The third trimester of pregnancy spans week 29 through week 40. It’s the final stretch before delivery, and it brings the most rapid fetal growth, the most noticeable physical changes for the pregnant person, and the most frequent prenatal visits. By the start of this trimester, the baby weighs roughly 2¼ pounds. By the end, that number climbs to around 7½ pounds.
How the Baby Grows Week by Week
The third trimester is when a baby does most of its growing. At week 28, the baby measures about 10 inches from crown to rump and weighs around 2¼ pounds. By week 32, that jumps to about 11 inches and 3¾ pounds. At week 34, the baby reaches roughly 12 inches and over 4½ pounds. And by week 40, the typical baby measures around 14 inches crown to rump and weighs about 7½ pounds, though some babies weigh closer to 9 pounds by this point.
Weight isn’t the only thing changing. The lungs are maturing throughout this trimester, producing the substance they’ll need to inflate properly after birth. The brain is developing rapidly, adding the folds and grooves that increase its surface area. Fat layers build up under the skin, helping the baby regulate body temperature once outside the womb. These final weeks of organ maturation are a major reason why babies born even a few weeks early can face health challenges that full-term babies don’t.
Physical Changes You’ll Notice
The third trimester is the most physically demanding phase of pregnancy, and most of the discomfort traces back to two things: hormones loosening your joints and ligaments, and a growing uterus pressing on everything around it.
Back pain is one of the most common complaints. Pregnancy hormones relax the connective tissue that holds your pelvic bones in place, while the expanding uterus stretches your abdominal muscles. Together, these shifts pull your center of gravity forward and strain your lower back. Frequent urination also picks up again (or gets worse) as the baby moves deeper into the pelvis and puts more pressure on your bladder.
Swelling in the feet, ankles, and hands is normal during these months. Your body retains more fluid, and the weight of the uterus can slow blood return from your legs. Elevating your feet and staying active helps. That said, there’s a difference between the gradual, mild swelling most people experience and the kind that signals a serious problem (more on that below).
Other common symptoms include heartburn from the uterus crowding the stomach, shortness of breath as it pushes up against the diaphragm, trouble sleeping due to sheer discomfort, and hemorrhoids from increased pelvic pressure.
Prenatal Visits and Screenings
Prenatal appointments become more frequent in the third trimester. You’ll typically go every two to four weeks from week 28 through 35, then every one to two weeks from week 36 until delivery. These visits usually include blood pressure checks, urine tests, fundal height measurements (the distance from your pubic bone to the top of the uterus), and monitoring the baby’s heart rate.
One important screening happens around weeks 36 or 37: the Group B strep test. Group B streptococcus is a type of bacteria that about 25% of healthy adults carry, usually without symptoms. It’s harmless to you, but it can be dangerous if passed to a newborn during delivery. The test is a simple swab. If the result is positive, you’ll receive antibiotics during labor to protect the baby.
Weight Gain Guidelines
Steady weight gain matters more in the second and third trimesters than in the first. If you started pregnancy at a healthy weight, the general guideline is to gain about 1 pound per week through the rest of pregnancy. If you started at a higher weight, the recommendation drops to about half a pound per week. These are averages, not rigid targets. Some weeks you’ll gain more, some less. Your provider tracks the overall trend rather than any single weigh-in.
When the Baby Drops
Toward the end of the third trimester, the baby settles lower into the pelvis in a process called lightening. For first-time pregnancies, this can happen a few weeks before labor begins. For people who’ve given birth before, it often doesn’t happen until days before labor or even during labor itself.
You’ll likely notice several changes when it happens. Breathing gets easier because there’s less pressure on the diaphragm. Heartburn may improve for the same reason. On the other hand, pelvic pressure increases, urination becomes even more frequent, and you may develop a wider, waddling gait. Some people notice increased vaginal discharge, lower back pain, or a visible change in the shape of their belly.
Braxton Hicks vs. Real Contractions
Braxton Hicks contractions are common in the third trimester and can be confusing if you’re wondering whether labor has started. The key differences come down to pattern, pain, and location.
- Pattern: Braxton Hicks come at random intervals and don’t get closer together. Real labor contractions develop a steady rhythm, intensify over time, and the gaps between them shorten.
- Pain: Braxton Hicks are usually painless or feel like mild menstrual cramps. Labor contractions are painful and progressively get worse. Changing position, drinking water, or resting can relieve Braxton Hicks. Nothing makes true labor contractions go away.
- Location: Braxton Hicks are felt across the front of the belly. Real labor pain typically radiates through the hips, lower back, and cervix.
Real labor also comes with other signs: your water breaking, loss of the mucus plug (a thick discharge that may be tinged with blood), or light bleeding as the cervix begins to dilate.
Warning Signs That Need Immediate Attention
Most third-trimester discomfort is normal, but certain symptoms can signal a life-threatening condition like preeclampsia or placental problems. Seek care immediately if you experience any of the following:
- Vision changes: Flashes of light, bright spots, blind spots, blurriness, double vision, or temporary loss of sight.
- Extreme swelling of the hands or face: This isn’t the mild puffiness most pregnant people get. It means swelling so severe that you can’t bend your fingers, your rings won’t fit, your eyes look puffy and hard to open, or your lips and mouth feel numb and swollen.
- Severe headache that won’t go away
- Pain in your upper belly, especially on the right side
- Sudden, significant decrease in the baby’s movement
Preeclampsia can develop without warning, even in people who’ve had an uncomplicated pregnancy up to that point. The frequent blood pressure checks at your prenatal visits are specifically designed to catch early signs before symptoms appear.