The third trimester of pregnancy starts at week 28 and lasts through delivery, which typically happens around week 40. Most major health organizations agree on this cutoff, though you may occasionally see week 27 mentioned. If you’re counting by months, week 28 marks the beginning of month seven.
How the Trimesters Break Down
Pregnancy is divided into three roughly equal stretches. The first trimester covers weeks 1 through 12, the second runs from week 13 through 27, and the third picks up at week 28. That gives you about 12 to 13 weeks in the final trimester, though the exact length depends on when you deliver.
Once you reach the final weeks, your pregnancy falls into more specific categories. Weeks 37 through 38 and 6 days are considered “early term.” A “full term” pregnancy is 39 weeks through 40 weeks and 6 days. Delivering at 41 weeks is “late term,” and anything from 42 weeks onward is “post-term.” These distinctions matter because babies born even a few weeks early can face different health outcomes than those born at full term.
How Your Baby Grows in the Third Trimester
At the start of week 28, the baby is roughly 10 inches long (measured from head to tailbone) and weighs about 2.25 pounds. From here, growth accelerates quickly. By week 30, the baby reaches about 3 pounds. By week 32, that jumps to nearly 3.75 pounds. And by week 34, the baby weighs over 4.5 pounds and measures close to 12 inches from head to tailbone.
The final weeks are all about finishing touches. The lungs mature, the brain develops rapidly, and the baby adds layers of fat that help regulate body temperature after birth. Bones harden, though the skull stays flexible to make delivery possible. By the time you reach full term, most babies weigh somewhere between 6 and 9 pounds.
What Your Body Feels Like
The third trimester brings a distinct set of physical changes, most of them driven by the baby’s size and the hormones preparing your body for labor.
Back pain is one of the most common complaints. Pregnancy hormones loosen the connective tissue in your pelvis, while your growing uterus stretches the abdominal muscles that normally support your spine. The combination puts extra strain on your lower back. Shortness of breath is also typical, caused by upward pressure on your rib cage as the baby takes up more room. You may notice you get winded more easily or feel out of breath when lying on your back.
Frequent urination picks up again as the baby drops deeper into your pelvis and presses on your bladder. Leaking urine when you laugh, sneeze, or cough is common. Heartburn and constipation often worsen because pregnancy hormones slow digestion, and the uterus compresses your intestines. Many people also develop varicose veins in the legs or hemorrhoids from increased blood volume and pressure.
Sleep gets harder. The baby’s size and position make it difficult to find a comfortable resting position, and between bladder pressure, back pain, and heartburn, uninterrupted sleep becomes rare. Heart palpitations, feeling like a flutter or skipped beat, can also show up because blood returns to the heart more slowly as the uterus grows. These are usually harmless but worth mentioning at your next visit if they’re frequent.
Braxton Hicks vs. Real Contractions
Starting in the third trimester, you’ll likely feel Braxton Hicks contractions: a tightening sensation in your abdomen that comes and goes. They tend to happen more in the afternoon or evening, after physical activity, or after sex. As your due date approaches, they get stronger and more frequent, which can make it hard to tell whether labor has actually started.
The key difference is coordination. Real labor contractions start at the top of the uterus and travel downward in a wave. Braxton Hicks tend to stay focused in one area and don’t follow a pattern. The other giveaway is timing. True labor contractions don’t stop, and the gaps between them get shorter. A useful rule of thumb: if contractions are coming every 5 minutes, each one lasts at least 1 minute, and this pattern has been going for 1 hour, that’s likely real labor.
Prenatal Visits and Screenings
Your appointment schedule picks up in the third trimester. From week 28 through 35, you’ll typically see your provider every two weeks. Starting at week 36, visits shift to once a week until delivery. These visits track your weight, blood pressure, and the baby’s position, and give you a chance to flag any new symptoms.
One important screening happens around weeks 36 to 37: the Group B Strep test. This is a simple swab that checks for bacteria that are harmless to you but could cause infection in your baby during delivery. If you test positive, you’ll receive antibiotics during labor to protect the baby.
Your provider may also ask you to start tracking fetal movement at home, sometimes called “kick counts.” There are different methods, but the basic idea is to get familiar with your baby’s normal activity pattern so you can notice if movement drops off. If you feel significantly less movement than usual, that’s worth a call to your provider rather than waiting for your next appointment.