A meta-analysis of early and late postpartum weight retention estimated that postpartum weight retention (as measured by BMI) averages 2.5 kg/m² (about 6 kg) at 6 weeks, 1.25 kg/m² (about 3 kg) at 6 months, and 0.5 kg/m² (about 1.25 kg) at 1 year [22]. A major limitation of practically all pregnancy cohort design studies is that they rely on maternal recall of body weight before conception and/or early first trimester weight measurements. Self-report of prepregnancy weight is biased toward underreporting to a greater extent in overweight or obese women and thereby inflate the estimates of postpartum weight retention for high-BMI groups. Pregnancy cohort studies based on self-reported pregravid weight may introduce substantial bias because high-BMI groups underreport body weight by about 5 kg versus 1 kg for other groups [23, 24]. Therefore, reporting bias affects estimates of gestational weight gain and postpartum weight retention to a greater extent for high- than low-BMI groups. An underestimate of 5 kg may overestimate gestational weight gain by almost 50 % for obese women, and an error of 1 kg may overestimate postpartum weight change by more than 100–200 %.

Furthermore, postpartum weight retention may be overestimated in the high-BMI groups due to the higher trajectory of weight gain established before conception that may continue at the similar accelerated pace after pregnancy. Thus, weight retention at 1–2 years postpartum may not be due to retention of gestational gain but weight gain after delivery. Previous studies with serial postpartum weight measurements have distinguished between weight retention and subsequent weight gain by prepregnancy body size [20].

Longitudinal cohort studies focusing on the natural history of childbearing among women of reproductive age suggest that a first birth has adverse effects on overall adiposity, while increasing number of births (parity) has cumulative effects on central adiposity. In CARDIA women, waist circumference increased with each subsequent birth controlling for prepregnancy measurements [25], while body weight increased primarily after the first birth [25, 26]. The findings suggest that the greatest impact on overall fat stores is after the first birth but subsequent births increase central obesity. The Coronary Artery Risk Development in Young Adults (CARDIA) study estimated pregnancy-related weight gain from before to after pregnancy controlling for secular trends in weight gain and to assess differences within prepregnancy BMI (overweight or obese) and parity groups (primiparas) [25]. The unique strengths of this study include the longitudinal design, standardized research measurements of body weight from before to after pregnancy (3- to 5-year intervals), and high retention rates. The large sample size enabled investigators to determine that pregnancy-related weight gain depended on the first birth (primiparity) and by maternal prepregnancy overweight. In CARDIA, postpartum weight retention averaged 1 kg for women who were not overweight before pregnancy (BMI <25) and about 3–6 kg for women who were already overweight or obese before pregnancy (BMI ≥25) controlling for sociodemographics, prepregnancy BMI, and lifestyle factors [25, 27]. Being overweight before pregnancy signifies the predisposition for weight gain, and pregnancy may exacerbate this tendency for many women. The CARDIA study provides most accurate estimates of long-term weight gain due to pregnancy and its aftermath, by accounting for secular trends in weight gain during the same period among women who did not bear children.

Certain lifestyle behaviors are also likely to influence women’s risk of becoming overweight or obesity in midlife [6, 28]. For example, among nonsmokers in CARDIA, giving birth was associated with a twofold greater risk of becoming overweight within several years versus not giving birth [6]. Yet, among smokers, women who had given birth were half as likely to become overweight as those who had never given birth; OR = 0.41(95 % CI: 0.17,0.96) for women delivering one birth only and 0.36 (95 % CI: 0.08,1.65) for women delivering two or more births [6]. Other modifiable risk factors include postpartum sleep, dietary practices, and physical activity, although biological risk factors such age at menarche and primiparity also appear to play key roles.

