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To examine the relation between childbearing and breastfeeding and subsequent body mass index (BMI) in middle-aged women.
Cross-sectional analysis within a large prospective study.
One million UK women recruited into the Million Women Study during 1996–2001. Participants completed questionnaires collecting information on personal and lifestyle characteristics, medical and reproductive history including questions on childbearing and breastfeeding, anthropometry, physical activity, and socio-demographic in factors.
Main Outcome Measures
Mean BMI stratified by parity and breastfeeding and adjusted for age, geographical region, socioeconomic status, smoking status, strenuous physical activity, age at first birth, and years since last birth.
980 474 women were included in the main analysis. 87% of the study population were parous, mean parity was 2.1 (1.2) and 68% of parous women ever breastfed. The mean lifetime duration of breastfeeding per child was 3.8 (SD 3) months. Mean BMI increased significantly with each birth from 25.8 (95% CI 25.7 to 25.8) for nullips to 28.1 (95% 28.1 to 28.3) for women with five or more births. Parous women who breastfed had significantly lower BMIs than their non-breastfeeding counter parts. The effect was attenuated by adjusting for socio-economic status but remained significant even after full adjustment The mean BMI decreased as lifetime duration of breastfeeding increased: compared with women who never breastfed, mean BMI decreased by 0.8 (95% CI 0.71 to 0.82)kg/m2 in those with a lifetime breastfeeding duration of more than 9 months. This relationship was statistically significant (p<0.0001) and maintained at each parity level.
In this analysis including one million middle-aged women in the UK we found that BMI increased with increasing parity, but that this increase would be offset if women breastfed. These relationships were independent of socioeconomic status, geographic region, smoking, exercise, age at first birth and time since last birth. These findings contribute to the body of evidence that childbearing and breastfeeding have sustained long term effects on the health status of women.