The third trimester begins at week 28 of pregnancy (28 weeks and 0 days) and continues through week 40, according to the American College of Obstetricians and Gynecologists. This final stretch, roughly 13 weeks long, is when your baby gains most of its weight and its organs finish maturing for life outside the womb.
How the Trimesters Break Down
Pregnancy is divided into three roughly equal periods. The first trimester covers weeks 1 through 12, the second runs from week 13 through week 27, and the third spans week 28 through week 40 (or delivery, if that comes later). Some sources round the start of the third trimester to week 27, which causes confusion, but the clinical definition from ACOG places it at 28 weeks and 0 days.
If you’re counting by months, week 28 falls near the start of the seventh month. Because calendar months and four-week pregnancy “months” don’t line up perfectly, the month labels are always a little messy. Weeks are the more reliable way to track where you are.
What’s Happening With Your Baby
At the start of the third trimester, your baby weighs roughly two to two and a half pounds. Over the next 12 to 13 weeks, it will more than triple that weight, adding layers of fat and building the lung tissue and brain connections it needs to survive after birth. Organs that formed earlier, like the kidneys and liver, spend this trimester refining how they function.
Somewhere between 34 and 38 weeks in a first pregnancy, the baby typically “drops” lower into the pelvis, a shift called lightening. You may notice you can breathe a little easier once this happens, though pelvic pressure increases. In subsequent pregnancies, this drop often doesn’t happen until labor actually starts.
Common Physical Changes
The third trimester brings a wave of symptoms driven by the baby’s increasing size and your body’s preparation for delivery. Most are normal, even if they’re uncomfortable.
Back pain is one of the most common complaints. Your center of gravity shifts forward as your belly grows, straining the lower back. Chairs with good back support help, and if you’re standing for a while, resting one foot on a low stool takes pressure off. Abdominal support garments, sold in most maternity stores, can also make a noticeable difference. Walking regularly keeps back muscles conditioned, and low-heeled shoes with good arch support are better than both flats and heels.
Heartburn tends to worsen because the growing uterus pushes the stomach upward. Eating smaller meals more frequently helps, and it’s worth avoiding fried foods, citrus, chocolate, spicy dishes, and carbonated drinks. Try not to lie down right after eating.
Swelling in the feet, ankles, and hands is common, especially late in the day. Propping your legs up when you can, doing light exercise, and wearing compression socks all help. Avoid standing for long stretches or sitting with your legs crossed.
Constipation often gets worse as well. Fiber-rich foods (whole grains, raw vegetables, dried fruit), plenty of fluids, and light walking are the first-line solutions. Don’t take stool softeners or laxatives without checking with your provider.
Your Prenatal Visit Schedule Changes
During the third trimester, appointments become more frequent. You’ll typically have a prenatal visit every two weeks from week 28 until week 36. After week 36, visits shift to once a week until delivery. These more frequent check-ins let your provider monitor blood pressure, track the baby’s growth, and catch complications early.
The CDC recommends getting a Tdap vaccine (which protects against whooping cough) between weeks 27 and 36 of each pregnancy, ideally in the earlier part of that window. Getting it during this period allows your body to produce antibodies that cross the placenta and give the baby some protection in its first weeks of life.
Warning Signs to Recognize
Preeclampsia is one of the more serious conditions that can develop in the third trimester. It involves high blood pressure, defined as a reading of 140/90 or higher, along with signs that organs like the kidneys or liver are under stress. Symptoms to watch for include swelling of the face or hands (beyond normal ankle puffiness), a headache that won’t go away, seeing spots or changes in vision, pain in the upper abdomen or shoulder, sudden weight gain, nausea and vomiting that appear for the first time in late pregnancy, and difficulty breathing. If you notice any of these, contact your provider promptly. Preeclampsia can progress quickly, and early detection makes a significant difference in outcomes.
Braxton Hicks vs. Real Labor
Practice contractions, called Braxton Hicks, can start weeks before your due date and sometimes feel strong enough to make you think labor has begun. They tend to be irregular, meaning the time between them varies, and they often show up more at the end of the day.
True labor contractions follow a pattern. They come at regular intervals and gradually get closer together and more intense over time. A simple test: rest, drink water, and time the contractions. If they fade with rest and hydration, they’re practice contractions. If they keep coming in a steady, tightening rhythm regardless of what you do, labor is likely starting.