women diagnosed at age 50+ years and 15+ years after the index pregnancy. Results were similar in analyses restricted to first pregnancies, those closest to diagnosis, and when excluding in situ disease.

CONCLUSION

These data suggest that multiple gestation pregnancies are protective, whereas delivering larger infants increases risk for later development of maternal breast cancer. Placental abnormalities that result in bleeding in pregnancy also may reverse the long-term protection in postmenopausal women associated with parity.

IMPACT

Certain pregnancy characteristics seem to be associated with later maternal breast cancer risk.

Pregnancy characteristics and maternal breast cancer risk: a review of the epidemiologic literature

The short- and long-term effects of pregnancy on breast cancer risk are well documented. Insight into potential biological mechanisms for these associations may be gained by studying breast cancer risk and pregnancy characteristics (e.g., preeclampsia, twining), which may reflect hormone levels during pregnancy. To date, no review has synthesized the published literature for pregnancy characteristics and maternal breast cancer using systematic search methods. We conducted a systematic search to identify all published studies. Using PUBMED (to 31 July 2009), 42 relevant articles were identified. Several studies suggest that multiple births may be associated with a lowered breast cancer risk of about 10-30%, but results were inconsistent across 18 studies. The majority of 13 studies suggest about a 20-30% reduction in risk with preeclampsia and/or gestational hypertension. Six of seven studies reported no association for infant sex and breast cancer risk. Data are sparse and conflicting for other pregnancy characteristics such as gestational age, fetal growth, pregnancy weight gain, gestational diabetes, and placental abnormalities. The most consistent findings in a generally sparse literature are that multiple births and preeclampsia may modestly reduce breast cancer risk. Additional research is needed to elucidate associations between pregnancy characteristics, related hormonal profiles, and breast cancer risk.

Exploring the underlying hormonal mechanisms of prenatal risk factors for breast cancer: a review and commentary

Prenatal factors have been hypothesized to influence subsequent breast cancer development. Directly evaluating the associations of in utero exposures with risk, however, presents several methodologic and theoretical challenges, including the long induction period between exposure and disease and the lack of certainty regarding the critical timing of exposure. Indirect evaluation of these associations has been achieved by use of proxies such as gestational and neonatal characteristics. Evidence suggests that preeclampsia is associated with a reduced breast cancer risk, whereas high birth weight and dizygotic twinning seem associated with an increased risk. Asians born in Asia have substantially lower breast cancer risks than women born in the West. Although data thus far are few, what exists is not consistent with a unifying hypothesis for a particular biological exposure (such as estrogens or androgens) during pregnancy as mediating the observed associations between pregnancy factors and breast cancer risk. This suggests that additional studies of prenatal factors should seek to broaden the range of hormones, growth, and other endocrine factors that are evaluated in utero. Once candidate biomarkers are identified, assessing them with respect to breast cancer and with intermediate end points in carcinogenesis should be a priority. In addition, investigations should explore the possibility that in utero exposures may not act directly on the breast, but may alter other physiologic pathways such as hormone metabolism that have their effect on risk later in life.

Intrauterine environment and breast cancer risk in a population-based case-control study in Poland

High estrogen exposure in utero may increase breast cancer risk later in life. However, studies of the associations between perinatal factors presumed to affect the fetal hormonal environment and breast cancer risk are inconsistent. We used data from a population-based case-control study of 2,386 incident breast cancers and 2,502 controls in Poland to evaluate risks associated with various perinatal characteristics. After adjusting for confounders, we found a significant trend (p = 0.01) of breast cancer risk with birth weight (OR = 1.54, 95% CI 1.08-2.19 for birth weights >4,000 g vs. <2,500 g). Subjects with a high birth order (> or =6) were at reduced risk (OR = 0.81, 0.61-1.06) when compared with first born subjects. Birth weight was somewhat a stronger risk predictor among subjects whose cancers were diagnosed at 50 years of age or older (OR = 1.84, 1.19-2.85) than among those with cancers diagnosed at younger ages (OR = 1.14, 0.61-2.12). Subjects whose mothers smoked during their pregnancies were at slightly higher risk than those who never smoked (OR = 1.21, 0.99-1.47), but the risk was similar to mothers who only smoked at other times (OR = 1.22, 0.81-1.84). Breast cancer risk was not related to paternal smoking, maternal age, gestational age or twin status. Our results add support to the growing evidence that some perinatal exposures may relate to breast cancer risk. Additional studies are needed to confirm associations and clarify the biologic mechanisms underlying these associations.

