Bigger may not be better for baby girls, as higher weight and longer length at birth are linked with an increased risk of developing breast cancer, new research suggests.
Researchers from the London School of Hygiene and Tropical Medicine have found that every 0.5 kg increase in birth weight was associated with a 7 per cent increased risk of breast cancer.
Birth length and head circumference were also linked with an increase in breast cancer risk.
This is not the first study to link birth size and breast cancer, though research results have been mixed.
Researchers believe that birth size is an indicator of the prenatal environment, which may be exposing babies to some cancer risk factors, such as the hormone estrogen.
However, these most recent findings are independent of those risk factors.
"Little is known on how the prenatal environment may affect breast cancer risk later in life," lead study author Isabel dos Santos Silva said in a statement. "Further research is needed to unravel the biological mechanisms underlying the birth size-breast cancer association."
The study was conducted by researchers at the London School of Hygiene and Tropical Medicine. The findings are published in the journal PLoS Medicine.
The research included data from 32 studies of more than 600,000 women, many of them in developed countries.
For accuracy, the scientists only used birth measurement data from official birth records, which are considered more accurate than information gleaned from surveys or questionnaires.
Abstract:
Birth Size and Breast Cancer Risk: Re-analysis of Individual Participant Data from 32 Studies
Isabel dos Santos Silva, Bianca De Stavola, Valerie McCormack
Background: Birth size, perhaps a proxy for prenatal environment, might be a correlate of subsequent breast cancer risk, but findings from epidemiological studies have been inconsistent. We re-analysed individual participant data from published and unpublished studies to obtain more precise estimates of the magnitude and shape of the birth size-breast cancer association.
Methods and Findings: Studies were identified through computer-assisted and manual searches, and personal communication with investigators. Individual participant data from 32 studies, comprising 22,058 breast cancer cases, were obtained. Random effect models were used, if appropriate, to combine study-specific estimates of effect. Birth weight was positively associated with breast cancer risk in studies based on birth records (pooled relative risk [RR] per one standard deviation [SD] [= 0.5 kg] increment in birth weight: 1.06; 95% confidence interval [CI] 1.02-1.09) and parental recall when the participants were children (1.02; 95% CI 0.99-1.05), but not in those based on adult self-reports, or maternal recall during the woman's adulthood (0.98; 95% CI 0.95-1.01) (p for heterogeneity between data sources = 0.003). Relative to women who weighed 3.000-3.499 kg, the risk was 0.96 (CI 0.80-1.16) in those who weighed < 2.500 kg, and 1.12 (95% CI 1.00-1.25) in those who weighed 4.000 kg (p for linear trend = 0.001) in birth record data. Birth length and head circumference from birth records were also positively associated with breast cancer risk (pooled RR per one SD increment: 1.06 [95% CI 1.03-1.10] and 1.09 [95% CI 1.03-1.15], respectively). Simultaneous adjustment for these three birth size variables showed that length was the strongest independent predictor of risk. The birth size effects did not appear to be confounded or mediated by established breast cancer risk factors and were not modified by age or menopausal status. The cumulative incidence of breast cancer per 100 women by age 80 y in the study populations was estimated to be 10.0, 10.0, 10.4, and 11.5 in those who were, respectively, in the bottom, second, third, and top fourths of the birth length distribution.
Conclusions: This pooled analysis of individual participant data is consistent with birth size, and in particular birth length, being an independent correlate of breast cancer risk in adulthood.
