The third trimester of pregnancy lasts about 12 to 13 weeks, starting at week 28 and ending when you give birth, which is typically around week 39 or 40. That works out to roughly three months, though the exact length varies because not every pregnancy ends on the same schedule.
When the Third Trimester Starts and Ends
Most major health organizations define the third trimester as weeks 28 through 40. It begins at the start of your seventh month of pregnancy and covers the final stretch before delivery. While 40 weeks is the standard benchmark, the average birth happens somewhere in weeks 39 or 40, so your third trimester may be slightly shorter or longer than the textbook 12 weeks.
Not all weeks within this window are equal in terms of how ready your baby is. Medical guidelines break the end of pregnancy into more specific categories: 37 weeks through 38 weeks and 6 days is considered “early term,” 39 weeks through 40 weeks and 6 days is “full term,” 41 weeks through 41 weeks and 6 days is “late term,” and 42 weeks or beyond is “post-term.” A baby born at 37 weeks is technically within the third trimester, but those final two weeks between 37 and 39 matter for lung and brain development.
What Happens to Your Baby During These Weeks
At the start of the third trimester (week 28), your baby’s eyelids can partially open. Over the next 12 weeks, the baby gains most of its birth weight, builds up fat stores, and finishes developing the lungs and brain. The rapid weight gain during this period is the reason you’ll feel increasingly heavy and notice more dramatic changes in your body than in earlier months.
How Your Body Changes
The third trimester brings the most noticeable physical discomfort of pregnancy. You may experience pelvic pressure, swelling in your feet and ankles, and difficulty sleeping as your belly grows. Braxton Hicks contractions, which are mild, irregular tightening sensations in your uterus, often show up during the last few months. These are not labor contractions. They tend to be weak or start strong and then fade, and they don’t fall into a regular pattern.
Weight gain during the second and third trimesters follows a fairly steady pace. If you started pregnancy at a healthy weight, the general guideline is about 1 pound per week. If you started at a higher weight, the recommendation drops to about half a pound per week. These are averages, and individual weeks will fluctuate.
Prenatal Visits Pick Up
Your appointment schedule gets busier once the third trimester begins. From week 28 to week 36, you’ll typically see your provider every two weeks. After week 36, visits shift to once a week until delivery. At each visit, your provider will check your weight, measure your abdomen to track the baby’s growth, monitor your blood pressure, and test your urine. A few weeks before your due date, you’ll also be screened for group B strep, a common bacteria that can affect the baby during delivery if untreated.
Tracking Your Baby’s Movement
Sometime in the third trimester, your provider may ask you to start counting kicks. The goal is to feel at least 10 movements within one hour. You pick a time when your baby is usually active, sit or lie down, and count each kick, roll, or jab. If you reach 10 within an hour, you’re done. If not, extend the window to two hours. A noticeable drop in movement is worth reporting to your provider, since it can signal that the baby needs evaluation.
Signs That Labor Is Approaching
In the final weeks of the third trimester, your body starts preparing for delivery in ways you can feel. “Lightening” happens when the baby’s head drops deeper into your pelvis, sometimes changing the shape of your belly and making it easier to breathe but harder to walk. This shift can happen a few weeks before labor or just hours before.
Your cervix also begins to change. It starts out about 3.5 to 4 centimeters long and gradually softens, shortens, and thins in a process called effacement. It needs to be completely thinned out before vaginal delivery. Your provider may check for these changes at your late-pregnancy appointments.
The clearest signs that labor has started are regular contractions and your water breaking. True labor contractions follow a pattern, getting closer together over time (generally 2 to 5 minutes apart) and growing stronger. Braxton Hicks contractions, by contrast, stay irregular and either stay mild or start intense and then weaken. When your water breaks, you might feel a slow trickle of fluid or a sudden gush.