The third trimester lasts about three months, covering months seven through nine of pregnancy. It begins at week 28 and ends when your baby is born, typically around week 40. Of the three trimesters, it’s the shortest in terms of weeks (roughly 12 to 13), though it often feels like the longest as your body prepares for delivery.
How Weeks Map to Months
Pregnancy math can be confusing because months and weeks don’t divide evenly. Here’s how the third trimester breaks down:
- Weeks 28 to 31: Month seven
- Weeks 32 to 35: Month eight
- Weeks 36 to 40: Month nine
Your due date falls at 40 weeks, but babies born anywhere from 39 weeks through 40 weeks and 6 days are considered full term. The National Institutes of Health defines 37 to 38 weeks and 6 days as “early term,” 41 weeks through 41 weeks and 6 days as “late term,” and 42 weeks or beyond as “post-term.” These distinctions matter because even a week or two of extra time in the womb can make a measurable difference in a baby’s readiness at birth.
What’s Happening With Your Baby
The third trimester is when your baby finishes building the systems needed to survive outside the womb. At week 28, the central nervous system has developed enough to regulate body temperature, and your baby begins practicing breathing movements visible on ultrasound. The eyelids can partially open around this time as well.
By week 33, your baby’s pupils can change size in response to light, and the bones are hardening, though the skull stays soft and flexible to pass through the birth canal. The final weeks are largely about gaining weight and putting finishing touches on lung development. At 39 weeks, the chest broadens and the baby is considered full term.
Physical Changes You’ll Notice
The third trimester brings a wave of new symptoms, most of them driven by the simple fact that your baby is getting much bigger and your body is working harder to support the extra weight.
Back pain is one of the most common complaints. Pregnancy hormones loosen the connective tissue in your pelvic area, and the growing uterus stretches your abdominal muscles. Those two changes together put extra strain on your lower back. Shortness of breath also picks up as the baby presses against your rib cage. Your lungs actually expand their total capacity during pregnancy, so you may breathe faster and feel winded more easily.
Frequent urination gets worse as the baby drops deeper into your pelvis, putting pressure on your bladder. Leaking urine when you laugh, cough, or sneeze is normal during this stage. Heartburn and constipation are also common because pregnancy hormones slow digestion while the uterus presses on your intestines. Some women develop varicose veins on their legs or hemorrhoids due to increased blood flow and pressure.
Heart palpitations, those fluttering or skipped-beat feelings, can happen because the larger uterus slows blood returning to the heart. They typically become less frequent as pregnancy progresses.
Braxton Hicks vs. Real Contractions
You’ll likely start feeling Braxton Hicks contractions during the third trimester. These are slight tightening sensations in your abdomen that come and go without a regular pattern. They tend to show up in the afternoon or evening, after physical activity, or after sex, and they get more frequent as your due date approaches.
Real labor contractions are different. They come at regular intervals, and between contractions your uterus relaxes completely. Several signs suggest labor is starting: the baby drops lower in your pelvis (called lightening), you lose the mucus plug that sealed your cervix, your water breaks, and contractions become steady and progressively stronger. Lightening can happen anywhere from a few weeks to a few hours before active labor. If you’re having more than six contractions per hour and they’re getting stronger, contact your care provider.
Prenatal Visit Schedule
Your appointment schedule picks up significantly during the third trimester. From week 28 through week 36, you’ll typically see your doctor or midwife every two weeks. After week 36, visits shift to once a week until delivery. These more frequent checkups let your provider monitor your blood pressure, track the baby’s position, and catch any late-pregnancy complications early.
Nutrition and Weight Gain
Your calorie needs increase in the third trimester. The CDC recommends adding about 450 extra calories per day during these final months, up from roughly 340 extra in the second trimester. That’s the equivalent of a substantial snack or small meal, not eating for two in any dramatic sense.
Total recommended weight gain over the entire pregnancy depends on your pre-pregnancy BMI. For women carrying one baby, the general targets are 25 to 35 pounds for a normal-weight starting point, 15 to 25 pounds if you started overweight, and 11 to 20 pounds if you started obese. A good portion of that total gain happens in the third trimester as the baby puts on the most weight.