The third trimester starts at week 28 of pregnancy (28 weeks and 0 days) and continues through week 40. This final stretch is when your baby gains most of its weight and the organs finish maturing in preparation for birth.
Why Week 28 Specifically
The American College of Obstetricians and Gynecologists defines the third trimester as 28 weeks and 0 days through 40 weeks and 6 days. This cutoff isn’t arbitrary. Around week 28, fetal development shifts from building new structures to refining and growing what’s already there. The lungs, brain, and digestive system all enter critical final stages of maturation. A baby born at 28 weeks has a reasonable chance of survival with intensive medical support, which is part of why this milestone marks a distinct phase of pregnancy.
You may see some sources list week 27 as the start of the third trimester. The one-week discrepancy comes from different ways of dividing 40 weeks into three equal periods. ACOG’s definition of week 28 is the most widely used clinical standard.
How Your Baby Grows in the Third Trimester
At week 28, your baby measures roughly 10 inches from the top of the head to the tailbone and weighs about 2¼ pounds. Over the next 12 weeks, that weight more than triples. By week 32, the baby is around 11 inches long and 3¾ pounds. By week 40, those numbers jump to about 14 inches and 7½ pounds.
Most of this weight gain is fat. Your baby builds layers of body fat that will help regulate temperature after birth. The brain also grows rapidly during this period, developing the folds and grooves that increase its surface area. The lungs produce a substance that keeps the air sacs from collapsing, which is one of the last major developmental steps before a baby can breathe independently.
What “Full Term” Actually Means
Not all weeks in the third trimester are equal when it comes to readiness for birth. Medical organizations 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
These distinctions matter because babies born even a week or two before 39 weeks have higher rates of breathing problems and feeding difficulties compared to those born at full term. The final weeks of pregnancy allow the brain and lungs to finish developing in ways that make a measurable difference.
Physical Changes You Can Expect
The third trimester brings a noticeable shift in how your body feels, mostly because your baby is now large enough to press against surrounding organs.
Backaches become common as pregnancy hormones loosen the connective tissue in your pelvis, and your growing uterus stretches the abdominal muscles that normally support your spine. Shortness of breath often increases because the baby pushes up against your rib cage, reducing the space your lungs have to expand. You may also notice your heart fluttering or pounding occasionally. As the uterus grows, blood returns to the heart more slowly, which can create a sensation of skipped beats.
Heartburn and constipation tend to worsen because pregnancy hormones slow digestion while the uterus presses on the intestines. Frequent urination picks up again (or gets worse) as the baby drops lower into your pelvis and puts direct pressure on the bladder. Leaking urine when you laugh, cough, or sneeze is also common in these final months.
Braxton Hicks contractions, the practice contractions that feel like a tightening across your belly, typically become more frequent and stronger as your due date approaches. They tend to show up in the afternoon or evening, after physical activity or sex. They’re irregular and go away on their own. If you’re having more than six in an hour and they’re getting steadily stronger, that’s worth a call to your provider, as it could signal early labor.
Prenatal Visits and Screening
Your appointment schedule picks up in the third trimester. From week 28 through week 36, you’ll typically see your provider every two weeks. After week 36, visits shift to weekly until delivery.
One key screening happens during week 36 or 37: the Group B strep test. Your provider swabs the vagina and rectum and sends the sample to a lab. Group B strep is a type of bacteria that’s harmless to you but can cause serious infection in a newborn during delivery. If the test comes back positive, you’ll receive antibiotics during labor to protect the baby.
Your provider will also start checking the baby’s position, monitoring your blood pressure more closely, and discussing your birth plan as you get closer to your due date.
Tracking Your Baby’s Movement
Fetal movement becomes an important indicator of well-being in the third trimester. Most providers recommend paying attention to your baby’s activity patterns starting around week 28. You’ll get familiar with when your baby is most active, and that baseline matters more than hitting a specific number.
Your provider may ask you to do periodic kick counts, though the specific method varies. The key thing to watch for is a change from your baby’s normal pattern. A noticeable decrease in movement, or a sense that the baby has stopped moving, is one of the warning signs that warrants immediate contact with your provider.
Warning Signs to Know
Most third-trimester discomfort is normal, but certain symptoms signal something more serious. Swelling of the hands or face that’s severe enough to make it hard to bend your fingers or open your eyes fully can indicate preeclampsia, a dangerous blood pressure condition. The same goes for a headache that won’t go away or gets progressively worse, especially if it comes with vision changes like seeing bright spots, flashes of light, or blurry vision.
Other signs that need prompt medical attention include vaginal bleeding that’s more than spotting, fluid leaking from the vagina, a fever of 100.4°F or higher, severe belly pain that starts suddenly, and sudden or worsening shortness of breath. Swelling, redness, or pain in one leg, particularly the calf, can indicate a blood clot and also requires immediate evaluation.