Is "Baby Weight" a Myth?: Longitudinal analysis of pregnancy weight retention

Pregnancy weight retention, or the increase in the woman's body weight after pregnancy relative to her pre-pregnancy weight, has long been viewed as an important contributor to long-term weight gain among women of childbearing age. However, large empirical studies of long-term pregnancy effects on female body weight are lacking. Additionally, the existing small-sample studies rarely account for the tendency of body weight to increase over time due to secular trends and age-related physiological and other changes unrelated to childbirth. This leads to potential false attribution of non-pregnancy related weight gain to pregnancy weight retention. We use data from Peridata.Net(r), a platform of hospital vital birth records linked to unique maternal identifiers from 31 Wisconsin hospitals between 2006-2013.  We track maternal weight longitudinally using data on maternal weight pre-pregnancy, at delivery, and pre-pregnancy in the following pregnancy from 36,334 adjacent births. We find that, on average, weight gain after pregnancy is similar in magnitude to weight gain that can be expected in the absence of pregnancy. Underweight women retain the most weight in first and subsequent pregnancies, while overweight and obese women, on the contrary, lose weight following second and third pregnancies. These findings do not support the notion that pregnancy-related weight gain contributes significantly to life-time weight accumulation for most women. Therefore, gestational weight gain does not explain high obesity rates in women of childbearing age.