The third trimester runs from week 29 through week 40, spanning roughly 12 weeks. It begins at the start of your seventh month of pregnancy and ends when you deliver, though not every pregnancy reaches exactly 40 weeks.
When Each Trimester Starts and Ends
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 week 28, and the third trimester picks up at week 29 and continues through week 40. The National Institute of Child Health and Human Development defines the third trimester as week 29 to week 40.
Your due date is calculated at 40 weeks from the first day of your last menstrual period. If you deliver on your due date, the third trimester lasts exactly 12 weeks. In practice, healthy pregnancies can end anywhere from 37 to 42 weeks, so your third trimester could be as short as 9 weeks or as long as 14.
How “Term” Pregnancy Is Classified
Not all deliveries at or after 37 weeks are considered equal. The American College of Obstetricians and Gynecologists breaks it down further:
- 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
- Postterm: 42 weeks and beyond
These distinctions matter because babies born at 39 weeks or later tend to have fewer breathing and feeding difficulties than those born at 37 or 38 weeks, even though both are technically “term.” This is one reason providers avoid elective inductions or scheduled deliveries before 39 weeks unless there’s a medical reason.
What Happens If the Baby Comes Early
The third trimester is when a baby’s organs finish maturing, so every additional week in the womb makes a difference. Survival rates improve dramatically as the weeks pass. Babies born at 28 weeks have an 80 to 90 percent survival rate. By 32 weeks, that climbs to about 95 percent. After 34 weeks, babies generally have the same long-term health outcomes as those born at full term.
Lung development is a key factor. The lungs produce a substance that keeps the tiny air sacs from collapsing with each breath, and this typically reaches adequate levels around 35 weeks. Babies born before that threshold are at higher risk for respiratory distress and may need breathing support in the NICU.
How the Baby Grows in the Third Trimester
At the start of the third trimester, around week 29, most babies weigh roughly 2.5 pounds. Over the next 12 weeks, weight gain accelerates considerably. Starting around week 35, the baby puts on about 8 to 12 ounces per week, which is the most rapid growth period of the entire pregnancy. By 40 weeks, the average baby weighs between 6 and 9 pounds.
Beyond weight, the third trimester is when the brain develops its characteristic folds and ridges, fat layers build up under the skin to help regulate body temperature after birth, and bones harden (though the skull stays flexible enough for delivery). The baby also settles into a head-down position, usually by around 36 weeks.
What Your Body Feels Like
The physical demands of the third trimester are noticeably different from earlier months. As the baby grows, your uterus presses against your bladder, lungs, and digestive tract. Frequent urination, shortness of breath, and heartburn are all common from week 29 onward.
Braxton Hicks contractions, the irregular tightening sensations across your abdomen, tend to pick up during the third trimester. They’re more likely to show up in the afternoon or evening, after physical activity, or after sex, and they become more frequent as your due date approaches. Unlike true labor contractions, they don’t follow a regular pattern and usually ease up with rest or a change in position.
In the final weeks, the baby drops lower into your pelvis, a shift sometimes called “lightening.” This relieves some pressure on your lungs (breathing gets easier), but increases pressure on your bladder. You may notice you’re making even more trips to the bathroom than before.
Tracking Fetal Movement
Kick counting becomes an important habit starting at week 28. The goal is to feel 10 movements, including kicks, flutters, swishes, or rolls, within a two-hour window. Most women feel 10 movements within a single hour. Pick a time of day when your baby is typically active, sit or lie down, and count.
If two hours pass and you haven’t reached 10 movements, contact your provider. A temporary slowdown can be normal, but consistently reduced movement can signal that the baby is under stress. Tracking daily gives you a reliable baseline so you’ll notice changes early.
Tests and Appointments in the Third Trimester
Prenatal visits ramp up during these final weeks. Most providers schedule appointments every two weeks from 28 to 36 weeks, then weekly from 36 weeks until delivery. These visits typically check your blood pressure, urine, fundal height (the measurement from your pubic bone to the top of your uterus), and the baby’s heart rate.
If you had your glucose screening in the second trimester (usually between 24 and 28 weeks) and the results were borderline, you may be retested early in the third trimester. Between 35 and 37 weeks, most providers also test for Group B strep, a common bacterium that can be passed to the baby during delivery. A positive result simply means you’ll receive antibiotics during labor. Your provider will also likely recommend the Tdap vaccine (for whooping cough) during weeks 27 through 36 to pass protective antibodies to the baby before birth.