A 2011 meta-analysis examined average postpartum weight retention with several time periods [29]. On average, at 6 months or less postpartum, women who experienced gestational weight gain above IOM recommendations retained an average of 4 kg more than those gaining within recommendations. By 6–12 months postpartum, the weight retention averaged 2.5 kg in the group with excessive gestational weight gain. By 3 years postpartum, the retention was estimated to be 3 kg greater. However, these estimates are based on very few studies, in which serial measurements of postpartum weights were not available, and had variable sample characteristics which may explain the higher “retention” for the longer period of follow-up. The variability in estimates of average postpartum weight retention may be related to maternal characteristics such as race, sociodemographics and economic status, age, smoking, and levels of gestational weight gain.

Large pregnancy cohort studies (n > 400) generally report that 13–20 % of pregnant women (Table 5.1) experience substantial weight retention by 1 year postpartum (defined as body weight of 5 kg above preconception weight). Substantial postpartum weight retention has been reported among 7–52 % of women at 1 year postpartum when studies including special populations, such as low-income groups and pregnant adolescents, are included. However, the percentages with substantial postpartum weight retention correlate closely with the prevalence of prepregnancy overweight and obesity for the specific cohort. For example, northern European and US cohorts with lower rates of maternal prepregnancy overweight or obesity (7–25 %) reported lower proportion of women with substantial weight retention (10–20 %). The US cohorts that focused largely on women from low socioeconomic groups reported much higher rates of maternal prepregnancy overweight or obesity (24–52 %) and reported the highest proportions of women experiencing substantial postpartum weight retention (20–50 %) [6, 28, 30–33].

Correlates of substantial postpartum weight retention based on epidemiologic studies include high gestational gain, pregravid overweight, primiparity, black race, low socioeconomic status, smoking cessation, and fewer than 5 h of sleep per day [19, 33, 34]. The strongest predictors of postpartum weight retention include maternal overweight or obesity before pregnancy, excessive gestational weight gain exceeding the IOM recommendations, and primiparity [27, 35, 36]. Maternal characteristics associated with a two- to threefold higher risk of becoming overweight after pregnancy independent of gestational weight gain include young age at menarche (less than 12 years), short interval (less than 8 years) from menarche to first birth, maternal age 24–30 years [28], and short sleep duration (<5 h per 24 h period) at 6 months postpartum [34]. These risk factors may indirectly represent either genetic or biologic influences on adult body weight prior to pregnancy, socioeconomic differences in maternal age when childbearing begins, and/or postpartum behavior changes. Although the strength of associations with postpartum weight retention for these traits is similar to total gestational weight gain, for some risk factors, their lower prevalence in a population may result in relatively lower attributable risk for postpartum weight retention than gestational weight gain or maternal body size.

A two- to threefold greater risk of becoming overweight after pregnancy has been associated with reproductive factors such as excessive gestational weight gain, young age at menarche (<12 years), and pregnancy within 8 years of menarche [28]. Very few studies have examined the risk of becoming overweight due to pregnancy, although some have linked gestational weight gain to weight status more than a decade later [37, 38]. These data suggest that weight gain trajectories may be influenced by genetic factors influencing reproductive maturation and that pregnancy at a young age exacerbates the risk of becoming overweight to the same extent as excessive gestational weight gain.

Evidence from childbearing cohorts and pregnancy cohorts suggests that a first birth and maternal prepregnancy body size are key predictors of long-term weight retention and that gestational weight gain is likely to mediate these associations. The next sections critically examine these risk factors and their impact on weight changes after pregnancy.

Although gestational weight gain is linked to postpartum weight retention, primiparity and larger body size before pregnancy exert important influences that modify these relationships. Evidence that prepregnancy BMI influences weight retention has varied by attributes of the populations studied and the inadequate sample size to assess effect modification by prepregnancy BMI categories in the gestational weight gain and postpartum weight association. Women who are overweight or obese before pregnancy are generally more likely to have excessive as well as inadequate gestational weight gains [21, 39]. Excessive gestational weight gain increases the risk of postpartum weight retention, but the effect may depend on prepregnancy body size, maternal age, and primiparity. Yet, very few studies have addressed the joint effects of these key risk factors. Weight retention or gain following delivery may also be highly variable and strongly influenced by both the weight gain trajectory that preceded pregnancy (i.e., high prepregnancy BMI) and excessive gestational weight gain.