The third trimester of pregnancy spans weeks 28 through 40, and it’s the period of the most dramatic growth for your baby and the most intense physical changes for you. Your baby gains most of its birth weight, its lungs prepare to breathe air, and your body shifts to accommodate a rapidly growing uterus. Here’s what to expect during these final months.
How Your Baby Grows Week by Week
At the start of the third trimester (week 28), the average fetus weighs roughly 2 to 2.5 pounds and measures about 14 to 15 inches long. Over the next 12 weeks, your baby will more than triple in weight, typically reaching 6 to 9 pounds by week 40. Much of that weight comes from fat deposits that form under the skin, helping your baby regulate body temperature after birth.
Between weeks 28 and 32, the bones harden (though the skull stays flexible for delivery), the eyes can open and close, and the brain develops rapidly, forming billions of new neural connections. By week 32, your baby can respond to light, sound, and even taste through the amniotic fluid. By week 35 or 36, most of the major organ systems are functioning well enough to support life outside the womb, though the lungs and brain continue maturing right up until delivery.
When the Lungs Are Ready to Breathe
Lung development is one of the most critical milestones of the third trimester. The lungs begin producing surfactant, a slippery substance that keeps the tiny air sacs from collapsing, as early as 24 weeks. But the lungs don’t produce enough surfactant to work reliably until around 32 weeks. This is why premature babies born before that point face a significant risk of respiratory distress. At less than 24 weeks, that risk is around 60%. After 34 weeks, it drops to less than 5%. Full lung maturation continues well after birth and isn’t truly complete until a child is around 8 years old.
Your Baby’s Position Before Birth
Throughout much of the third trimester, your baby is still somersaulting and shifting around. But by around week 36, most babies settle into a head-down (vertex) position, which is the ideal alignment for a vaginal delivery. If your baby remains in a breech position (feet or buttocks down), your provider will likely discuss options, which may include an external version (a hands-on technique to rotate the baby) or a planned cesarean delivery. Breech positioning itself doesn’t pose a risk to the baby before labor, but it does make vaginal delivery significantly more complicated and less safe.
Physical Changes You’ll Notice
The third trimester brings a collection of symptoms that are largely caused by two things: a much larger uterus pressing on your organs and the hormonal shifts that prepare your body for labor.
Heartburn and constipation are among the most common complaints. Pregnancy hormones slow digestion, and your expanding uterus presses against your stomach and intestines, making both problems worse. Eating smaller, more frequent meals and staying upright after eating can help with heartburn. For constipation, fiber-rich foods and plenty of water make the biggest difference.
Swelling in your feet, ankles, and hands is normal and results from increased blood volume and fluid retention. You may also notice spider veins (small red or purple marks), varicose veins in your legs, or hemorrhoids, all driven by the extra blood your body is circulating. Propping your legs up when resting and staying moderately active can reduce swelling.
Sleep often becomes difficult. Between the size of your belly, the need to urinate frequently, leg cramps, and general discomfort, many people find the last few weeks of pregnancy exhausting. Back pain and pelvic pressure are common as the baby drops lower into your pelvis in preparation for delivery.
Weight Gain and Nutrition
For someone who started pregnancy at a normal weight (BMI of 18.5 to 24.9), the total recommended weight gain across the entire pregnancy is 25 to 35 pounds. The third trimester is when the most noticeable gain occurs, typically about a pound per week. Your body needs roughly 450 extra calories per day during these final months to support your baby’s rapid growth. That’s a modest increase, roughly equivalent to an extra sandwich or a substantial snack, not “eating for two” in the way people often imagine.
Braxton Hicks vs. Real Contractions
Starting sometime in the third trimester, you’ll likely feel Braxton Hicks contractions. These feel like a tightening in one area of your abdomen, similar to mild menstrual cramps. They’re irregular, don’t increase in strength, and usually fade on their own, especially if you change position or drink water.
True labor contractions are different in several clear ways. They come at regular intervals and get progressively closer together. They increase in intensity over time rather than staying the same or fading. And the pain typically starts in your midback and wraps around to the front of your abdomen, rather than staying in one spot. If your contractions are getting stronger, more frequent, and more rhythmic, that’s real labor.
Prenatal Visits and Screenings
Your appointment schedule picks up in the third trimester. From week 28 to 36, you’ll see your provider every two weeks. After week 36, visits shift to once a week until delivery. These visits typically include blood pressure checks, urine tests, measuring your belly, and listening to the baby’s heartbeat.
One important screening happens during week 36 or 37: a test for Group B Streptococcus (GBS), a type of bacteria that about 25% of healthy women carry. GBS is harmless to you but can cause serious infection in a newborn during delivery. The test is a simple swab, and if you test positive, you’ll receive antibiotics during labor to protect your baby.
Tracking Your Baby’s Movement
As your baby gets bigger, you’ll become more aware of its movement patterns. By the third trimester, most providers encourage you to pay attention to how often and when your baby moves. One common approach is the “count to 10” method: pick a time when your baby is usually active, lie on your side, and count how long it takes to feel 10 movements. Most babies reach 10 within an hour or two. If your baby takes much longer than usual, or if you notice a sudden and clear decrease in movement from what’s normal for your baby, contact your provider. A noticeable drop in movement can be an early sign that the baby needs to be evaluated.
Warning Signs of Preeclampsia
Preeclampsia is a serious pregnancy complication that usually develops after 20 weeks, most commonly in the third trimester. It involves high blood pressure and signs of organ stress, particularly to the kidneys and liver. Many women with preeclampsia have no symptoms at first, which is why those frequent prenatal visits and blood pressure checks matter so much.
When symptoms do appear, they can include severe headaches that don’t respond to typical remedies, vision changes like blurriness or light sensitivity, pain in the upper right abdomen (under the ribs), shortness of breath, and nausea or vomiting. Sudden swelling in your face and hands, or rapid unexplained weight gain, can also be signs. Some swelling is completely normal in pregnancy, but a sudden change from your baseline is worth reporting. Preeclampsia can progress quickly and become dangerous for both you and your baby, so these symptoms should never be brushed off.