The third trimester spans roughly three months, covering weeks 28 or 29 through 40 of pregnancy. In calendar terms, that’s months 7, 8, and 9. The reason you’ll see slightly different starting points is that pregnancy months don’t line up neatly with four-week blocks. A full pregnancy lasts 40 weeks, which is actually closer to 10 calendar months than nine, so the math gets fuzzy depending on the source.
Why the Week Count Doesn’t Match the Month Count
Most people think of pregnancy as nine months, but 40 weeks divided by 4 weeks per month equals 10. The discrepancy exists because calendar months average about 4.3 weeks, not exactly 4. So when a source says the third trimester starts at week 28 and another says week 29, the difference is just how they’re rounding.
The Office on Women’s Health defines the third trimester as weeks 29 through 40. Other organizations, including Australia’s Better Health Channel, place the start at week 28. Either way, you’re looking at approximately 12 to 13 weeks of pregnancy remaining, which works out to about three months.
What “Term” Actually Means
Not all weeks in the third trimester carry equal weight when it comes to delivery timing. The National Institutes of Health and the American College of Obstetricians and Gynecologists break it down further:
- 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
A baby born at 37 weeks is technically in the third trimester and past the preterm threshold, but those final two to three weeks matter. Lung maturity, for example, is generally considered clinically complete around 36 weeks, though babies born as early as 24 weeks can sometimes manage basic breathing. The lungs start producing the substance needed for independent breathing between 20 and 24 weeks, but the amount increases steadily all the way to the due date.
What Your Body Does During These Months
The third trimester brings the most dramatic physical changes of the entire pregnancy. Your uterus is now large enough to press on your bladder, lungs, and lower back simultaneously, which explains why the last few months can feel so much harder than the second trimester.
Braxton Hicks contractions become more noticeable. These are slight tightening sensations in your abdomen that come and go, often in the afternoon or evening. They tend to show up after physical activity or sex and get more frequent as your due date approaches. They’re not labor contractions, but they can feel alarming the first few times.
Pelvic pressure increases as the baby drops lower into your pelvis in the final weeks. This shift relieves some pressure on your lungs (breathing gets a little easier) but puts more weight on your bladder. Leaking urine when you laugh, cough, sneeze, or lift something becomes common. More frequent trips to the bathroom, especially at night, are typical for the entire trimester.
Weight Gain in the Third Trimester
If you started pregnancy at a healthy weight, the general guideline is to gain about 1 pound per week through the second and third trimesters. For those who started overweight or obese, the recommendation drops to about half a pound per week. These are averages. Some weeks you’ll gain more, others less, and that’s normal as long as the overall trend is steady. Your provider will track this at each visit.
How Prenatal Visits Change
Appointments become more frequent once you hit the third trimester. From week 28 onward, you’ll typically see your provider every two weeks. After week 36, visits shift to every single week until delivery. These later appointments focus on the baby’s position, your blood pressure, and signs that labor may be approaching.
One specific screening happens during weeks 36 or 37: a test for Group B Strep bacteria. This is a simple swab test recommended by ACOG for every pregnancy, regardless of your history. About 1 in 4 pregnant women carry this bacteria without any symptoms, and knowing your status before labor helps your medical team plan accordingly.
A Quick Reference by Week and Month
- Month 7: approximately weeks 28 through 31
- Month 8: approximately weeks 32 through 35
- Month 9: approximately weeks 36 through 40
These ranges are approximate because, again, weeks and months don’t divide evenly. If someone asks how far along you are, giving the week number is always more precise than the month. Your provider will use weeks exclusively, and most pregnancy apps do the same.