First pregnancy characteristics and subsequent breast cancer risk among young women

There is growing evidence that perinatal factors associated with altered gestational hormones may influence subsequent breast cancer risk in the mother. Events occurring during the first pregnancy may be particularly important. In this matched case-control study, we investigated the relation between characteristics of a woman's first pregnancy and her later breast cancer risk using linked records from the New York State birth and tumor registries. Cases were 2,522 women aged 22 to 55 diagnosed with breast cancer between 1978 and 1995 and who had also completed a first pregnancy in New York State (NY) at least 1 year prior to diagnosis. Controls were 10,052 primiparous women not diagnosed with breast or endometrial cancer in NY and matched to cases on county of residence and date of delivery. Information on factors characterizing the woman's first pregnancy was obtained from the pregnancy record of each subject. The association of these factors to breast cancer risk was assessed using conditional logistic regression. Extreme prematurity (< 32 weeks gestational age) was associated with elevated maternal breast cancer risk [adjusted odds ratio (OR)=2.1, 95% confidence interval (CI) 1.2,3.9], as were abruptio placentae (OR = 1.8, CI 1.1,3.0) and multifetal gestation (OR=1.8, CI 1.1,3.0). Preeclampsia was associated with a marked reduction in breast cancer risk among women who bore their first child after age 30 (OR=0.3, CI 0.2,0.7) and in the first 3 years after delivery (OR=0.2 (0.1-0.9). These findings suggest that certain perinatal factors influence maternal breast cancer risk and offer indirect support for a role of gestational hormones, and particularly gestational estrogens, in the etiology of breast cancer.

Social, biological and reproductive characteristics of mothers of twins: implications for breast cancer risk

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.

Breast cancer risk in mothers of twins

The risk of breast cancer associated with delivering a twin birth was examined in a population-based nested case-control study of nearly 4800 Swedish women with breast cancer and 47000 age-matched control subjects. All were aged less than 50 years and parous. After adjustment for age at first birth and parity, a 29% reduction in breast cancer risk was observed in mothers of twins relative to those who were not (odds ratio = 0.71, 95% confidence interval 0.55-0.91). These results provide evidence that women who bear twins are at reduced risk of breast cancer, one explanation for which may be their unusual levels of hormonal exposure.

Maternal risk of breast cancer following multiple births: a nationwide study in Sweden

The association between multiple births and subsequent maternal breast cancer risk was explored in a nested case-control study in Sweden encompassing 19,368 parous women with breast cancer diagnosed up to age 65 years, and 100,459 parous controls. Among cases and controls, there were 329 and 2,031 women, respectively, with a history of at least one live multiple birth. Compared with singleton mothers, breast cancer risk was 12 percent lower (odds ratio = 0.88, 95 percent confidence interval = 0.78-0.99) in women who had had a multiple birth. After stratification for age at diagnosis, evidence of a significant inverse association was found only in women aged 54 years or younger. Birth order of the multiple pregnancy had no apparent risk-modifying effect. Age at earliest multiple birth was unrelated to breast cancer risk. The inverse association between twinning and breast cancer risk may reflect protective physiological features of twin pregnancies. Further research is needed to investigate the role, if any, of increased levels of steroid hormone-binding globulins in mothers of twins and the proposed inhibitory effects of human chorionic gonadotropin and alpha-fetoprotein, both of which are increased during multiple gestations, on breast carcinogenesis. Breast feeding patterns in mothers of twins also may modify their risk of developing breast cancer.