Department of Research & Evaluation News

For years, researchers have known that many people struggle to lose the weight they gain during pregnancy. Up to 1 in 4 individuals retain at least 10 pounds 1 to 2 years after giving birth, and this lingering weight can raise their long‑term risk of hypertension, cardiovascular disease, and other metabolic problems.

Despite the importance of the issue, scientists have had limited data on how physical activity before and during pregnancy shapes postpartum weight outcomes. A new multi‑region study offers the clearest picture to date of how staying active affects postpartum weight retention.

“People who maintained higher levels of physical activity from preconception through pregnancy had significantly less weight retention a year after giving birth compared to those who didn’t exercise as much,” said the study’s lead author Deborah Young, PhD, MBA, director of the Division of Behavioral Research at the Kaiser Permanente Southern California Department of Research & Evaluation.

“Our results highlight that keeping up regular physical activity throughout pregnancy is a valuable way to lower the chances of holding onto extra weight after giving birth and to reduce weight-related health risks over time,” she added.

The study was published in November 2025 in Obesity.

Study analyzed nearly 60,000 pregnancies

The research team analyzed 58,828 pregnancies from 2009 to 2020 within Kaiser Permanente Southern California. Using electronic health records, researchers linked measured BMI (Body Mass Index) to self‑reported physical activity collected through the Exercise Vital Sign. This is a Kaiser Permanente tool that asks patients how many days per week and minutes per session they engage in moderate or strenuous activity. This approach allowed investigators to track physical activity patterns beginning up to 3 years before pregnancy and continuing through each trimester.

Their analysis was broken into 3 physical activity trajectories: low, medium, and high. Nearly two‑thirds of the cohort fell into the medium‑activity group, while 32% were categorized as low‑activity and only 3% maintained high levels of activity from preconception through pregnancy.

Exercising regularly during pregnancy was beneficial

Despite representing the smallest proportion of the cohort, the high‑activity group demonstrated the most striking outcomes. At 360 days postpartum, these individuals had significantly lower postpartum BMI percentages than their less active peers. They were also far more likely to return to or fall below their pre‑pregnancy BMI 1 year after delivery.

Compared with individuals in the low‑activity trajectory, those in the high‑activity group had 31% higher odds of reaching their pre‑pregnancy BMI. Even when compared with the medium‑activity group, the high‑activity individuals remained substantially more likely to return to baseline weight. The findings were consistent across racial, ethnic, and educational backgrounds.

“These results indicate that physical activity during the months leading up to pregnancy, and continuing throughout pregnancy, may be one of the most effective tools for reducing postpartum weight retention,” Dr. Young said.

Few pregnant patients maintained recommended activity

The researchers note that although guidelines from the American College of Obstetricians and Gynecologists recommend at least 150 minutes per week of moderate activity during pregnancy, very few patients met this standard. In their analysis, only 3% of the entire cohort achieved this level of activity before and throughout pregnancy, compared with nearly half of women in the high‑activity trajectory.

“This research carries important implications for clinicians, public health leaders, and patients planning pregnancy,” Dr. Young said. “Encouraging people to establish consistent physical activity habits before conception, and supporting them in continuing those habits while pregnant, may help counter rising rates of obesity and cardiometabolic disease.”

Authors in addition to Dr. Young include Kimberly K. Vesco, MD, MPH; Erin LeBlanc, MD, MPH; and Meghan Mayhew, MPH, of the Center for Health Research, Kaiser Permanente Northwest; Caryn E. Oshiro, PhD, RD, of the Center for Integrated Health Care Research, Kaiser Permanente Hawaii; Margo A. Sidell, PhD; and Botao Zhou, MS, of the Department of Research & Evaluation; Natalie Rosenquist, PhD; and Janne Boone‑Heinonen, PhD, MPH, of the Institute for Health Policy Studies, University of California, San Francisco.