The third trimester lasts about three months, covering weeks 28 through 40 of pregnancy. That works out to roughly 13 weeks, starting at the beginning of month seven and stretching through months eight and nine until delivery. Because pregnancy months and calendar months don’t line up perfectly, you’ll sometimes see slight variations in how sources count, but the week range is consistent: week 28 is where the third trimester begins, and week 40 is a full-term due date.
How the Weeks Break Down by Month
Pregnancy is 40 weeks total, divided into three trimesters. The simplest way to map the third trimester onto months:
- Month 7: Weeks 28 through 31
- Month 8: Weeks 32 through 35
- Month 9: Weeks 36 through 40
The confusion comes from the fact that a calendar month averages about 4.3 weeks, not exactly four. So “nine months of pregnancy” is really closer to ten calendar months from the first day of your last period. The third trimester itself spans roughly 12 to 13 weeks depending on when you deliver, which is just over three calendar months.
What “Full Term” Actually Means
Not all weeks within the third trimester are equal when it comes to your baby’s readiness. The National Institutes of Health and the American College of Obstetricians and Gynecologists use specific categories to describe the end of pregnancy:
- 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 week or two before 39 weeks can have less mature lungs and brains than those who go to full term. The final weeks of the third trimester are when critical finishing touches happen.
How Your Baby Grows During These Months
At the start of the third trimester, around week 28, your baby’s eyes are beginning to partially open. From this point forward, the focus shifts from forming new organs to growing bigger and maturing the organs that are already in place. Fat layers build up under the skin, the brain develops rapidly, and bones harden.
Lung development is one of the most important milestones of this period. The cells responsible for producing surfactant, the substance that keeps the lungs from collapsing after birth, begin maturing around mid-pregnancy but don’t reach full clinical maturity until about 36 weeks. Babies born at 24 weeks can sometimes exchange oxygen well enough to survive, but the risk of breathing problems drops significantly after 36 weeks.
By week 40, a full-term baby typically measures around 14 inches from crown to rump and weighs about 7.5 pounds. Most of that weight gain happens in the final two months, which is why the third trimester can feel physically intense.
What Your Body Feels Like
The third trimester brings a distinct set of physical changes, many of them driven by the baby’s increasing size and the extra blood volume your body is maintaining.
Braxton Hicks contractions are common, especially in the afternoon or evening and after physical activity. These feel like a tightening across your abdomen, sometimes similar to menstrual cramps. They’re irregular, don’t get progressively stronger, and usually ease up if you change positions or walk around. They tend to become more frequent as your due date approaches, which can make it harder to tell them apart from the real thing.
True labor contractions are different in a few key ways: they start at the top of the uterus and move downward in a coordinated wave, they get stronger and closer together over time, and they don’t stop when you move. A useful guideline is the 5-1-1 rule: contractions every 5 minutes, each lasting at least 1 minute, continuing for 1 hour.
Pelvic pressure increases as the baby drops lower, which puts more weight on your bladder. Expect to urinate more frequently and possibly leak urine when you laugh, cough, or sneeze. Swelling in the ankles and feet is also common from fluid retention and weight gain. Elevating your legs and staying active can help manage it.
Prenatal Visits Pick Up
Your appointment schedule changes in the third trimester. From week 28 through week 35, you’ll typically see your provider every two weeks. After week 36, visits shift to once a week until delivery. These more frequent check-ins allow your provider to monitor blood pressure, the baby’s position, and signs of labor.
One important screening happens at week 36 or 37: testing for Group B Streptococcus, a type of bacteria that about 1 in 4 women carry without symptoms. The test is recommended for every pregnancy, even if a cesarean birth is planned, because the bacteria can be passed to the baby during delivery. Testing is done late in pregnancy because GBS status can change, and the results are most useful when they reflect what’s happening close to delivery.
Counting From Where You Are
If you’re trying to figure out how much time you have left, the math is straightforward. Entering the third trimester at 28 weeks means you have roughly 12 weeks, or about three months, until your due date. In practice, most babies arrive somewhere between 39 and 41 weeks, so the actual time remaining could be anywhere from 11 to 13 weeks once you cross the 28-week mark. Your provider will give you a more precise estimate based on early ultrasound dating, which is more accurate than counting from your last period alone.