The third trimester of pregnancy is approximately 12 to 13 weeks long, starting at week 28 and lasting until delivery, which typically happens around week 39 or 40. Most major health organizations agree on this week 28 starting point, though the exact length depends on when your baby arrives.
Week-by-Week Breakdown
The third trimester covers the final three months of pregnancy: months seven, eight, and nine. Here’s how the weeks map out:
- Month 7: Weeks 28 through 31
- Month 8: Weeks 32 through 35
- Month 9: Weeks 36 through 40
Not every pregnancy ends at exactly 40 weeks. The National Institutes of Health breaks the final stretch into more specific categories: 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), and post-term (42 weeks and beyond). A baby born at 37 weeks is in the third trimester but technically early term, while one arriving at 41 weeks is considered late term. So for some people, the third trimester lasts closer to 9 weeks; for others, it stretches past 14.
How Your Baby Develops During These Weeks
The third trimester is when your baby gains the most weight and finishes developing the systems needed for life outside the womb. At week 28, the nervous system can regulate body temperature and trigger practice breathing movements visible on ultrasound. The brain is maturing rapidly during this entire stretch.
By week 32, your baby weighs roughly 3¾ pounds and measures about 11 inches from crown to rump. The fine layer of downy hair that covered the skin during earlier months starts falling off, and toenails become visible. Four weeks later, at week 36, fat is filling in under the skin, making the limbs look chubby. Most babies have turned head-down by this point, settling into position for delivery.
At 40 weeks, a typical baby measures around 14 inches from crown to rump and weighs about 7½ pounds. That means your baby roughly doubles in weight between weeks 32 and 40, gaining close to half a pound per week in the final stretch.
Common Physical Symptoms
Your body is doing significant work during these 12-plus weeks, and you’ll feel it. Back pain is one of the most common complaints. Pregnancy hormones loosen the connective tissue in your pelvic area while your growing uterus stretches out abdominal muscles, pulling your center of gravity forward.
Shortness of breath picks up as the baby presses upward against your rib cage. You may find it harder to breathe when lying on your back and more comfortable resting on your side. Heart palpitations, those fluttering or pounding sensations in your chest, can also show up because the larger uterus slows blood returning to the heart. They’re usually harmless.
Frequent urination gets worse as the baby drops deeper into your pelvis and puts pressure on your bladder. Leaking urine when you laugh, cough, or sneeze is common. Heartburn and constipation often intensify too, because pregnancy hormones continue slowing digestion while the uterus presses on your intestines. Varicose veins, hemorrhoids, and small spider veins on the face, neck, or arms can appear from increased blood flow. Most of these changes fade after delivery.
Braxton Hicks vs. True Labor Contractions
You’ll likely start feeling Braxton Hicks contractions during the third trimester, sometimes weeks before actual labor. These “practice” contractions cause a tightening in your abdomen that can range from barely noticeable to genuinely painful. They tend to show up in the afternoon or evening, after physical activity, or after sex, and they become more frequent as your due date nears.
The key differences: Braxton Hicks contractions have no regular pattern and don’t get closer together over time. They often stop if you walk, rest, change position, or drink water. True labor contractions follow a pattern, come at regular intervals that get shorter, and last about 60 to 90 seconds each. They continue regardless of what you do. If you’re unsure, timing your contractions while resting and hydrating is the simplest test. If they go away, it’s not labor yet.
Prenatal Visits and Screening
Your appointment schedule picks up once the third trimester starts. You’ll have prenatal visits every two weeks from week 28 until week 36, then switch to weekly visits until delivery. These more frequent check-ins let your provider monitor blood pressure, baby’s position, and growth.
One specific screening happens during weeks 36 or 37: a test for Group B Strep bacteria. About 1 in 4 pregnant women carry this bacteria, which is harmless to you but can cause serious infection in a newborn during delivery. The test is a simple swab, and if you test positive, you’ll receive treatment during labor to protect the baby.
Nutrition and Energy Needs
Your calorie needs are higher during the third trimester than at any other point in pregnancy. The general recommendation is about 300 extra calories per day throughout pregnancy, though the third trimester is when your baby is gaining the most weight and your body is working hardest to support that growth. Those extra calories are roughly equivalent to a large banana with peanut butter or a cup of yogurt with granola. Focus on protein, iron, and calcium-rich foods, since your baby is building bone density and muscle mass rapidly during these final weeks.