Intentionally losing weight during pregnancy is not recommended, even for women who start pregnancy at a higher weight. A large population-based study of over 157,000 pregnant individuals with obesity found that gestational weight loss was associated with a 31% increased risk of adverse outcomes for the baby compared to gaining within the recommended range. That said, some weight loss in early pregnancy is common and usually not a cause for alarm, so the answer depends heavily on when, why, and how much weight you’re losing.
What the Guidelines Actually Recommend
Current recommendations from the American College of Obstetricians and Gynecologists (ACOG), based on 2009 Institute of Medicine guidelines, set target weight gain ranges based on your pre-pregnancy BMI. Overweight women (BMI 25 to 29.9) are advised to gain 15 to 25 pounds total. Women with obesity (BMI 30 or higher) are advised to gain 11 to 20 pounds. Women at a normal weight typically aim for 25 to 35 pounds, and underweight women are advised to gain 28 to 40 pounds.
Notice that even at the highest BMI categories, the recommendation is still to gain some weight, not lose it. Your body needs to build a placenta, increase blood volume, grow a baby, and store energy for breastfeeding. Those processes require calories and nutrients that weight loss can compromise.
Why Early Weight Loss Is Usually Normal
Mild weight loss in the first trimester is extremely common and typically harmless. Research tracking weight changes across pregnancy found that women in every BMI category above normal weight actually lost a small amount of weight on average during the first trimester. The losses ranged from about 0.01 kg per week in overweight women to 0.11 kg per week in women with class III obesity. Morning sickness, food aversions, and shifting appetite all contribute, and your caloric needs don’t actually increase during these first 12 weeks.
This type of loss tends to resolve on its own as nausea fades and appetite returns in the second trimester, when your body needs roughly 340 extra calories per day. By the third trimester, that rises to about 450 extra calories daily.
When Weight Loss Becomes a Problem
There’s an important difference between losing a pound or two from morning sickness and losing a significant amount of weight. Hyperemesis gravidarum, the severe form of pregnancy nausea, is typically defined as vomiting accompanied by a loss of at least 5% of your pre-pregnancy weight. Extreme cases can involve losses of 15% or more and may require medical treatment for dehydration and nutritional deficiencies.
If you’re losing weight in the second or third trimester, that’s a stronger signal that something needs attention. Research shows that low weight gain in the second trimester is most strongly associated with having a baby that’s small for gestational age, particularly for women who started pregnancy at a normal weight. In that group, low second-trimester weight gain raised the risk of a small baby by 62%. The association weakened in women with higher BMIs and was much less pronounced in the third trimester.
The Risks of Weight Loss for Women With Obesity
Because excess weight carries its own pregnancy risks (gestational diabetes, preeclampsia, cesarean delivery), it’s natural to wonder whether losing weight might actually help. The evidence suggests otherwise. A 2024 population-based study found that among pregnant women with obesity, about 6% experienced overall gestational weight loss. The more severe the obesity, the more likely weight loss was: roughly 4% of women in class I obesity lost weight, compared to nearly 16% in class V obesity.
Across every obesity class, weight loss was linked to more complications, not fewer. The relationship between weight change and outcomes followed a U-shaped curve, meaning both extremes (losing weight and gaining too much) carried elevated risk. The lowest risk sat in the middle, right around the recommended gain of 11 to 20 pounds for women with obesity.
For women with higher BMIs, there is one potential silver lining in the data. Gaining less weight in the first and second trimesters appears to reduce the chance of having an unusually large baby without a proportional increase in the chance of having a small baby. That’s a meaningful distinction: gaining on the lower end of the recommended range is different from actually losing weight.
What Happens to the Baby When a Mother Loses Weight
Poor gestational weight gain is one of several factors associated with fetal growth restriction, a condition where the baby doesn’t grow as expected in the womb. Growth restriction raises the risk of complications at birth, including breathing difficulties, low blood sugar, and trouble regulating body temperature. Over the longer term, studies have found higher rates of cognitive delays, learning difficulties, attention problems, and fine and gross motor challenges in children who were growth-restricted.
When the body doesn’t get enough calories during pregnancy, it shifts into a fat-burning state that produces molecules called ketones. Animal research suggests this metabolic shift can alter fetal growth, particularly in later pregnancy when the baby’s nutrient demands spike. In mouse studies, mothers on very low-carbohydrate diets showed dramatically low blood sugar and very high ketone levels near the end of pregnancy, and their offspring had measurable growth differences. While this doesn’t translate directly to humans, it illustrates why sustained calorie restriction during pregnancy raises red flags.
Managing Weight Safely During Pregnancy
If you’re concerned about gaining too much, the goal isn’t weight loss. It’s gaining at a steady, appropriate pace. The CDC recommends eating a balanced diet rich in whole grains, vegetables, fruits, low-fat dairy, and lean protein while limiting added sugars, fried foods, and sugary drinks. These changes help manage weight gain without restricting the nutrients your body needs.
Staying active also helps. The recommendation for most pregnant women is at least 150 minutes of moderate-intensity activity per week, like brisk walking. You can break that into shorter sessions throughout the day. Regular exercise during pregnancy helps regulate weight gain and also reduces the risk of gestational diabetes and preeclampsia.
Tracking your weight regularly gives you and your provider an early signal if you’re gaining too quickly or too slowly. If you started pregnancy above a healthy weight, your provider can help you aim for the lower end of the recommended range rather than pursuing weight loss. That approach gives you the best chance of a healthy outcome for both you and your baby.