The third trimester of pregnancy runs from week 28 through week 40, making it approximately 12 to 13 weeks long. The American College of Obstetricians and Gynecologists (ACOG) defines it precisely as 28 weeks and 0 days through 40 weeks and 6 days. Of course, not every pregnancy reaches 40 weeks, so your third trimester ends whenever you deliver.
Why the Third Trimester Is 12 Weeks, Not Exactly One-Third
Pregnancy is divided into three trimesters, but they aren’t perfectly equal. The first trimester covers weeks 1 through 12, the second spans weeks 13 through 27, and the third picks up at week 28. That gives the first trimester 12 weeks, the second 15 weeks, and the third roughly 12 to 13 weeks depending on when delivery happens. The uneven split exists because the trimesters are organized around developmental milestones rather than strict calendar math.
What “Full Term” Actually Means
Reaching the third trimester doesn’t mean the baby is ready to be born. The National Institutes of Health breaks down the final stretch into more specific categories that reflect how prepared a baby’s organs are for life outside the womb:
- Early term: 37 weeks through 38 weeks and 6 days
- Full term: 39 weeks through 40 weeks and 6 days
- Late term: 41 weeks through 41 weeks and 6 days
- Post-term: 42 weeks and beyond
These distinctions matter because babies born even a couple of weeks early can face more breathing and feeding difficulties than those born at 39 weeks or later. Your due date is simply an estimate of when your pregnancy will reach 40 weeks, not a guarantee of when labor will start.
How Your Baby Grows During These Weeks
The third trimester is when your baby gains the most weight and the organs finish maturing so they can function independently after birth. At week 28, your baby’s eyelids can partially open. Over the following weeks, the lungs continue developing, fat layers build up under the skin, and the brain grows rapidly. By week 40, a baby typically measures about 14 inches from crown to rump and weighs around 7.5 pounds.
This rapid growth is why the third trimester feels so physically demanding. Your baby is essentially tripling or quadrupling in weight during these final 12 weeks, which puts increasing pressure on your bladder, lungs, and lower back.
What to Expect at Prenatal Visits
Prenatal appointments become more frequent once you hit the third trimester. Most providers shift from monthly visits to every two weeks around week 28, then weekly visits starting around week 36. These check-ins monitor your blood pressure, your baby’s position, and how your body is preparing for labor.
One specific test to expect: Group B strep screening, typically done during week 36 or 37. This is a simple swab test to check for a type of bacteria that’s harmless to you but could cause infection in your baby during delivery. If the test comes back positive, you’ll receive antibiotics during labor to protect your newborn.
Tracking Your Baby’s Movement
Kick counting becomes especially important during the third trimester. Starting around week 28, paying attention to how often your baby moves gives you a simple way to check on their well-being. There are two common approaches: count how many kicks you feel in one hour, or time how long it takes to feel 10 movements.
If you haven’t felt 10 movements within two hours, contact your provider. That doesn’t necessarily mean something is wrong, but it’s worth a check. Every pregnancy has its own normal pattern, so the most useful thing you can do is get familiar with your baby’s typical activity level and notice when something changes.
Braxton Hicks vs. Real Contractions
Sometime during the third trimester, you’ll likely feel your uterus tighten and release. These are Braxton Hicks contractions, often called practice contractions. They feel like mild menstrual cramps or a random tightening across your abdomen. You should still be able to walk and talk through them, and they usually stop if you change positions.
Real labor contractions are different in three key ways: they come at regular intervals, they get stronger and closer together over time, and they don’t go away when you move around. A general guideline is that contractions occurring less than five minutes apart for one hour, especially ones intense enough that you can’t walk or talk through them, signal that labor has likely started.