The third trimester starts at week 28 of pregnancy (28 weeks and 0 days) and runs through week 40. That said, you may see some sources round up slightly, listing it as starting at week 29. The American College of Obstetricians and Gynecologists uses the 28-week mark, which is the most widely referenced standard. This final stretch covers roughly 13 weeks of rapid fetal growth and significant physical changes for you.
Why Sources Sometimes Disagree
If you’ve seen conflicting answers, you’re not imagining it. ACOG defines the third trimester as 28 weeks 0 days through 40 weeks 6 days. The U.S. Department of Health and Human Services lists it as week 29 through week 40. The difference is small, just a matter of how each organization rounds the cutoff, but it can be confusing when you’re tracking milestones. For practical purposes, 28 weeks is the most commonly cited start point in clinical settings.
How Your Baby Grows From Week 28 to 40
At the start of the third trimester, your baby is roughly 10 inches long (measured from the top of the head to the tailbone) and weighs about 2.25 pounds. By week 32, that jumps to around 11 inches and nearly 3.75 pounds. By the time you reach your due date at 40 weeks, expect a baby around 14 inches crown-to-rump and about 7.5 pounds.
Weight gain is only part of the picture. The organs are maturing so they can function outside the womb. Lung cells begin producing surfactant, a coating that prevents the tiny air sacs from collapsing after birth. This process starts around week 26 and continues refining throughout the third trimester. The brain develops most rapidly during weeks 29 through 32 and keeps growing through month nine. At week 35, the brain still weighs only about two-thirds of what it will at birth, which gives you a sense of how much development is packed into those final weeks. By month nine, growth is mostly about adding body fat and putting finishing touches on brain maturity.
Your baby also gains the ability to regulate its own body temperature around week 30. These overlapping milestones explain why every additional week in the womb matters, particularly for babies born early in the third trimester.
Physical Changes You’ll Notice
The third trimester brings a cluster of new symptoms, most of them driven by your baby’s increasing size pressing on surrounding organs.
Shortness of breath is common as your growing uterus pushes up against your rib cage. Your lungs can still expand to their full capacity, but the pressure means you breathe a bit faster and get winded more easily. Lying on your back tends to make it worse. Resting on your side often helps.
Swelling in the feet, ankles, and hands picks up as your body retains more fluid. Elevating your legs when you can and staying moderately active both help reduce it. Braxton Hicks contractions also become more frequent. These feel like a tightening across your belly, often appearing in the afternoon or evening, after physical activity, or after sex. They tend to get stronger and more noticeable as your due date approaches, but they’re not labor contractions.
Braxton Hicks vs. True Labor
Knowing the difference between practice contractions and real labor saves a lot of unnecessary trips to the hospital. True labor contractions come at regular intervals, last about 60 to 90 seconds each, and get progressively closer together and stronger over time. The pain typically starts in the back and moves to the front. They continue no matter what you’re doing.
Braxton Hicks contractions are irregular, don’t follow a pattern, and don’t get closer together. They tend to stay weak or start strong and then fade. A simple test: rest and drink water. If the contractions stop, they aren’t the real thing. True labor contractions keep coming regardless of rest or movement.
Sleep Position After 28 Weeks
Once you enter the third trimester, sleeping on your side is the safest position for your baby. Research has linked falling asleep on your back after 28 weeks with an increased risk of stillbirth. This applies to nighttime sleep, naps, and any time you fall back asleep after waking in the middle of the night. If you wake up and find yourself on your back, there’s no need to panic. Just roll onto your side and settle back in.
Prenatal Tests in the Third Trimester
Prenatal visits become more frequent during the final stretch, typically shifting from monthly to every two weeks and then weekly as you approach your due date. A glucose screening for gestational diabetes is usually done between weeks 24 and 28, right around the time the third trimester begins (or earlier if you have risk factors). Later in the trimester, you’ll be screened for Group B strep, a common bacterium that can be passed to the baby during delivery. A positive result simply means you’ll receive treatment during labor to protect your newborn.
These visits also track your baby’s position, your blood pressure, and fetal heart rate, all of which become more clinically significant as delivery approaches.