The third trimester of pregnancy starts at week 28 and lasts through week 40, or whenever you deliver. It covers the final three months of pregnancy and is when your baby gains the most weight, finishes developing key organs, and prepares for life outside the womb.
How the Trimester Breaks Down
Weeks 28 through 40 make up the third trimester, and most major health organizations agree on that range. Your first trimester covers weeks 1 through 12, and the second trimester spans weeks 13 through 27. So week 28, which marks the start of your seventh month, is when you officially enter the home stretch.
Within the third trimester, there’s an important distinction once you reach 37 weeks. The American College of Obstetricians and Gynecologists breaks the end of pregnancy into more specific categories:
- Early term: 37 weeks through 38 weeks, 6 days
- Full term: 39 weeks through 40 weeks, 6 days
- Late term: 41 weeks through 41 weeks, 6 days
- Postterm: 42 weeks and beyond
These distinctions matter because babies born at 39 weeks have better outcomes than those born at 37 weeks, even though both fall within the third trimester. Most people deliver around 39 or 40 weeks.
How Your Baby Grows During Weeks 28 to 40
Your baby roughly triples in weight over the course of the third trimester. At week 28, the average baby measures about 10 inches from head to rump and weighs around 2¼ pounds. By week 32, they’ve grown to about 11 inches and 3¾ pounds. At week 40, the average baby is around 14 inches from head to rump and weighs approximately 7½ pounds.
Much of this weight gain comes from fat deposits that help your baby regulate body temperature after birth. The brain also grows rapidly during this period, developing the folds and grooves that support everything from breathing to processing sensory information.
Lung development is one of the most critical milestones. By 36 weeks, your baby’s lungs are fully formed and producing surfactant, a substance that keeps the tiny air sacs in the lungs open so they can take in oxygen. This is a big reason why babies born before 36 weeks often need breathing support, while those born after typically do not.
What You’ll Likely Feel
The third trimester brings some of the most noticeable physical changes of pregnancy. As your baby takes up more space, you may feel pressure on your bladder, shortness of breath, lower back pain, and swelling in your feet and ankles. Heartburn and trouble sleeping are common, especially in the final weeks.
Many people start experiencing Braxton Hicks contractions during this trimester. These feel like a tightening sensation across the front of your belly, sometimes similar to mild menstrual cramps. They’re sometimes called “practice contractions” because they don’t lead to labor. Unlike real labor contractions, Braxton Hicks are irregular, don’t get stronger or closer together, and typically ease up when you change positions or take a walk. Dehydration, physical activity, and a full bladder can all trigger them.
If your contractions become regular, get progressively more intense, or don’t stop when you rest, those are signs of actual labor rather than Braxton Hicks.
Tracking Your Baby’s Movement
Starting around week 28, your provider will likely ask you to pay attention to your baby’s movements. The general guideline is to feel for 10 movements (kicks, rolls, or flutters) within a two-hour window. Most people notice 10 movements in well under an hour. Picking a consistent time each day when your baby tends to be active makes tracking easier. A noticeable decrease in movement is worth reporting to your provider promptly.
Prenatal Visits and Screenings
Your appointment schedule picks up during the third trimester. From weeks 28 through 35, you’ll typically see your doctor or midwife every two weeks. Starting at week 36, visits shift to once a week until delivery. These visits usually include checking your blood pressure, measuring your belly, and monitoring the baby’s heart rate.
One key screening happens during week 36 or 37: a test for Group B Streptococcus, a type of bacteria that about 1 in 4 pregnant people carry. The bacteria are harmless to you but can cause serious infection in a newborn during delivery. The test is a simple swab, and if you test positive, you’ll receive antibiotics during labor to protect the baby.
Weight Gain in the Final Months
If you were at a healthy weight before pregnancy, the typical recommendation for the second and third trimesters is gaining between half a pound and 1 pound per week. That rate varies depending on your pre-pregnancy weight. Some weeks you may gain more than others, and that’s normal. The overall trend matters more than any single weigh-in. Much of the weight gain in the third trimester comes from your baby’s growth, increased blood volume, and amniotic fluid rather than from body fat alone.