The third trimester spans weeks 29 through 40, making it roughly 12 weeks long. For most pregnancies, that means the final stretch begins at the start of your seventh month and lasts until delivery, which typically happens between 39 and 40 weeks.
How the Third Trimester Fits Into Pregnancy
Pregnancy is divided into three trimesters of roughly equal length. The first trimester covers weeks 1 through 12, the second runs from week 13 through 28, and the third picks up at week 29 and continues through week 40. In practice, the third trimester can be shorter or longer than 12 weeks depending on when your baby arrives. A delivery at 39 weeks means your third trimester lasted about 10 weeks, while going to 41 weeks stretches it to 13.
The American College of Obstetricians and Gynecologists breaks down the final weeks into more specific categories. Early term is 37 weeks through 38 weeks and 6 days. Full term is 39 weeks through 40 weeks and 6 days. Late term is 41 weeks through 41 weeks and 6 days, and post-term begins at 42 weeks. These distinctions matter because babies born even a couple of weeks early can face different health outcomes than those born at full term.
What Happens to Your Baby During These 12 Weeks
The third trimester is when your baby puts on the most weight. Around week 35, the rate of growth peaks at roughly 8 to 12 ounces per week. Much of that weight comes from fat deposits that will help your baby regulate body temperature after birth. By 40 weeks, most babies weigh between 6 and 9 pounds.
Lung development is one of the most important milestones in this trimester. The lungs begin producing surfactant, a substance that keeps the air sacs from collapsing, around week 26. But surfactant doesn’t reach adequate levels until about week 35. This is a major reason why babies born before 35 weeks often need breathing support.
Around week 36, most babies rotate into the head-down position in preparation for delivery. If your baby hasn’t turned by 37 weeks, your provider may suggest a procedure to gently rotate the baby from the outside. Not every baby cooperates, and a baby that stays in a breech position may change delivery plans.
Physical Changes You Can Expect
The third trimester brings a collection of symptoms driven by two forces: a growing uterus and shifting hormones. Some are mild annoyances, others can genuinely disrupt your day.
Bladder pressure increases as the baby drops lower into your pelvis, especially in the final weeks. You may find yourself making more bathroom trips than at any other point in pregnancy. Shortness of breath is also common because the expanding uterus pushes up against the rib cage, leaving less room for your lungs to fully expand. Many women notice this improves slightly in the last few weeks as the baby drops lower.
Back pain tends to worsen during this trimester. Pregnancy hormones loosen the connective tissue in your pelvic area, and the growing uterus stretches the abdominal muscles that normally help support your spine. Heartburn and constipation often intensify too, because those same hormones slow digestion while the uterus presses on the intestines.
Swelling in the feet and ankles is normal, caused by increased blood volume and fluid retention. Elevating your legs and staying active can help. Some women also develop varicose veins in the legs or small spider veins on the skin from the increased blood flow. Heart palpitations occasionally happen because the larger uterus can slow blood returning to the heart.
Braxton Hicks vs. Real Contractions
Braxton Hicks contractions often become more noticeable in the third trimester, and they can be strong enough to make you think labor has started. These “practice” contractions tend to show up in the afternoon or evening, after physical activity, or after sex. They feel like a tightening across the abdomen that varies in strength.
The key difference is pattern. True labor contractions come at regular intervals and get progressively closer together over time. Braxton Hicks contractions are irregular and don’t follow a consistent rhythm. True contractions also persist when you rest, change positions, or drink water. If lying down and hydrating makes the tightening stop, it’s not real labor. Braxton Hicks can begin many weeks before delivery, so experiencing them at 30 or 32 weeks doesn’t mean anything is happening early.
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 once a week until delivery. These more frequent check-ins allow your provider to monitor your blood pressure, track the baby’s position, and watch for signs of complications like preeclampsia.
One key screening happens during week 36 or 37: a test for Group B streptococcus, a type of bacteria that can be present in the vaginal or rectal area. About 1 in 4 pregnant women carry it, and while it’s harmless to you, it can cause serious infection in a newborn during delivery. A positive result simply means you’ll receive antibiotics during labor to protect the baby.
Counting the Weeks
If you’re trying to figure out exactly where you are in the third trimester, the math is straightforward. Subtract 28 from your current week of pregnancy. At 32 weeks, you’re 4 weeks into the third trimester with roughly 8 weeks to go. At 36 weeks, you’re 8 weeks in and entering the home stretch. At 39 weeks, you’ve hit the full-term window where most providers are comfortable with delivery proceeding naturally.
Keep in mind that only about 5% of babies arrive on their exact due date. The 40-week mark is an estimate, not a deadline. Most first-time mothers deliver somewhere between 39 and 41 weeks, so the actual length of your third trimester will vary by a week or two in either direction.