The third trimester of pregnancy runs from week 28 through week 40, or whenever you deliver. It’s the final stretch, covering roughly 12 weeks of rapid fetal growth and significant physical changes for you. While you may occasionally see sources listing week 27 as the starting point, most major health organizations agree that week 28 marks the beginning.
How the Trimesters Break Down
Pregnancy is divided into three trimesters of roughly 13 weeks each. The first trimester covers weeks 1 through 12, the second spans weeks 13 through 27, and the third trimester picks up at week 28 and continues until birth. A typical pregnancy lasts about 40 weeks from the first day of your last menstrual period, though delivery anywhere between 37 and 42 weeks falls within the recognized range.
Within the third trimester, medical guidelines further distinguish how “term” your pregnancy is. Early term is 37 weeks through 38 weeks and 6 days. Full term is 39 weeks through 40 weeks and 6 days. Late term covers 41 weeks through 41 weeks and 6 days, and post-term begins at 42 weeks. These distinctions matter because outcomes for babies improve with each additional week closer to full term, which is why providers generally prefer to let labor begin on its own when possible.
What Your Baby Is Doing in the Third Trimester
The third trimester is when your baby gains the most weight and finishes developing the systems needed for life outside the womb. By week 28, the central nervous system can regulate body temperature and trigger breathing movements, which are sometimes visible on ultrasound. Fat begins accumulating under the skin earlier in pregnancy, but this process accelerates significantly in the final weeks. By week 36, that added fat smooths out the skin, and by week 39, fat is being deposited all over the body to help your baby stay warm after birth.
Lung maturation is one of the most critical developments during this period. The lungs are among the last organs to fully mature, which is a key reason why babies born earlier in the third trimester often need breathing support. Brain development also continues at a rapid pace, with the brain roughly tripling in weight between week 28 and week 40. This is part of why those final weeks of pregnancy, even when uncomfortable, carry real developmental significance.
Physical Changes You Can Expect
Your body undergoes substantial shifts during these last 12 weeks. The growing uterus pushes against your diaphragm, ribs, and bladder, which can make breathing feel harder and send you to the bathroom more often. Back pain becomes common as pregnancy hormones loosen the connective tissue that holds your pelvic bones in place, and the expanding uterus stretches your abdominal muscles. Together, these changes shift your center of gravity and put extra strain on your lower back.
Braxton Hicks contractions, those irregular tightening sensations in your abdomen, tend to show up more frequently in the third trimester. They’re more likely in the afternoon or evening, after physical activity, or after sex, and they become stronger and more noticeable as your due date approaches. Unlike true labor contractions, they’re irregular and usually stop when you change positions or rest.
Swelling in your legs and feet is another hallmark of the third trimester. Varicose veins may appear on your legs, and hemorrhoids (swollen veins in the rectal area) are also common. Regular movement and propping your legs up when you rest can help manage the swelling.
Sleep Position Matters More Now
Earlier in pregnancy, your sleep position doesn’t significantly affect outcomes. But as the uterus grows heavier in the third trimester, sleeping on your back can compress major blood vessels, specifically the large artery and vein that supply and drain blood from your lower body. Studies have linked back sleeping in late pregnancy with higher risks of stillbirth, reduced fetal growth, and preeclampsia. Sleeping on your side, particularly your left side, is the standard recommendation for the third trimester. A pillow between your knees or behind your back can help you stay comfortable.
Prenatal Visits Pick Up
Your appointment schedule changes 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 every week until delivery. These more frequent checkups allow your provider to monitor your blood pressure, track your baby’s position, and catch any late-pregnancy complications early.
One important screening happens late in the third trimester: the Group B Strep (GBS) test. GBS is a type of bacteria that can live in the vagina and rectum without causing any symptoms for you, but it can pass to your baby during labor. Without treatment, about 1 to 2 out of every 100 babies born to GBS-positive mothers become infected. If you test positive, you’ll receive antibiotics during labor to protect your baby.
Signs That Labor Is Approaching
In the final weeks of the third trimester, your body starts preparing for delivery. Lightening is the sensation of your baby dropping lower into your pelvis. It can happen weeks or just hours before labor starts, and you may notice it’s suddenly easier to breathe since there’s less pressure on your diaphragm.
Other signals include losing your mucus plug, a thick clump of discharge that sealed your cervix throughout pregnancy. As your cervix begins to open, you may notice clear, pink, or slightly bloody discharge. Your water breaking, the rupture of the fluid-filled sac surrounding your baby, is another sign. This can feel like a gush or a slow trickle. Contractions feel like pain in your back or pelvis, similar to menstrual cramps, and they grow more regular and intense over time.
You should head to the hospital if your water breaks and contractions haven’t started, if you’re bleeding heavily, if you have constant severe pain with no relief between contractions, or if you notice your baby is moving less than usual.