The third trimester of pregnancy spans from week 28 through week 40, or whenever you deliver. It covers the final three months and is the stretch where your baby finishes developing, gains weight rapidly, and settles into position for birth. For you, it brings the most noticeable physical changes and the shift toward preparing for labor and delivery.
When It Starts and Ends
Most major health organizations, including the American College of Obstetricians and Gynecologists, define the third trimester as beginning at 28 weeks of pregnancy. It lasts until birth, which on average happens around week 39 or 40. The third trimester is sometimes broken down further by how delivery timing is classified: 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 is 42 weeks and beyond. These distinctions matter because babies born at full term (39 weeks or later) tend to have better outcomes than those born even a week or two earlier.
How Your Baby Develops
By the time you enter the third trimester, your baby has completed most major organ development. The focus now shifts to growth, maturation, and fine-tuning. Around week 28, the central nervous system reaches a milestone: it can regulate body temperature on its own. By week 30, the eyes can open wide and red blood cells begin forming in the bone marrow rather than in other tissues.
Week 31 marks the transition to rapid weight gain. Your baby will roughly double or even triple in weight between now and birth, going from around 3 pounds to an average of 6 to 9 pounds at delivery. The soft, downy hair covering the skin (called lanugo) starts falling off around week 32. By week 33, the pupils can change size in response to light, and the bones are hardening, though the skull stays flexible to fit through the birth canal. Small details fill in too: toenails become visible by week 32, and fingernails reach the fingertips by week 34.
Your baby also becomes increasingly active during these weeks, kicking, stretching, and making grasping motions. You’ll likely feel more defined movements than the flutters of earlier months. Paying attention to this movement matters, because a noticeable decrease can signal a problem.
Your Baby’s Position Before Birth
Somewhere between weeks 32 and 36, most babies rotate into a head-down position, which is the ideal alignment for vaginal delivery. By week 36, most have settled there. If your baby remains in a breech position (feet or bottom down) past 36 weeks, your provider will discuss options, which may include a procedure to turn the baby or planning a cesarean delivery.
Physical Changes You Can Expect
The third trimester is the most physically demanding phase of pregnancy. Your uterus is now large enough to press against your diaphragm and rib cage, which causes shortness of breath, especially when lying on your back. Sleeping on your side helps. Your lungs actually expand their capacity during pregnancy, so you breathe a bit faster and get winded more easily even during light activity.
As the baby drops lower into your pelvis in the later weeks, you’ll feel increasing pressure on your bladder. Frequent urination becomes constant, and leaking urine when you laugh, cough, sneeze, or bend over is common. Swelling in the legs, ankles, and feet is also typical, caused by increased blood volume and pressure on veins. Varicose veins may appear on the legs. Propping your feet up and staying active can help manage the swelling.
Other common experiences include back pain, heartburn, difficulty sleeping, and Braxton Hicks contractions, which are practice contractions your uterus uses to prepare for labor. These are irregular, usually felt in the front of your belly, and stop when you change positions or walk around. They vary in length and aren’t particularly painful. Real labor contractions, by contrast, get consistently stronger and closer together, last 30 to 90 seconds each, don’t stop with movement, and may be felt in your lower back, cervix, and throughout your body.
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 during the third trimester. If you started out overweight or obese, the recommendation drops to about half a pound per week. These are averages. Some weeks you’ll gain more, others less. The overall trajectory matters more than any single week.
Prenatal Visits and Screening
Prenatal visits become more frequent in the third trimester. You’ll typically shift from monthly appointments to every two weeks around week 28, then weekly from about week 36 until delivery. These visits track your blood pressure, the baby’s heart rate, your weight, and the baby’s position.
One key screening happens during weeks 36 or 37: a test for Group B strep bacteria. About 1 in 4 pregnant people carry this bacteria, which is harmless to adults but can cause serious infection in newborns during delivery. The test is a simple swab. If the result is positive, you’ll receive antibiotics during labor to protect the baby.
Braxton Hicks vs. Real Labor
Distinguishing practice contractions from real labor is one of the most common concerns in the third trimester. Here’s how they differ:
- Pattern: Braxton Hicks are irregular and don’t get closer together. Real contractions follow a consistent pattern, gradually increasing in frequency.
- Response to movement: Walking or changing positions stops Braxton Hicks. Real contractions continue or intensify regardless of what you do.
- Intensity: Braxton Hicks are uncomfortable but manageable. You can walk, talk, and carry on. Real contractions grow strong enough that talking or walking through them becomes difficult.
- Location: Braxton Hicks are felt in the front of your belly. Real contractions often radiate from the lower back or cervix and can be felt throughout the body.
- Other signs: Real labor may come with your water breaking, a bloody show, or losing your mucus plug. Braxton Hicks come with none of these.
A reliable rule of thumb: if you’re having strong contractions less than five minutes apart for an hour, or contractions intense enough that you can’t walk or talk through them, it’s time to call your provider.
Warning Signs That Need Immediate Attention
Most third trimester discomfort is normal, but certain symptoms signal potentially dangerous complications like preeclampsia, placental problems, or infection. Seek medical care right away if you experience any of the following:
- Severe headache: One that won’t go away with medication or fluids, starts suddenly with intense pain, throbs on one side of your head, or comes with blurred vision or dizziness.
- Vision changes: Flashes of light, bright spots, blind spots, temporary vision loss, blurriness, or double vision.
- Extreme swelling of face or hands: Not the mild puffiness common in late pregnancy, but swelling severe enough that you can’t bend your fingers, wear rings, or fully open your eyes.
- Severe belly pain: Sharp, stabbing, or cramp-like pain that doesn’t go away, starts suddenly, or gets progressively worse. This includes severe chest, shoulder, or back pain.
- Decreased fetal movement: A noticeable change in how much your baby moves. There’s no single “normal” number of movements, but a clear slowdown or stop in your baby’s usual pattern is what to watch for.
- Vaginal bleeding or fluid leaking: Any bleeding heavier than spotting, fluid leaking from your vagina, or discharge with a foul smell.
These symptoms don’t always mean something is wrong, but they overlap with conditions that can become life-threatening quickly. Getting checked promptly is always the right call.