Most people gain about 1 pound per week during the third trimester, which adds up to roughly 10 to 14 pounds between week 28 and delivery. That number shifts depending on your pre-pregnancy weight, whether you’re carrying multiples, and how your body handles the massive increase in blood volume and fluid that happens in the final stretch.
Weekly Weight Gain by Pre-Pregnancy BMI
The rate you’re expected to gain in the third trimester depends on where your weight started. For someone who began pregnancy at a normal weight (BMI of 18.5 to 24.9), the target is about 1 pound per week through delivery. If you started out overweight or obese, the recommendation drops to about half a pound per week during both the second and third trimesters.
These weekly targets are part of broader total-pregnancy goals set by the Institute of Medicine and referenced by the CDC:
- Underweight (BMI under 18.5): 28 to 40 pounds total
- Normal weight (BMI 18.5 to 24.9): 25 to 35 pounds total
- Overweight (BMI 25.0 to 29.9): 15 to 25 pounds total
- Obese (BMI 30.0 to 39.9): 11 to 20 pounds total
Since most people gain only 1 to 4 pounds in the first trimester and then ramp up during the second, the third trimester typically accounts for the largest single chunk of total pregnancy weight. If your overall goal is 25 to 35 pounds and you’ve already gained around 20 by week 28, you’re looking at gaining another 5 to 15 pounds before delivery. That range is wide, and it’s normal.
Where the Weight Actually Goes
A common worry is that third trimester weight is “all fat,” but most of it is baby and the biological infrastructure keeping your pregnancy going. Your baby alone enters the third trimester weighing roughly 2 pounds and leaves it at an average of 6.5 to 8 pounds. Starting around week 35, the baby is in its most rapid growth phase, packing on 8 to 12 ounces every single week.
On top of the baby’s weight, your body is carrying a heavier placenta (which grows throughout pregnancy), more amniotic fluid, a significantly expanded blood supply, breast tissue preparing for milk production, and additional fluid in your tissues. The uterus itself gains several pounds of muscle. Together, these components explain why a healthy 30-pound total gain might only include 5 to 8 pounds of actual body fat.
Why the Scale Fluctuates So Much
Don’t be surprised if your weight jumps 3 pounds in a few days, then barely moves for a week. Fluid retention is a major factor in the third trimester. Your body holds significantly more water in late pregnancy, and things like sodium intake, how long you’ve been standing, and even the weather can cause noticeable swings on the scale. A single salty meal can shift your weight by a couple of pounds overnight, none of which is fat or cause for concern.
The third trimester also calls for about 450 extra calories per day compared to your pre-pregnancy intake. That’s roughly the equivalent of a sandwich or a generous snack. Eating more than that consistently will accelerate gain; eating less can slow it. But week-to-week fluctuations driven by fluid are far more common than real changes in fat or tissue.
Weight Gain With Twins
If you’re carrying twins, the numbers are substantially higher. The CDC recommends a total of 37 to 54 pounds for someone who started at a normal weight, 31 to 50 pounds for someone who was overweight, and 25 to 42 pounds for someone with a BMI of 30 or above. Because twin pregnancies often deliver earlier (around 36 to 37 weeks), the third trimester may be shorter, but the weekly rate of gain is faster. You’re growing two babies, two sets of support systems, and carrying even more blood volume and fluid.
When Weight Gain Signals a Problem
Gradual, steady gain is the goal. What raises concern is a sudden spike. Gaining more than 3 to 5 pounds in a single week can be a sign of preeclampsia, a serious blood pressure condition that typically appears after week 20. Preeclampsia causes the body to retain fluid rapidly, which shows up as sudden swelling in the face, hands, and feet alongside the jump on the scale.
On the other end, gaining very little or losing weight in the third trimester can indicate that the baby isn’t getting adequate nutrition, or that something else is going on with your health. Consistent gain that tracks your weekly target, even if individual weeks vary, is the pattern that matters most. A single “off” week is rarely meaningful. A trend over two or three weeks is worth paying attention to.