The third trimester of pregnancy starts at week 28 and lasts until week 40, or whenever you give birth. The American College of Obstetricians and Gynecologists (ACOG) defines it precisely as 28 weeks and 0 days through 40 weeks and 6 days.
Why Some Sources Say Week 27
If you divide 40 weeks evenly into three trimesters, you get roughly 13.3 weeks each. That math doesn’t land on a clean number, which is why you’ll occasionally see sources list the third trimester as starting at week 27. However, most major health organizations, including ACOG and the Cleveland Clinic, place the cutoff at week 28. The standard breakdown looks like this:
- First trimester: conception through 12 weeks
- Second trimester: 13 to 27 weeks
- Third trimester: 28 to 40 weeks
One detail worth noting: pregnancy is counted from the first day of your last menstrual period, not the date of conception. Conception typically happens about two weeks later, so your baby has actually been developing for around 26 weeks when you hit the 28-week mark.
What’s Happening With Your Baby at Week 28
At 28 weeks, your baby’s eyelids can partially open. The third trimester is the period when fetal weight increases significantly and organs mature in preparation for life outside the womb. Lung development is a major focus during these final months, and the brain, liver, and immune system all continue to refine their function.
Babies born at or just after 28 weeks are considered very preterm. Survival rates at this stage are considerably higher than earlier in pregnancy, and the risk of serious complications drops consistently with each additional week of gestation from 27 through 31 weeks. Still, every extra week in the womb gives organs more time to mature, which is why the full third trimester matters so much for long-term health outcomes.
Physical Changes You Can Expect
The third trimester brings a noticeable shift in how your body feels. Some of these symptoms start right around week 28, while others build gradually as your due date approaches.
Back pain is one of the most common complaints. Pregnancy hormones loosen the connective tissue in your pelvis, and the growing uterus stretches your abdominal muscles, leaving your back to pick up the slack. Shortness of breath also becomes more frequent as the baby takes up space beneath your rib cage, reducing your lung capacity. Lying on your back can make this worse, so sleeping on your side often helps.
Braxton Hicks contractions, those mild tightening sensations in your abdomen, tend to show up more in the afternoon or evening, especially after physical activity. They become more frequent and stronger as you get closer to your due date, but they’re irregular and not a sign of labor on their own.
Heartburn and constipation are common because pregnancy hormones slow digestion while the uterus presses on your intestines. Frequent urination picks up again as the baby settles deeper into your pelvis and puts pressure on your bladder. You may also notice urine leaking when you laugh, cough, or sneeze. Varicose veins, hemorrhoids, and small spider veins on the face, neck, or arms can appear due to increased blood volume. Heart palpitations, those fluttering or pounding sensations in your chest, sometimes happen because the larger uterus slows blood returning to the heart. These are almost always harmless and tend to ease later in pregnancy.
How Prenatal Visits Change
Once you enter the third trimester, your prenatal visit schedule picks up. You’ll see your provider every two weeks from week 28 until week 36, then switch to weekly visits from week 36 until delivery. These more frequent check-ins allow your provider to monitor your blood pressure, track the baby’s position and growth, and watch for complications like preeclampsia or preterm labor.
One key screening happens between weeks 36 and 38: a Group B Strep test, which involves a simple swab of the vagina and rectum. About 1 in 4 pregnant women carry this bacteria without symptoms, and knowing your status before labor helps your care team protect the baby during delivery.
Counting Your Weeks Accurately
If you’re unsure exactly how far along you are, the most reliable reference point is the estimated due date your provider set at your first ultrasound or based on your last menstrual period. From there, you can count forward: week 28 falls exactly 12 weeks before your due date. Most pregnancy apps track this automatically, but if the numbers ever seem off between your app and your provider’s records, go with your provider’s dating. Early ultrasound measurements are accurate to within about five days, while apps rely on the dates you entered and can drift if your cycle was irregular.