The third trimester starts at week 28 of pregnancy and lasts through week 40, giving you roughly 12 to 13 weeks in the final stretch. Most major health organizations agree on week 28 as the starting point, though your baby could arrive a bit before or after that 40-week mark.
When the Third Trimester Starts and Ends
Most providers recognize week 28 as the beginning of the third trimester. You may see slight variations online, with some sources placing it at week 27, but the medical consensus lands firmly on 28. The trimester runs through week 40, which is your estimated due date. Of course, “40 weeks” is simply an estimate of when your pregnancy will reach full duration. Plenty of babies arrive earlier or later.
The American College of Obstetricians and Gynecologists breaks down the final weeks into more specific categories. A delivery between 37 weeks and 38 weeks, 6 days is considered “early term.” A baby born between 39 weeks and 40 weeks, 6 days is “full term.” Deliveries at 41 weeks through 41 weeks, 6 days fall into “late term,” and anything at 42 weeks or beyond is “post-term.” These distinctions matter because outcomes tend to be best for babies born in the full-term window.
How Your Baby Grows During These 12 Weeks
Your baby enters the third trimester weighing roughly 2 to 2.5 pounds and exits it at an average of about 7.5 pounds, with a crown-to-rump length of around 14 inches. That’s a dramatic amount of growth packed into the final stretch. Much of the weight gain comes from fat deposits that help regulate body temperature after birth.
At week 28, your baby’s eyelids can partially open for the first time. Over the following weeks, the lungs continue maturing, bones harden (though the skull stays flexible for delivery), and the brain develops rapidly. By the end of the trimester, your baby has cycled through sleep-wake patterns, can respond to light and sound, and has shifted into a head-down position in most cases.
What Your Body Goes Through
The third trimester brings a distinct set of physical changes, most of them driven by two forces: your growing uterus and the hormones that loosen your joints and slow your digestion.
Back pain is one of the most common complaints. Pregnancy hormones relax the connective tissue holding your pelvic bones in place, while the expanding uterus stretches your abdominal muscles. Together, those shifts put extra strain on your lower back.
Shortness of breath picks up as your baby presses upward against your rib cage. Your lungs can still hold their full capacity, but the pressure makes each breath feel shallower. You may get winded more easily, especially when lying on your back.
Heartburn and constipation tend to worsen because pregnancy hormones slow digestion, and the uterus presses directly on your intestines. Frequent urination also ramps up as the baby drops lower into your pelvis, putting steady pressure on your bladder. Leaking urine when you laugh, cough, or sneeze is common and normal during these final weeks.
Varicose veins, spider veins, and hemorrhoids can all appear as blood volume increases and circulation slows. Heart palpitations sometimes occur because blood returns to the heart more slowly as the uterus grows. These symptoms are typically harmless but worth mentioning at your next prenatal visit if they feel persistent or severe.
Braxton Hicks vs. Real Contractions
Sometime during the third trimester, you’ll likely feel Braxton Hicks contractions: mild tightening sensations in your abdomen that come and go without a pattern. They tend to show up in the afternoon or evening, after physical activity, or after sex, and they become more frequent as your due date approaches. Walking or changing positions usually makes them stop.
Real labor contractions behave differently in several key ways. They come at regular intervals and get closer together over time. Each one lasts between 30 and 90 seconds. They grow stronger and more intense rather than fading, and changing positions does nothing to relieve them. If you can still walk and talk comfortably through a contraction, it’s more likely Braxton Hicks.
Prenatal Visits Pick Up
Your appointment schedule gets busier in the third trimester. From week 28 through week 36, you’ll see your provider every two weeks. After week 36, visits shift to once a week until delivery. These appointments typically include blood pressure checks, urine tests, belly measurements, and monitoring your baby’s position.
One important screening happens during week 36 or 37: a swab test for Group B Streptococcus, a type of bacteria that about 1 in 4 pregnant people carry without symptoms. Carriers receive antibiotics during labor to prevent passing the bacteria to the baby during delivery. The test is quick and painless, and both ACOG and the CDC recommend it for every pregnancy.
Tracking the Weeks
If you’re trying to figure out exactly where you are, the simplest way is to count from the first day of your last menstrual period, which is how your provider calculates gestational age. Week 28 marks the start of month seven, and weeks 28 through 40 span roughly the seventh, eighth, and ninth months of pregnancy. The month-to-week conversion is never perfectly clean because calendar months vary in length, which is why providers stick to weeks.
Your due date at 40 weeks is an estimate, not a deadline. Only about 5% of babies arrive on their exact due date. The full-term sweet spot, where outcomes are best, falls between 39 weeks and 40 weeks, 6 days. If your pregnancy extends past 41 weeks, your provider will monitor you more closely and discuss whether induction makes sense for your situation.