Factors that Affect Postpartum BMI Gain or Loss
by Anne Eglash MD, IBCLC, FABM
What are the factors that determine weight loss or weight retention postpartum?
It has been unclear whether lactation facilitates postpartum weight loss, as the research findings have been mixed. There seem to be several factors associated with changes in postpartum BMI, such as race, income, education, and pre-pregnancy BMI.
The researchers for this study hypothesized that lactating mothers would have a different trajectory of postpartum BMI changes as compared to mothers who never lactated, and that it might depend on the individual’s pre-pregnancy BMI status and their eating behaviors.
The researchers evaluated BMI gain or loss during the first year postpartum among 208 women by evaluating their lactation history, pre-pregnancy BMI, various demographic factors, and their eating behavior (e.g. dietary restraint, disinhibition, and susceptibility to hunger). Among the 208 subjects, 96 lactated for at least 4.5 months, with 62% having lactated for 1 year, and 112 exclusively formula fed. All subjects were categorized into pre-pregnancy BMI categories as follows: Healthy (≤ 24.9 kg/m2), Overweight (25-29.9 kg/m2), and Obese (≥ 30.0 kg/m2).
They obtained BMI measurements from each subject 8-13 times during the first year postpartum.
They found that the effect of lactation on postpartum BMI depended on psychological eating behavior traits, with greater BMI loss among those with higher dietary restraint, higher disinhibition, and lower susceptibility to hunger.
The effect of lactation on change in postpartum BMI also depended on the pre-pregnancy BMI. For more information, see the question!
- Among all subjects (lactating and nonlactating), those in the pre-pregnancy Overweight BMI category lost less BMI and at slower rates than subjects in the pre-pregnancy Healthy BMI category.
- On average, BMI increased at 1 year postpartum among all subjects (lactating and nonlactating) in the pre-pregnancy Obese BMI category.
- BMI loss or gain was similar between lactating and nonlactating women at one year postpartum if their pre-pregnancy BMI was Healthy or Obese.
- Subjects who practiced dietary restraint postpartum had significantly more BMI loss by 1 year postpartum, no matter the BMI category.
See the Answer
We tested the hypotheses that mothers of infants who exclusively breastfed would differ in the trajectories of postpartum BMI changes than mothers of infants who exclusively formula fed, but such benefits would differ based on the maternal BMI status prepregnancy (primary hypothesis) and that psychological eating behavior traits would have independent effects on postpartum BMI changes (secondary hypothesis). To these aims, linear mixed-effects models analyzed measured anthropometric data collected monthly from 0.5 month (baseline) to 1 year postpartum from two groups of mothers distinct in infant feeding modality (Lactating vs. Non-lactating). While infant feeding modality group and prepregnancy BMI status had independent effects on postpartum BMI changes, the benefits of lactation on BMI changes differed based on prepregnancy BMI. When compared to lactating women, initial rates of BMI loss were significantly slower in the non-lactating women who were with Prepregnancy Healthy Weight (β = 0.63 percent BMI change, 95% CI: 0.19, 1.06) and with Prepregnancy Overweight (β = 2.10 percent BMI change, 95% CI: 1.16, 3.03); the difference was only a trend for those in the Prepregnancy Obesity group (β = 0.60 percent BMI change, 95% CI: −0.03, 1.23). For those with Prepregnancy Overweight, a greater percentage of non-lactating mothers (47%) gained ≥ 3 BMI units by 1 year postpartum than did lactating mothers (9%; p < 0.04). Psychological eating behavior traits of higher dietary restraint, higher disinhibition, and lower susceptibility to hunger were associated with greater BMI loss. In conclusion, while there are myriad advantages to lactation, including greater initial rates of postpartum weight loss regardless of prepregnancy BMI, mothers who were with overweight prior to the pregnancy experienced substantially greater loss if they breastfed their infants. Individual differences in psychological eating behavior traits hold promise as modifiable targets for postpartum weight management.
This was a very detailed study, that essentially found that the biggest impact of lactation on BMI change at 1 year postpartum was among women who were in the Overweight category before pregnancy. The difference in BMI loss at 1 year was greater when compared to those who didn’t lactate.
Lactating women who had a healthy pre-pregnancy BMI demonstrated a similar BMI change by 1 year postpartum compared to those who didn’t lactate.
Women who were in the Obese category before pregnancy tended to gain BMI by 1 year postpartum, regardless of lactation.
The authors discussed that women in the pre-pregnancy Obese category had more gestational weight gain during pregnancy with higher fat mass gain, leading to more insulin resistance, and a harder time losing weight postpartum. This was based on data collected on these subjects that was reported in other studies.
Women who had more eating restraint had more BMI loss in the first year, no matter the BMI category. It seems logical to offer nutritional counseling for women who are struggling to lose weight postpartum, especially given the evidence that women with a pre-pregnancy BMI >30 have a harder time losing weight postpartum.