The third trimester of pregnancy starts at week 28 and lasts through week 40, or whenever you deliver. Most major health organizations agree on this cutoff, though you may occasionally see week 27 mentioned. If your provider uses a slightly different number, don’t worry. The difference is minor, and the important thing is that this final stretch covers roughly the last 12 to 13 weeks of a full-term pregnancy.
How the Trimesters Break Down
Pregnancy is divided into three roughly equal blocks. The first trimester runs from week 1 through week 12, the second from week 13 through week 27, and the third from week 28 through week 40. These divisions aren’t arbitrary. Each trimester represents a distinct phase of fetal development and a shift in what’s happening in your body.
Entering the third trimester is a meaningful milestone. At 28 weeks, your baby’s central nervous system can regulate body temperature and trigger rhythmic breathing movements visible on ultrasound. The eyelids can partially open. Your baby is roughly 10 inches long (measured from crown to rump) and weighs about 2¼ pounds. From here, the main job is rapid weight gain and organ maturation, especially the lungs.
What Your Baby Is Doing in the Third Trimester
Growth accelerates dramatically over these final weeks. At week 30, your baby’s eyes can open wide and weight climbs to nearly 3 pounds. By week 32, that number reaches about 3¾ pounds. At week 33, the pupils start responding to light, changing size the way they will after birth.
By week 34, your baby weighs more than 4½ pounds. At week 38, the average is around 6½ pounds, though some babies are already close to 9 pounds at this point. By week 40, the typical weight is about 7½ pounds with a crown-to-rump length of around 14 inches. Size varies quite a bit from baby to baby, and a wide range is completely normal.
The lungs are one of the last organs to fully mature. This is a big reason why every additional week in the womb matters, particularly between weeks 28 and 36. Babies born in the early third trimester often need breathing support, while those born closer to 39 or 40 weeks rarely do.
How Your Body Changes
As your baby grows, the physical demands on your body increase. Shortness of breath becomes common because the expanding uterus pushes up against your diaphragm, leaving less room for your lungs to fully expand. Swelling in your feet and ankles is typical, especially later in the day. Heartburn, back pain, and trouble sleeping are all frequent complaints, driven by the sheer size of the uterus and the hormonal changes that loosen your joints and relax the valve between your stomach and esophagus.
You’ll also start noticing Braxton Hicks contractions if you haven’t already. These are short, irregular tightening sensations that usually happen a few times a day and last less than a minute. They often stop when you change position. If you’re walking and sit down, or resting and start moving, they tend to fade. They’re your uterus practicing, not a sign of labor.
Real labor contractions behave differently. They become regular, longer, and progressively more painful. The pain typically starts at the top of the uterus and radiates down toward the pubic bone, sometimes wrapping around into your lower back. If you’re having contractions more than four times an hour before 37 weeks, that could signal preterm labor and warrants a call to your provider right away.
Prenatal Visits and Testing
Your appointment schedule picks up once the third trimester begins. You’ll typically go in every two to four weeks from week 28 onward, then every one to two weeks starting at week 36. These visits focus on monitoring blood pressure, checking your baby’s position, and tracking growth.
Two specific screenings happen during this stretch. The CDC recommends the Tdap vaccine (which protects against whooping cough) between weeks 27 and 36, ideally on the earlier side. Getting vaccinated during this window allows your body to produce antibodies that pass to your baby before birth, offering protection during those vulnerable first months of life.
At 36 or 37 weeks, you’ll be screened for Group B strep, a type of bacteria that about 25% of healthy women carry. The test is simple: a swab of the vagina and rectum, sent to a lab. If you test positive, you’ll receive antibiotics during labor to prevent passing the bacteria to your baby during delivery.
Tracking Your Baby’s Movement
Kick counts become an important daily habit in the third trimester. The standard method is straightforward: pick a time when your baby is usually active, lie on your left side or sit with your feet up, and count each kick, flutter, swish, or roll until you reach 10. Ideally, you should feel 10 movements within two hours. Most people reach 10 well before that.
If two hours pass without 10 movements, contact your provider. It doesn’t always mean something is wrong, but a noticeable drop in movement is one of the few things you can monitor at home that gives real-time information about how your baby is doing.
Weight Gain in the Final Stretch
If you started pregnancy at a healthy weight, the general guideline is to gain about 1 pound per week through the second and third trimesters. If you started at a higher weight, the recommendation drops to about half a pound per week. Steady, gradual gain matters more than hitting an exact number on any given week. Sudden weight gain, especially if paired with swelling in your face or hands, can be a warning sign of preeclampsia rather than normal pregnancy weight.
Warning Signs to Watch For
Preeclampsia is one of the most serious complications of the third trimester, and it’s defined by high blood pressure along with signs of organ stress. Beyond elevated blood pressure readings at your prenatal visits, symptoms you might notice on your own include severe headaches that don’t respond to rest or hydration, vision changes like blurriness or light sensitivity, sudden swelling of the face and hands, pain under the ribs on the right side, shortness of breath, and nausea or vomiting that feels different from typical pregnancy discomfort.
None of these symptoms alone confirms preeclampsia, but any of them in the third trimester is worth reporting promptly. The condition can progress quickly, and early detection gives your care team the most options for keeping you and your baby safe.