The third trimester starts at week 28 and runs through week 40, giving you roughly 12 to 13 weeks in the final stretch of pregnancy. Both the American College of Obstetricians and Gynecologists (ACOG) and the Cleveland Clinic define it as beginning at exactly 28 weeks and 0 days. Of course, not every pregnancy reaches 40 weeks, so the actual length of your third trimester depends on when your baby arrives.
Why Some Sources Say Different Weeks
You may have noticed that some pregnancy apps or websites place the start of the third trimester at week 27. This comes from dividing 40 weeks into three equal chunks of about 13.3 weeks each, which puts the cutoff slightly earlier. In clinical practice, though, the standard definition is week 28. ACOG specifically defines the third trimester as 28 weeks and 0 days through 40 weeks and 6 days. If your provider or app uses week 27, the difference is just one week of rounding, not a medical disagreement.
How Your Baby Grows During These Weeks
At the start of the third trimester (week 28), your baby measures about 10 inches from head to tailbone and weighs roughly 2¼ pounds. By week 32, that jumps to about 11 inches and 3¾ pounds. At 40 weeks, the average baby is around 14 inches crown to rump and 7½ pounds.
Most of that weight gain happens because the baby is building fat stores and maturing organs. The brain grows especially fast during these final weeks, increasing roughly fourfold in size. Critical brain structures and neural connections form during this period, which is one reason full-term delivery (reaching at least 39 weeks) matters so much. The lungs also finish developing during the third trimester, producing a substance that keeps the air sacs from collapsing after birth.
What Your Body Goes Through
The third trimester brings a predictable set of physical changes, most of them driven by your baby’s increasing size and your body’s preparation for labor.
Braxton Hicks contractions are practice tightenings in your abdomen that tend to show up in the afternoon or evening, after physical activity, or after sex. They come and go irregularly and get more frequent as your due date approaches. They’re uncomfortable but not usually painful. If you’re having more than six in an hour and they’re getting steadily stronger, contact your provider.
Pelvic pressure and bladder changes happen as the baby drops lower into your pelvis. You’ll likely urinate more often and may leak urine when you laugh, cough, or sneeze. This is normal and temporary.
Swelling in your feet, ankles, and hands is common. Propping your legs up, staying active, and wearing compression socks can help. Avoid standing for long stretches or sitting with your legs crossed.
Prenatal Visits Pick Up
Your appointment schedule gets busier once the third trimester starts. From week 28 onward, you can expect visits every two to four weeks. Starting at 36 weeks, that increases to every one to two weeks until delivery. These visits typically include blood pressure checks, urine tests, belly measurements, and monitoring fetal position.
One specific screening happens during weeks 36 or 37: the Group B Strep test. This is a simple swab that checks for a type of bacteria that’s harmless to you but could be passed to the baby during delivery. About 1 in 4 pregnant women carry it. If you test positive, you’ll receive treatment during labor to protect the baby.
Braxton Hicks vs. Real Labor
One of the biggest third-trimester questions is whether contractions are the real thing. Braxton Hicks contractions tend to stay in one area of the abdomen, come at irregular intervals, and taper off on their own. They don’t get progressively stronger or closer together.
True labor contractions start at the top of the uterus and move downward in a coordinated wave. They’re painful, they get stronger over time, and they don’t stop. A useful guideline is the 5-1-1 rule: contractions every 5 minutes, each lasting at least 1 minute, for at least 1 hour. If your contractions fit that pattern, it’s time to head to the hospital.
Warning Signs to Take Seriously
Most third-trimester discomfort is normal, but certain symptoms need immediate attention. The CDC lists these as urgent maternal warning signs: a severe headache that won’t go away or comes with blurred vision, sudden vision changes like flashing lights or blind spots, extreme swelling of the hands or face (enough to make it hard to bend your fingers or open your eyes), chest pain or a racing heartbeat, sharp or stabbing belly pain that doesn’t resolve, and any vaginal bleeding beyond light spotting or fluid leaking.
A noticeable decrease in your baby’s movement also warrants a call to your provider. By the third trimester you’ll be familiar with your baby’s patterns, and a significant drop-off from their normal activity level is worth checking out promptly.
Practical Prep Before Week 36
Since babies sometimes arrive early, it’s smart to have your hospital bag packed by around 35 to 36 weeks. That gives you a buffer before the weekly appointment schedule kicks in and things start moving quickly. If you can count the remaining weeks on one hand, you’ve waited long enough. Beyond the bag, this is also the window to finalize your birth plan, install the car seat, and sort out any leave or childcare logistics while you still have the energy and time to do it.