The third trimester starts at week 28 of pregnancy. The American College of Obstetricians and Gynecologists defines it precisely as 28 weeks and 0 days through 40 weeks and 6 days, making it the final stretch of roughly 13 weeks before your due date.
How Trimesters Break Down by Week
ACOG divides pregnancy into three distinct periods based on fetal development:
- First trimester: First day of your last menstrual period through 13 weeks and 6 days. This is when fertilization and major organ formation happen.
- Second trimester: 14 weeks and 0 days through 27 weeks and 6 days. A period of rapid growth.
- Third trimester: 28 weeks and 0 days through 40 weeks and 6 days. The fetus gains weight and organs mature in preparation for life outside the womb.
You might see slightly different cutoffs on different websites or apps, with some placing the start at week 27. The variation comes from rounding, but ACOG’s week-and-day breakdown is the standard used in clinical care.
Where Your Baby Is at Week 28
At the start of the third trimester, your baby is about 10 inches long (measured from head to tailbone) and weighs roughly 2ΒΌ pounds. By the end of the seventh month, that grows to 14 to 15 inches and 2 to 3 pounds. From here, weight gain accelerates significantly as your baby builds fat stores and the organs finish maturing.
Several important milestones cluster right around this transition. At 26 weeks, the lungs begin producing surfactant, a substance essential for breathing air after birth. By week 27, your baby can open and blink their eyes. At 28 weeks, the eyelids can partially open, and the central nervous system has developed enough to regulate body temperature and trigger rhythmic breathing movements visible on ultrasound. These breathing motions aren’t moving air yet, but they’re practice for the real thing.
What Changes in Your Body
The third trimester is when most people start feeling noticeably more uncomfortable. Your uterus is large enough to shift your center of gravity, stretch your abdominal muscles, and put pressure on your bladder, lungs, and lower back. Several symptoms tend to appear or intensify around week 28.
Back pain is one of the most common complaints. Pregnancy hormones loosen the connective tissue in your pelvis to prepare for delivery, and the growing uterus stretches abdominal muscles that normally help support your spine. The combination makes lower back pain almost universal in the final trimester.
Braxton Hicks contractions often become noticeable in the third trimester. These feel like mild tightness or cramping in your abdomen, and they’re irregular. They tend to show up more in the afternoon or evening, after physical activity, or after sex. Walking, changing positions, or drinking water usually eases them. They’re not a sign of labor unless you’re having more than six in an hour and they’re getting steadily stronger.
Sleep gets harder. Between frequent bathroom trips, difficulty finding a comfortable position around your belly, and general restlessness, insomnia is a hallmark of the final months.
Weight Gain in the Third Trimester
For someone who started pregnancy at a normal weight, the general target is about 1 pound per week through the second and third trimesters. That works out to roughly an extra 300 calories a day above your pre-pregnancy intake. If you started pregnancy overweight or obese, the recommended gain drops to about half a pound per week during the same period. Steady, gradual gain matters more than hitting an exact number each week.
Tests and Appointments to Expect
Prenatal visits become more frequent once you enter the third trimester. Most providers shift from monthly to every-two-week appointments around week 28, then weekly visits starting at week 36.
Glucose screening for gestational diabetes typically happens between 24 and 28 weeks, so you may have already completed it or will do so right as the third trimester begins. Group B strep screening comes later in the trimester, and your provider will let you know when that’s scheduled.
Term Pregnancy Categories
Not all deliveries within the third trimester carry the same outlook. Medical guidelines break down the final weeks into specific categories that reflect how ready a baby is for life outside the womb:
- Early term: 37 weeks through 38 weeks and 6 days
- Full term: 39 weeks through 40 weeks and 6 days
- Late term: 41 weeks through 41 weeks and 6 days
- Post-term: 42 weeks and beyond
A baby born before 37 weeks is considered preterm. The distinction between early term and full term matters because organs, particularly the brain and lungs, continue developing right up to 39 weeks. Even a couple of extra weeks can make a meaningful difference in outcomes.