The third trimester begins at 28 weeks and 0 days of pregnancy, as defined by the American College of Obstetricians and Gynecologists (ACOG). It runs through 40 weeks and 6 days, covering roughly the final 12 to 13 weeks before your due date. If you’re counting by months, that’s the start of your seventh month of pregnancy.
Why 28 Weeks Is the Cutoff
Pregnancy is divided into three roughly equal stages. The first trimester covers conception through 12 weeks, the second runs from 13 to 27 weeks, and the third picks up at 28 weeks. You might occasionally see sources that say 27 weeks, but ACOG’s standard definition places the boundary at 28 weeks and 0 days. The difference comes down to whether a source counts week 27 as the last week of the second trimester or the first of the third. If your pregnancy app says something slightly different, this is why.
What Happens to Your Baby From Week 28 On
The third trimester is primarily about weight gain and organ maturation. Your baby roughly gains an average of 175 to 180 grams per week (about 6 ounces) during this stretch, though boys tend to gain slightly more than girls. A baby that weighs around 2 to 2.5 pounds at 28 weeks will typically reach 6.5 to 8 pounds or more by full term.
Lung development is one of the most critical milestones. Your baby’s lungs begin producing surfactant, the substance that keeps air sacs from collapsing, as early as 24 weeks. But adequate surfactant levels to support breathing outside the womb aren’t reached until around 32 weeks. This is a major reason why babies born very early in the third trimester often need breathing support, while those born closer to term generally don’t.
Other changes happening during these weeks include the brain forming its characteristic folds and grooves, the bones hardening (though the skull stays flexible for delivery), and the baby settling into a head-down position, typically by 36 weeks. Fat layers build up under the skin, helping your baby regulate temperature after birth.
Early Term, Full Term, and Late Term
Not all weeks in the third trimester are equal when it comes to readiness for birth. The National Institutes of Health breaks it down into specific categories:
- 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
This distinction matters because babies born at 37 or 38 weeks, while technically “term,” have higher rates of breathing difficulties and feeding problems than those born at 39 or 40 weeks. Those final two weeks allow the lungs, brain, and liver to finish critical development. This is why most providers avoid scheduling elective deliveries before 39 weeks unless there’s a medical reason.
What You’ll Likely Feel
The physical experience of the third trimester is shaped by the fact that your baby is now large enough to press on surrounding organs. Frequent urination picks up as the baby drops deeper into your pelvis and puts pressure on your bladder. Leaking urine when you laugh, cough, or sneeze is common during these weeks.
Braxton Hicks contractions, those irregular tightening sensations across your belly, tend to increase in frequency as the third trimester progresses. They’re more likely in the afternoon or evening, after physical activity, or after sex. Unlike true labor contractions, they don’t follow a regular pattern and they don’t get progressively stronger. Swelling in your feet and ankles is also typical, especially toward the end of the day. Propping your legs up and staying active both help.
Shortness of breath, heartburn, trouble sleeping, and back pain round out the list of common complaints. Most of these ease once the baby “drops” lower into the pelvis in the final weeks before delivery, though that trade-off brings even more bladder pressure.
Prenatal Visits and Screening
Your appointment schedule shifts in the third trimester. From 28 weeks until 36 weeks, you’ll typically see your provider every two weeks. After 36 weeks, visits become weekly until delivery. These more frequent check-ins allow your provider to monitor blood pressure, check the baby’s position, and track growth.
Two key screenings happen during the third trimester. The CDC recommends a Tdap vaccine (which protects against whooping cough) between 27 and 36 weeks, ideally on the earlier end of that window. This allows your body to produce antibodies that cross the placenta and protect your newborn during their first months of life. The second is Group B Strep screening, a simple vaginal and rectal swab done during week 36 or 37. About 1 in 4 pregnant women carry this bacteria, which is harmless to you but can cause serious infection in a newborn during delivery. A positive result means you’ll receive antibiotics during labor.
Tracking Fetal Movement
Starting around 28 weeks, your provider may ask you to begin tracking kick counts. The method recommended by ACOG is straightforward: pick a time when your baby is usually active, sit or lie down, and count how long it takes to feel 10 movements. Kicks, rolls, flutters, and swishes all count. You’re looking for 10 movements within two hours, though most women feel them much faster than that.
The point isn’t to hit an exact number every day. It’s to learn your baby’s normal pattern so you can notice if something changes. A sudden, sustained decrease in movement is worth reporting to your provider. Babies do tend to move less as space gets tighter near the end, but they shouldn’t stop moving altogether.