The third trimester starts at week 28 of pregnancy. According to the American College of Obstetricians and Gynecologists (ACOG), the second trimester runs through 27 weeks and 6 days, and the third trimester officially begins at 28 weeks and 0 days, lasting until delivery at around 40 weeks.
If you’ve seen some sources say week 27, that’s because a 40-week pregnancy doesn’t divide evenly into three equal parts. ACOG’s standard breakdown splits it this way: first trimester through week 13, second trimester from week 14 through week 27, and third trimester from week 28 through week 40. That puts roughly 13 to 14 weeks in each trimester.
Why Pregnancy Weeks Can Feel Confusing
Pregnancy is dated from the first day of your last menstrual period (LMP), not from the day you actually conceived. That means the first two weeks of your “pregnancy” are counted before fertilization even happens. So when you reach week 28, your baby has really been developing for about 26 weeks. This dating system is universal in obstetric care, but it does mean your week count will always run about two weeks ahead of actual fetal age.
What Your Baby Looks Like at Week 28
By the start of the third trimester, your baby has hit some important milestones. The lungs began producing surfactant around week 26, a substance that will eventually allow independent breathing after birth. By week 27, your baby can open and blink their eyes. At week 28, the eyelids can partially open.
The third trimester is primarily about weight gain and organ maturation. Your baby’s organs are largely formed by this point, but they need these final 12 or so weeks to grow, add body fat, and get the lungs and brain ready for life outside the womb.
Physical Changes You’ll Notice
The third trimester brings a noticeable shift in how pregnancy feels day to day. Many of the symptoms you experience are driven by the baby’s increasing size and the pressure that puts on your organs.
Braxton Hicks contractions become more common. These are mild, irregular tightenings across your belly that tend to show up in the afternoon or evening, after physical activity, or after sex. They get more frequent as your due date approaches but shouldn’t follow a regular pattern or increase in intensity the way labor contractions do.
Back pain is one of the most common third-trimester 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 happens as your baby presses upward against your rib cage. It can feel worse when you lie on your back. Heartburn and constipation also tend to ramp up because hormones slow digestion and the uterus crowds the intestines. Frequent urination returns (or intensifies) as the baby settles deeper into your pelvis and puts pressure on your bladder. Leaking urine when you laugh, cough, or sneeze is common during these final months.
You may also notice spider veins, varicose veins on your legs, or hemorrhoids. These are all related to the increased blood volume and pressure in your circulatory system. Heart palpitations, a fluttering or pounding feeling in your chest, can also occur because blood returns to the heart more slowly as the uterus grows. These are typically harmless.
Prenatal Visits and Screenings
Your appointment schedule picks up once the third trimester begins. Expect visits every two to four weeks through about week 36, then every one to two weeks until delivery. Your provider will monitor your blood pressure, measure your belly, and check the baby’s heart rate at each visit.
Two key things happen during the third trimester. You’ll be tested for Group B streptococcus (GBS), a type of bacteria that can be passed to the baby during delivery. If you test positive, you’ll receive antibiotics during labor to protect your baby. The CDC also recommends getting the Tdap vaccine between weeks 27 and 36, ideally on the earlier end of that window. This protects your newborn against whooping cough by passing antibodies through the placenta before birth. The recommendation applies to every pregnancy, even if your pregnancies are close together.
Warning Signs to Take Seriously
Most third-trimester discomfort is normal, but certain symptoms need immediate medical attention. Seek care right away if you experience a severe headache that won’t go away or gets worse over time, vision changes like flashing lights or blurry vision, or extreme swelling in your hands or face that makes it hard to bend your fingers or open your eyes. These can be signs of preeclampsia, a serious blood pressure condition.
Other red flags include sharp or persistent belly pain, vaginal bleeding beyond light spotting, fluid leaking from your vagina, a fever of 100.4°F or higher, and severe swelling or redness in one leg or arm. Pay attention to your baby’s movement patterns, too. If you notice the baby is moving less than usual or has stopped moving, that warrants a call to your provider right away. By the third trimester, you should be feeling regular movement, and a noticeable change is worth checking out promptly.