The third trimester of pregnancy starts at week 28 and lasts through week 40 (or whenever you deliver). This is the most widely used cutoff among major medical organizations, though some providers place the boundary at week 27. The difference comes down to how institutions round the weeks, but for practical purposes, week 28 is the standard starting point.
How the Trimesters Break Down
Pregnancy is divided into three roughly equal blocks. The first trimester covers weeks 1 through 12, the second trimester spans weeks 13 through 27, and the third trimester runs from week 28 until birth. Each trimester lasts about 13 to 14 weeks. Your due date is calculated at 40 weeks from the first day of your last menstrual period, so the third trimester covers the final three months of pregnancy.
What Your Baby Looks Like at Week 28
At 28 weeks, your baby weighs roughly 2.25 pounds (about 1,000 grams) and measures close to 10 inches from the top of the head to the tailbone. The eyelids can partially open, and the central nervous system has matured enough to regulate body temperature and trigger breathing movements visible on ultrasound.
Several important milestones happen right around this transition. At week 26, the lungs begin producing surfactant, a substance your baby will need to breathe independently after birth. By week 27, your baby can open its eyes, blink, and has grown eyelashes. From weeks 29 through 32, the brain develops most rapidly, growing and maturing at a faster pace than any other point in pregnancy. Somewhere between weeks 32 and 36, most babies shift into a head-down position in preparation for birth.
Symptoms That Ramp Up in the Third Trimester
The third trimester is when pregnancy starts to feel physically heavy. Your baby is gaining weight quickly, and the size and position of the baby inside your uterus can make it genuinely hard to get comfortable. Most people notice aches and pains increasing from this point forward.
Backaches are one of the most common complaints. Pregnancy hormones loosen the connective tissue holding your pelvic bones in place, while the growing uterus stretches your abdominal muscles. That combination puts extra strain on your lower back. Sleep also gets harder. Between frequent bathroom trips and difficulty finding a position that works around your belly, quality rest becomes elusive. An eye mask and your own pillows can help if you end up on a hospital ward later.
You’ll likely start feeling Braxton Hicks contractions, which are mild, irregular tightenings in your abdomen. They tend to show up in the afternoon or evening, after physical activity, or after sex. Unlike true labor contractions, Braxton Hicks don’t get progressively stronger or more painful. They feel more like menstrual cramps, and changing positions or walking usually makes them ease up. They become more frequent as your due date approaches.
Prenatal Visits Pick Up
Your appointment schedule changes once you hit the third trimester. From week 28 through week 36, you’ll see your doctor or midwife every two weeks instead of monthly. After week 36, visits increase to once a week until delivery. These more frequent check-ins let your provider monitor your blood pressure, track your baby’s growth and position, and catch any late-pregnancy complications early.
Nutrition and Calorie Needs
Your calorie needs go up slightly in the third trimester compared to the second. Most normal-weight pregnant women need about 2,400 calories per day in the third trimester, compared to roughly 2,200 in the second. That’s an increase of about 200 calories, or the equivalent of a small snack. The general guideline is roughly 300 extra calories per day above your pre-pregnancy baseline throughout the second and third trimesters, with the third trimester sitting at the higher end of that range.
Warning Signs to Watch For
Certain symptoms during the third trimester require immediate medical attention. The CDC identifies several urgent warning signs: a headache that won’t go away or feels like the worst you’ve ever had, vision changes like seeing flashes of light or blind spots, extreme swelling in your hands or face (enough to make it hard to bend your fingers or wear rings), and sudden or severe belly pain. A noticeable decrease in your baby’s movement is also a reason to call your provider right away.
Other red flags include vaginal bleeding heavier than spotting, fluid leaking from the vagina, shortness of breath that comes on suddenly, chest pain or a fast and irregular heartbeat, a fever of 100.4°F or higher, and swelling, redness, or warmth in one leg. These can signal conditions like preeclampsia or blood clots that need prompt treatment. If something feels wrong, it’s worth getting checked.
Getting Ready for Delivery
The third trimester is when most people start preparing in practical terms. Packing a hospital bag around weeks 34 to 36 gives you a buffer in case baby arrives early. Essentials include loose, comfortable clothes for labor, a front-opening nightgown if you plan to breastfeed, nursing bras, maternity pads, slippers or flip-flops for the shower, your toiletry bag, and a change of clothes for going home. Bringing your own pillows and a lip balm are small touches that make a real difference in comfort. Don’t forget your birth plan and hospital notes.