Third Trimester Week by Week — What to Expect (Weeks 28-40+)

Weeks 25-27 bridge the transition between the second and third trimesters. By week 25, your baby is about 13.5 inches long and weighs roughly 1.5 pounds — about the size of a cauliflower. The lungs are developing rapidly, forming more air sacs (alveoli) and producing increasing amounts of surfactant, though they're still immature. The baby responds to sound consistently now and may startle at loud noises.

At week 26, the eyes open for the first time, and the baby can perceive light and dark through the uterine wall. Brain wave patterns show distinct sleep and wake cycles, and the baby spends much of its time in REM sleep. The immune system is maturing as the baby receives antibodies from you through the placenta.

By week 27, the baby weighs about 2 pounds and is practicing breathing by inhaling and exhaling amniotic fluid. The brain is forming its characteristic grooves and folds, increasing surface area for those critical neural connections. If born at this stage, survival rates with NICU care exceed 90%, though significant support would still be needed.

For you, the growing baby means increased pressure on your bladder (frequent bathroom trips return), possible shortness of breath as the uterus pushes upward toward your diaphragm, and more intense Braxton Hicks contractions. Sleep may become more challenging — try sleeping on your left side with a pillow between your knees, which improves blood flow to the baby. Your prenatal visits will shift from every four weeks to every two weeks starting around now.

By week 28, you're officially in the third trimester. The baby is about 14.8 inches long and weighs roughly 2.25 pounds. A critical development during this period is rapid brain growth — the brain is adding billions of neurons and forming complex neural pathways. The baby can now dream during REM sleep, and brain scans of premature babies born at this age show recognizable patterns of brain activity.

At week 29, the baby is gaining weight more quickly now — about half a pound per week — as fat stores accumulate under the skin. This fat is essential for temperature regulation after birth. The baby's muscles and lungs are continuing to mature, and bones are hardening, though the skull remains soft and flexible to allow passage through the birth canal.

Week 30 marks another milestone: the baby is about 15.7 inches long and weighs about 3 pounds. The lanugo (fine body hair) is beginning to disappear as the baby's own fat stores take over temperature regulation. The bone marrow has completely taken over red blood cell production from the spleen. The baby's eyes can now focus, and it may turn toward or away from light shined on your belly.

Your provider will start monitoring your baby's position. Many babies are still in a breech position at 28-30 weeks, which is completely normal — most will turn head-down by week 36. You should begin doing regular kick counts now if you haven't already. Common discomforts include heartburn, insomnia, shortness of breath, and increasing pelvic pressure.

During weeks 31-33, your baby's growth and maturation continue at a remarkable pace. By week 31, the baby weighs about 3.3 pounds and is about 16 inches long. The five senses are now all functional — the baby can see light and dark, hear and respond to sounds, taste the amniotic fluid (which is flavored by what you eat), feel touch, and even smell. The baby's sleep cycles are more defined, with periods of quiet sleep, active sleep, and wakefulness.

At week 32, the baby is practicing all the skills needed for life outside the womb: breathing movements (inhaling and exhaling amniotic fluid), sucking, swallowing, and even coordinating these actions together. Toenails and fingernails have grown to the tips of the toes and fingers. The baby's skin is becoming less transparent and more opaque as fat accumulates.

By week 33, the baby weighs about 4 pounds. The bones of the skull are still soft and separated by gaps called fontanelles, which allow the skull to compress during delivery. The immune system is receiving a boost of antibodies from you — this passive immunity will protect the baby during the first few months of life until their own immune system matures.

You may notice your baby hiccupping regularly — these are normal and help the diaphragm develop. Space is getting tight in there, so movements may feel different — more rolling and stretching, less dramatic kicks. You might experience increasingly frequent Braxton Hicks contractions, difficulty sleeping, and the need to urinate more often. If you haven't started a birth plan, this is a good time to begin thinking about your preferences.

Weeks 34-36 are a critical time for lung maturation and final preparations for birth. By week 34, the baby weighs about 4.7 pounds and is approximately 17.7 inches long. The lungs are nearly mature — surfactant production is increasing rapidly, and babies born at 34 weeks, while still premature, generally do well with minimal NICU support. The central nervous system and lungs are the last major systems to fully mature.

At week 35, the kidneys are fully developed, the liver can process some waste products, and physical growth is mostly complete — from here, the baby is mainly gaining fat. Most babies have settled into the head-down position by now. Your provider may check the baby's position at your visit.

Week 36 is a key checkpoint. Your prenatal visits shift to weekly from now until delivery. Your provider will perform the Group B streptococcus (GBS) test — a simple vaginal and rectal swab. About 25% of women carry GBS bacteria, which is harmless to you but can cause serious infection in newborns. If you test positive, you'll receive IV antibiotics during labor to protect the baby.

