The third trimester of pregnancy spans weeks 28 through 40, covering the final three months before birth. It’s the stretch where your baby gains most of its weight, your body prepares for labor, and prenatal visits become more frequent. For most people, this trimester ends around week 39 or 40, though delivery any time after 37 weeks falls within the range of “term.”
When It Starts and Ends
Most major health organizations agree that the third trimester begins at 28 weeks and lasts until you give birth. While average pregnancies reach 39 or 40 weeks, the medical definition of “term” is more specific than many people realize. Deliveries between 37 and 38 weeks are classified as early term, 39 through 40 weeks as full term, 41 weeks as late term, and 42 weeks or beyond as postterm. These distinctions matter because babies born even a few weeks early can face more health challenges than those born at full term.
How Your Baby Grows
The third trimester is primarily about weight gain and organ maturation. At week 28, a baby measures roughly 10 inches from head to tailbone and weighs about 2.25 pounds. By week 32, that jumps to around 11 inches and 3.75 pounds. At week 40, most babies reach about 14 inches crown to rump and 7.5 pounds.
That rapid weight gain isn’t just size for its own sake. Fat layers build up under the skin, helping your baby regulate temperature after birth. The lungs are among the last organs to fully mature, which is one reason early deliveries carry more risk. The brain also undergoes a period of intense development during these final weeks, forming the connections that will support breathing, feeding, and responding to the world outside the womb.
What Your Body Goes Through
As your baby grows, you’ll feel the effects in ways that are uncomfortable but mostly normal. The uterus expands up under your rib cage, which puts pressure on your diaphragm and reduces the space your lungs have to expand. This is why shortness of breath becomes common, especially when lying on your back. You may also notice you get winded more easily during everyday activities, even though your lung capacity actually increases slightly during pregnancy.
Braxton Hicks contractions typically pick up during the third trimester. These are mild, irregular tightenings of the uterus that come and go, often in the afternoon or evening, after physical activity, or after sex. They tend to get more frequent as your due date approaches, which can make it tricky to tell them apart from early labor. The key difference: Braxton Hicks contractions are irregular in timing and don’t progressively get stronger or closer together.
Swelling in the feet and ankles is also common as your body retains more fluid and the growing uterus puts pressure on blood vessels. Back pain, trouble sleeping, and frequent urination round out the list of near-universal third trimester experiences.
Weight Gain in the Final Months
If you started pregnancy at a healthy weight, the general guideline is to gain about 1 pound per week through the second and third trimesters. For people who were overweight or obese before pregnancy, the recommendation drops to about half a pound per week. Steady, gradual gain matters more than hitting an exact number on any given week. Sudden, rapid weight gain (especially paired with swelling) can signal a complication and is worth mentioning to your provider.
Tests and Screenings to Expect
Prenatal visits shift from monthly to every two weeks around 28 to 32 weeks, then weekly from 36 weeks until delivery. These visits typically include blood pressure checks, urine tests, and measurements of your belly to track growth.
One important screening in the third trimester is the Group B strep (GBS) test, usually done between weeks 36 and 37. GBS is a common bacterium that lives in the vagina or rectum and poses no threat to you. But it can pass to your baby during delivery. Without treatment, about 1 to 2 out of every 100 babies born to GBS-positive mothers will become infected. If you test positive, you’ll receive antibiotics during labor to dramatically reduce that risk.
Tracking Your Baby’s Movement
By the third trimester, you’re familiar with your baby’s patterns of activity. Kick counts are a simple way to monitor fetal well-being at home. The standard approach: pick a time when your baby is usually active, sit or lie down, and count movements. You want to feel 10 movements within two hours. Kicks, rolls, and jabs all count.
If you don’t reach 10 in that window, try having a snack or shifting positions to wake the baby up, then try again. If movement still feels significantly reduced compared to your baby’s normal pattern, contact your provider. A noticeable decrease in movement doesn’t always mean something is wrong, but it’s one of the clearest signals your baby can send that something needs checking.
Warning Signs That Need Immediate Attention
Most third trimester discomfort is harmless, but certain symptoms require urgent evaluation. Preeclampsia, a condition involving dangerously high blood pressure, develops most often in the third trimester and can escalate quickly. Watch for a persistent headache that worsens over time, vision changes like flashing lights or blurry spots, and extreme swelling of the hands or face (not the mild ankle puffiness that’s normal).
Other red flags include:
- Vaginal bleeding beyond light spotting, or fluid leaking from the vagina
- Severe belly pain that is sharp or constant and doesn’t let up
- Sudden shortness of breath with chest tightness, particularly if it’s different from the gradual breathlessness of late pregnancy
- Fever of 100.4°F or higher
- Severe swelling, redness, or pain in one leg, which could indicate a blood clot
- A significant decrease in your baby’s movement
These symptoms don’t always mean something serious is happening, but they overlap with conditions that can become dangerous fast. Calling your provider or going to labor and delivery is always the right call when they appear.