The third trimester begins at week 28 of pregnancy (28 weeks and 0 days) and lasts through week 40. According to the American College of Obstetricians and Gynecologists, this period runs from 28 weeks and 0 days to 40 weeks and 6 days. It’s the final stretch, when your baby gains most of its weight and the organs mature in preparation for life outside the womb.
How Pregnancy Trimesters Break Down
The first trimester covers weeks 1 through 12, when the brain, spine, and major organs begin forming. The second trimester spans weeks 13 through 27, a period of rapid growth when movement becomes noticeable and fat starts forming under the skin. The third trimester picks up at week 28 and continues until delivery, which typically happens around week 40.
Some sources round slightly differently, placing the start of the third trimester at week 27 or week 29. The standard medical definition used by ACOG is 28 weeks and 0 days. If your provider mentions a different cutoff, it’s likely just a rounding difference rather than a disagreement.
How Your Baby Grows in the Third Trimester
At the start of the third trimester (week 28), a baby measures about 10 inches from the top of the head to the tailbone and weighs roughly 2¼ pounds. By week 32, that increases to about 11 inches and 3¾ pounds. By week 40, the average baby is around 14 inches crown to rump and weighs about 7½ pounds.
Most of that weight gain serves a purpose beyond size. Your baby is building fat stores that will help regulate body temperature after birth. The lungs are maturing so they can handle breathing air. The brain is developing rapidly, forming the connections needed for reflexes, sensory processing, and basic body functions. ACOG describes this trimester simply as the time when “the fetus’s weight increases and the organs mature so they will be ready to function after birth.”
What Your Body Feels Like
The third trimester brings a distinct set of physical changes, many of them driven by the baby’s increasing size and shifting position.
Braxton Hicks contractions are mild, irregular tightenings of the uterus that many people feel for the first time in this trimester. They tend to show up in the afternoon or evening, after physical activity, or after sex. They get more frequent as your due date approaches but don’t follow a regular pattern and don’t get progressively stronger. Changing positions or walking usually relieves them. If rest and hydration make contractions stop, they’re not real labor.
Swelling in the ankles, feet, and hands is common due to increased fluid retention and weight gain. Elevating your legs helps. Sudden or severe swelling, especially in the face, is different and worth flagging to your provider promptly.
Pelvic pressure increases as the baby drops lower in preparation for birth. This puts direct pressure on your bladder, which means more frequent bathroom trips and possible leaking when you sneeze, cough, or laugh. On the positive side, many people find breathing gets easier once the baby moves down and takes some pressure off the diaphragm.
Sleep Gets Harder
Finding a comfortable sleeping position becomes a real challenge in the third trimester. Side sleeping is the recommended position, with the left side considered ideal because it allows the best blood flow to the baby and improves kidney function. Lying flat on your back puts pressure on the vena cava, the large vein that returns blood from the lower body to the heart. That pressure can reduce blood flow and cause discomfort in your back and intestines. A pillow between your knees or under your belly can make side sleeping more comfortable.
Tests and Screenings to Expect
Two key screenings happen in or near the third trimester. Glucose screening, which checks for gestational diabetes, is typically done between weeks 24 and 28. You’ll drink a sugary solution and have blood drawn an hour later. If your blood sugar comes back high, a follow-up test confirms whether gestational diabetes is present.
Group B strep (GBS) screening happens between weeks 36 and 38. A swab is taken from the vagina and rectum and sent to a lab. About 1 in 4 pregnant people carry this bacteria without symptoms. If you test positive, you’ll receive antibiotics during labor to prevent the bacteria from passing to the baby during delivery.
Prenatal visits also become more frequent in the third trimester, shifting from monthly to every two weeks and then weekly as you approach your due date. These visits typically include checking the baby’s position, monitoring your blood pressure, and tracking the baby’s heart rate.
Braxton Hicks vs. True Labor
Knowing the difference between practice contractions and real labor matters most in the final weeks. Braxton Hicks contractions are irregular, stay mild to moderate in intensity, and tend to be felt mainly in the front of the abdomen. They can be surprisingly painful at times, which is why they send many people to the hospital before actual labor starts.
True labor contractions come at regular intervals and follow a predictable pattern. They get stronger, longer, and closer together over time. They don’t go away when you rest, change positions, or drink water. Sometimes the only way to confirm the difference is a vaginal exam to check for cervical changes, so there’s no reason to feel embarrassed about calling your provider if you’re unsure.