The third trimester officially begins at week 28 of pregnancy, or more precisely at 28 weeks and 0 days. This is the standard used by the American College of Obstetricians and Gynecologists (ACOG) and most major health organizations worldwide. It marks the final stretch of pregnancy, lasting from week 28 through week 40.
Why Some Sources Say Week 27
If you’ve seen different answers online, you’re not imagining it. The confusion comes from how you divide 40 weeks into three equal parts. Mathematically, 40 divided by 3 is about 13.3 weeks per trimester, which doesn’t split neatly. Some sources round down and place the start at week 27, while others begin at week 28. The widely accepted clinical standard is week 28, which puts the trimesters at weeks 1 through 12, weeks 13 through 27, and weeks 28 through 40.
There’s also the question of how weeks map to months. At 28 weeks, you’re roughly seven months pregnant, though pregnancy months don’t line up perfectly with calendar months. That’s because pregnancy is counted from the first day of your last menstrual period, not from conception, which typically happens about two weeks later. So the 40-week clock includes roughly two weeks before the baby was even conceived.
What Your Baby Looks Like at Week 28
At the start of the third trimester, your baby weighs about 2.25 pounds (1,000 grams) and measures nearly 10 inches from the top of the head to the rump. The next 12 weeks are all about gaining weight and maturing organs. Your baby will roughly triple in weight, or more, before birth.
Several important developmental milestones happen around this transition. The lungs begin producing surfactant, a substance that keeps the air sacs open and allows breathing after birth. This process actually starts around week 26 and continues maturing well into the third trimester. Between weeks 29 and 32, the brain develops most rapidly, and your baby can see, hear, and respond to stimuli from outside the womb. By week 30, the baby can regulate its own body temperature. You’ll also start noticing more distinct sleep and wake cycles around week 31.
Survival if Baby Arrives Early
One reason week 28 is a significant milestone: survival rates for babies born at this point are high. A Duke University study found that 94% of infants born at 28 weeks survived to hospital discharge. These babies still face real risks, though. When assessed at two years of age, nearly half had no or only mild developmental issues, but about 29% had moderate impairment and 21% had severe impairment. About 8% had moderate to severe cerebral palsy, and 15% needed mobility aids like braces or walkers.
Reaching the third trimester doesn’t mean the baby is ready for the outside world, but it does mean the odds improve dramatically compared to earlier weeks.
Physical Changes You’ll Notice
The third trimester brings a noticeable shift in how your body feels. Many of these changes come from the baby’s increasing size and the pressure it puts on your organs.
Backaches are one of the most common complaints. Pregnancy hormones loosen the connective tissue in your pelvis, and the growing uterus stretches your abdominal muscles, pulling your center of gravity forward. Shortness of breath often develops as the baby pushes up against your rib cage, and it tends to be worse when you lie on your back.
You’ll likely start feeling Braxton Hicks contractions: a gentle tightening across your belly that comes and goes. These are more common in the afternoon or evening, especially after physical activity. They become more frequent as your due date approaches but are different from true labor contractions, which get progressively stronger and closer together.
Heartburn and constipation often worsen because pregnancy hormones slow digestion while the uterus presses against your intestines. Frequent urination picks up again as the baby drops deeper into your pelvis and pushes on your bladder. Leaking urine when you cough, sneeze, or laugh is common and nothing to be alarmed about. Some people also notice spider veins, varicose veins in the legs, or hemorrhoids as blood volume increases.
Heart palpitations, that fluttering or skipped-beat feeling, can happen because the enlarged uterus slows blood flow back to the heart. These are usually harmless.
Medical Care in the Third Trimester
Your prenatal visit schedule typically increases during the third trimester, shifting from monthly to every two weeks and then weekly as you approach your due date. Two key medical events are commonly timed around the start of this trimester.
Gestational diabetes screening usually happens between weeks 24 and 28, so you may have just completed it or will be wrapping it up as the third trimester begins. The Tdap vaccine, which protects your newborn against whooping cough, is recommended between weeks 27 and 36, with the earlier end of that window being ideal. Getting the vaccine early in this range gives your body more time to produce antibodies and pass them to the baby before birth.