The third trimester starts at week 28 of pregnancy and lasts through week 40, giving you roughly 12 to 13 weeks before your due date. Some sources place the start at week 27, but most major health organizations, including those referenced by the Cleveland Clinic, agree that week 28 marks the beginning. Since few babies arrive exactly on their due date, the third trimester effectively ends whenever you give birth.
How the Weeks Break Down
Pregnancy is divided into three trimesters. The first covers weeks 1 through 12, the second spans weeks 13 through 27, and the third picks up at week 28 and runs to week 40. That 40-week mark is a full-term due date calculated from the first day of your last menstrual period, not from conception. Actual conception typically happens about two weeks later, which is why “40 weeks pregnant” really means roughly 38 weeks of fetal development.
Within the third trimester, providers further distinguish between early term (37 weeks through 38 weeks and 6 days), full term (39 weeks through 40 weeks and 6 days), and late term (41 weeks through 41 weeks and 6 days). These categories matter because outcomes improve with each additional week the baby stays in the womb up to 39 weeks. ACOG recommends against elective induction before 39 weeks for this reason.
How Your Baby Grows Week by Week
Your baby enters the third trimester weighing about 2.25 pounds and roughly 10 inches long (measured from the top of the head to the tailbone). Over the next 12 weeks, that weight more than triples. Here’s the general trajectory, based on Mayo Clinic estimates:
- Week 28: About 2.25 pounds (1,000 grams)
- Week 30: Nearly 3 pounds (1,300 grams)
- Week 32: About 3.75 pounds (1,700 grams)
- Week 34: More than 4.5 pounds (2,100 grams)
- Week 38: About 6.5 pounds (2,900 grams), though some babies weigh close to 9 pounds by this point
- Week 40: About 7.5 pounds (3,400 grams) and around 14 inches crown to rump
Much of this weight gain is fat. In the final weeks, your baby adds about half a pound per week, building the insulating layer needed to regulate body temperature after birth.
Key Developments Inside the Womb
The third trimester is when your baby’s lungs undergo their most critical maturation. Most of the surfactant, the substance that keeps the tiny air sacs in the lungs from collapsing, is produced after week 28. By around 35 weeks, lung development is typically sufficient for a baby to breathe independently. This is a major reason why every additional week in the womb before 35 weeks makes a significant difference for premature babies.
Your baby’s brain is also developing rapidly during this period, forming billions of new neural connections. The eyes can detect light, sleep-wake cycles become more defined, and bones harden (though the skull stays flexible for delivery). By 37 weeks, most babies have shifted into a head-down position. Only about 3 in 100 remain breech after that point.
What Changes in Your Body
As your baby gains weight quickly, you’ll likely notice that everyday tasks feel harder. Your uterus pushes up against your diaphragm, making deep breaths more difficult, and presses down on your bladder, increasing bathroom trips. Back pain, swollen ankles, and trouble sleeping are all common as you move through the final weeks.
Your body also starts preparing for breastfeeding. Colostrum, the concentrated first milk rich in antibodies, begins developing as early as week 16, but you may notice small amounts leaking from your breasts during the third trimester. This is normal and varies widely from person to person. Not leaking doesn’t mean your body isn’t producing it.
Prenatal Visits Pick Up
Your appointment schedule gets busier in the third trimester. From weeks 28 through 35, you can expect visits every two to four weeks. Starting at 36 weeks, that frequency increases to every one to two weeks until delivery. These visits typically include blood pressure checks, urine tests, measuring your belly to track growth, and checking your baby’s heart rate.
One important screening happens during weeks 36 or 37: a swab test for Group B strep, a type of bacteria that’s harmless to you but can cause serious infection in a newborn during delivery. About 1 in 4 pregnant people carry it, and a positive result simply means you’ll receive antibiotics during labor.
Braxton Hicks vs. Real Labor
Starting in the third trimester, you may feel your uterus tighten and release. These practice contractions, called Braxton Hicks, are irregular, don’t follow a pattern, and don’t get closer together over time. They often stop when you change positions or drink water.
True labor contractions are different in three key ways: they come at regular intervals, they get progressively closer together, and they increase in intensity. If your contractions form a consistent pattern and keep getting stronger regardless of what you do, that’s the real thing.
Conditions to Be Aware Of
Preeclampsia is one of the more serious conditions that can develop in late pregnancy. It’s diagnosed when blood pressure reaches 140/90 mm Hg or higher after 20 weeks of pregnancy, combined with signs like protein in the urine (indicating kidney strain), severe headaches, vision changes, or upper abdominal pain. Preeclampsia can develop even if your blood pressure has been normal throughout pregnancy, which is one reason those frequent third-trimester checkups include blood pressure monitoring every time.
Other things worth paying attention to include a noticeable decrease in your baby’s movement, sudden severe swelling in your face or hands, or any vaginal bleeding. Tracking your baby’s kick counts starting around week 28 gives you a baseline so you can notice changes.