The third trimester starts at week 28 and lasts through week 40, giving you roughly 12 to 13 weeks in the final stretch of pregnancy. Most major health organizations agree on week 28 as the starting point, though the end date depends on when you actually deliver. The average pregnancy wraps up around week 39 or 40.
How the Third Trimester Fits Into Pregnancy
Pregnancy divides 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 continues until birth. That makes the third trimester the longest stretch, at least on paper, since babies don’t always arrive on schedule.
Within those final weeks, the American College of Obstetricians and Gynecologists breaks things down further with specific labels for deliveries from week 37 onward:
- Early term: 37 weeks through 38 weeks, 6 days
- Full term: 39 weeks through 40 weeks, 6 days
- Late term: 41 weeks through 41 weeks, 6 days
- Post-term: 42 weeks and beyond
These distinctions matter because babies born even a couple of weeks early can have different outcomes than those born at full term. A baby delivered at 37 weeks is technically in the third trimester and past the preterm threshold, but is still classified as early term rather than full term.
What Your Baby Is Doing During These Weeks
At week 28, your baby’s eyes are partially open and the body is still lean. The real transformation happens over the next 12 weeks as fat stores build up, organs mature, and the baby prepares for life outside the womb.
Around week 31, rapid weight gain kicks in. By week 32, the fine body hair (called lanugo) that covered the skin starts to disappear. Week 33 brings the ability to detect light, and by week 35, the baby takes up most of the space in the amniotic sac. Around week 36, most babies have turned head-down into the position they’ll be in for delivery. By week 37, the head typically moves down into the pelvis. The final weeks are spent adding body fat and growing fingernails and toenails.
Physical Changes You Can Expect
The third trimester is when pregnancy feels its most physically demanding. Your body is supporting a rapidly growing baby, and that comes with a predictable set of changes.
Braxton Hicks contractions are common during these weeks. These are mild tightening sensations in your abdomen that come and go without a regular pattern. They tend to show up more in the afternoon or evening, after physical activity, or after sex, and they get more frequent as your due date approaches. They’re sometimes called “practice contractions” because they mimic labor without actually progressing into it.
Swelling in the feet, ankles, and legs is also typical, especially later in the trimester. Propping your legs up, staying active, and wearing compression socks can help. Avoid standing for long stretches or sitting with crossed legs.
Telling Braxton Hicks From Real Labor
One of the biggest questions in the third trimester is whether what you’re feeling is actual labor. Here’s how to tell the difference:
- Pattern: True labor contractions come at regular intervals and have a consistent rhythm. Braxton Hicks contractions are irregular and unpredictable.
- Duration: Real contractions last about 60 to 90 seconds. Practice contractions vary in length.
- Response to rest: If you lie down, drink water, or change position and the contractions stop, they’re likely Braxton Hicks. True labor continues regardless of what you do.
- Intensity: True labor contractions get steadily stronger over time. Braxton Hicks contractions stay about the same or fade.
If you’re having more than six contractions per hour and they’re getting stronger, contact your provider. That pattern suggests real labor may be starting.
Prenatal Visit Schedule in the Third Trimester
Your appointments pick up in frequency during these final weeks. From week 28 through week 36, you’ll typically see your doctor or midwife every two weeks. After week 36, visits shift to every week until delivery. These checkups monitor your blood pressure, weight, and any new symptoms.
Two vaccines are recommended during the third trimester. The whooping cough vaccine (Tdap) is generally given between weeks 27 and 36. The RSV vaccine, which helps protect your newborn from respiratory syncytial virus, is recommended between weeks 32 and 36 during RSV season (September through January in most of the continental U.S.). If you receive the RSV vaccine after 36 weeks, there likely won’t be enough time for protective antibodies to cross the placenta before delivery.
Warning Signs to Watch For
Preeclampsia is one of the most serious complications of the third trimester. Its defining feature is high blood pressure, but it can affect the kidneys, liver, and other organs. Symptoms to watch for include severe headaches, blurred vision or light sensitivity, pain in the upper right abdomen, sudden swelling in the face and hands, shortness of breath, and nausea or vomiting. Any of these warrant immediate medical attention, as preeclampsia can escalate quickly and poses risks to both mother and baby.