Your baby may 'drop' (engage) into your pelvis around this time, especially if it's your first pregnancy. This is called lightening, and while it relieves pressure on your diaphragm (making breathing easier), it increases pelvic pressure and the urge to urinate. You may also experience more intense Braxton Hicks contractions, a 'nesting' instinct to clean and organize, and increased vaginal discharge as your cervix begins to soften. If your baby is still breech at 36 weeks, your provider may discuss an external cephalic version (ECV) to try to turn the baby.

At week 37, your pregnancy is considered 'early term,' and at week 39, it's 'full term.' The distinction matters — research shows that babies born at 39 weeks or later have better outcomes than those born at 37-38 weeks, with fewer breathing problems, better temperature regulation, and stronger feeding skills. This is why ACOG recommends against elective induction or cesarean before 39 weeks unless medically indicated.

By week 37, the baby is about 19 inches long and weighs approximately 6.3 pounds. The brain and lungs are still maturing — the brain will increase by about one-third in size between weeks 35-39. The baby is adding about half an ounce of fat per day, building the chubby cheeks and plump body you'll see at birth.

At week 38, the baby's organ systems are all ready for life outside the womb. The baby continues to swallow amniotic fluid and produce meconium (the dark, tarry first stool). Melanin is accumulating in the skin — though final skin tone develops over months after birth.

Week 39 is considered the ideal time for delivery. The baby typically weighs 7-8 pounds and is about 20 inches long. The chest is more prominent, and the baby has enough fat to maintain body temperature after birth. You may notice the mucus plug dislodging (a thick, sometimes blood-tinged discharge), increased Braxton Hicks that may become more regular, and loose stools as your body prepares for labor.

Watch for true labor signs: regular contractions that get closer together, longer, and stronger over time; lower back pain that radiates to the front; and water breaking (which happens before labor starts in only about 10-15% of women).

First, know that due dates are estimates — only about 5% of babies are born on their exact due date. Going past 40 weeks is common, especially with first pregnancies. About 30% of pregnancies extend beyond 40 weeks, and the pregnancy isn't considered 'post-term' until after 42 weeks.

However, your provider will monitor you more closely after 40 weeks because risks increase gradually. The placenta has a finite lifespan, and after 41 weeks it may become less efficient at delivering oxygen and nutrients. Amniotic fluid levels can decrease, and the baby continues to grow, which can make vaginal delivery more challenging.

Your provider will likely recommend more frequent monitoring after your due date, typically including non-stress tests (NSTs) that measure the baby's heart rate in response to movement, amniotic fluid index (AFI) checks via ultrasound, and biophysical profiles that assess the baby's breathing, movement, muscle tone, and fluid levels. These tests help ensure the baby is still thriving.

Most providers recommend induction of labor between 41 and 42 weeks. The ARRIVE trial, a major study published in the New England Journal of Medicine, found that induction at 39 weeks for low-risk first-time mothers was associated with a lower rate of cesarean delivery, with no increased risk to the baby. However, the decision about when and whether to induce is individual and should be discussed with your provider.

Natural methods people try to encourage labor include walking, nipple stimulation, dates (the fruit — studies show eating 6 dates daily from week 36 may help), and sex (semen contains prostaglandins that may soften the cervix). While evidence is mixed, these are generally safe to try after 39 weeks with your provider's approval.

The third trimester brings significant physical challenges as your body supports a rapidly growing baby. Here are evidence-based strategies for the most common discomforts.

For heartburn and acid reflux (which affects over 50% of women in the third trimester), eat smaller, more frequent meals, avoid lying down for at least 30 minutes after eating, prop yourself up with pillows at night, and ask your provider about antacids like calcium carbonate (Tums) or famotidine, which are considered safe in pregnancy.

For insomnia and sleep disruption, establish a consistent bedtime routine, sleep on your left side with a pregnancy pillow for support, limit fluids in the evening (to reduce nighttime bathroom trips), and try relaxation techniques like guided meditation or gentle prenatal yoga before bed.

For back pain and pelvic pressure, wear a supportive maternity belt, practice pelvic tilts and cat-cow stretches, apply heat or cold to sore areas, and consider prenatal massage or chiropractic care. Swimming is particularly helpful because buoyancy relieves pressure on joints and muscles.

For swelling (edema) in the feet and ankles, elevate your feet when possible, stay hydrated (counterintuitively, drinking more water helps reduce swelling), wear compression socks, and avoid standing for long periods. However, report sudden or severe swelling — especially in the face or hands — immediately, as this can be a sign of preeclampsia.

For shortness of breath, practice good posture to give your lungs more room, sleep propped up on pillows, and take breaks during physical activity. The baby 'dropping' in the final weeks often provides relief.