A large population-based study of women in the UK was carried out to explore the long-term effects of women’s childbearing patterns on their body mass index (BMI). Data was captured from responses from 740,628 postmenopausal participants in the Million Women Study, recruited during the years 1996-2001.

The women reported their height, weight, reproductive histories, infant feeding history and other relevant factors. Women were aged 57.5 years on average, and had a mean BMI of 26.2; 88% were parous, with 2.1 children on average. The standardised mean BMI increased progressively with the number of births, from 25.6 in nulliparous women up to 27.2 for women with four or more births. Among the parous women, 70% had ever breastfed and their average total duration of breastfeeding was 7.7 months. At every parity level the standardised mean BMI was significantly lower among women who had breastfed than those who had not, decreasing by 0.22 for every 6 months of breastfeeding. In other words, women’s mean BMI was 1% lower for every 6 months that they had breastfed.

These associations were highly statistically significant and independent of the effects of other known factors that are associated with adiposity – socioeconomic group, region of residence, smoking and physical activity.

The authors conclude that patterns of childbearing have a persistent effect on adiposity in this population. The reduction in BMI associated with just 6 months breastfeeding in UK women could importantly reduce their risk of obesity-related disease and their costs as they age.

Bobrow KL, Quigley MA, Green J et al (2012). Persistent effects of women’s parity and breastfeeding patterns on their body mass index: results from the Million Women Study. International Journal of Obesity advance online publication, 10 July 2012; doi:10.1038/ijo.2012.76