No single organ “forms” for the first time in the third trimester. By week 28, nearly every organ already exists in some structural form. What the third trimester does is finish the job: the lungs, brain, bones, kidneys, and immune system all undergo critical maturation that makes life outside the womb possible. The lungs are the most dramatic example, since they don’t produce enough of a key breathing substance until around week 32.
Lungs: The Last Major Organ to Mature
The lungs are the organ most closely associated with third-trimester development, and for good reason. Although they begin forming early in pregnancy, they can’t support independent breathing until late in the third trimester. The lungs produce surfactant, a slippery coating that keeps the tiny air sacs from collapsing each time a baby exhales. Surfactant production starts around week 24, but adequate amounts don’t accumulate until about week 32. This is the single biggest reason premature birth is dangerous: without enough surfactant, a newborn’s lungs can’t stay open.
This timeline directly tracks with survival rates. A baby born at 28 weeks has roughly an 80 to 90 percent chance of survival with intensive care. By 32 weeks, survival exceeds 97 percent. Those extra four weeks of lung maturation make an enormous difference.
Brain Growth Accelerates Dramatically
The brain undergoes its most visible transformation during the third trimester. Early in pregnancy, the brain’s surface is smooth. Starting around week 20, it begins to fold, but the pace picks up sharply after week 28. Primary and secondary folds form during the third trimester, with a third wave of folding appearing near the time of birth. These folds massively increase the brain’s surface area, creating room for the billions of neural connections that support thinking, movement, and sensation.
By week 28, the central nervous system can regulate body temperature and trigger practice breathing movements visible on ultrasound. By week 29, the eyes can detect changes in light and the baby begins to blink. These milestones reflect a nervous system that’s rapidly wiring itself together, a process that continues well after birth but reaches a critical threshold in these final months.
Bones Harden With a Surge of Calcium
The skeleton exists in cartilage form much earlier in pregnancy, but the third trimester is when bones actually harden. About 80 percent of the baby’s total mineral content is deposited during the third trimester. In the final six weeks alone, calcium transfers from mother to baby at a rate of 300 to 350 milligrams per day, up from roughly 60 milligrams per day at week 24. That’s a fivefold increase in just a few months.
By week 33, bones are noticeably harder, though the skull remains flexible and soft on purpose. The skull bones stay slightly separated so they can compress and overlap during delivery, which is why newborns often have a slightly elongated head shape at birth.
The Immune System Gets a Head Start
A fetus doesn’t build a fully working immune system on its own before birth. Instead, it borrows one from its mother. Antibodies (specifically a type called IgG) cross the placenta throughout pregnancy, but the highest levels transfer during the third trimester. These maternal antibodies give the newborn temporary protection against infections the mother has already fought off or been vaccinated against.
This is one reason vaccines like the flu shot and whooping cough booster are recommended during pregnancy. Antibodies produced in response to those vaccines cross to the baby most efficiently in the final weeks, providing a shield that lasts for the first few months of life while the baby’s own immune system is still immature.
Kidneys Ramp Up Urine Production
The kidneys are functional well before the third trimester, but their workload increases substantially in the final months. Fetal urine becomes the primary source of amniotic fluid in the third trimester, with the baby producing measurable amounts each hour. This urine production, combined with fluid secreted by the lungs, maintains the pool of amniotic fluid that cushions the baby and supports continued lung development (the baby “breathes” amniotic fluid in and out as practice).
Interestingly, urine production slows in the final two weeks before labor begins, which contributes to the normal decline in amniotic fluid seen at the very end of pregnancy.
Fat, Skin, and Temperature Control
One of the most important third-trimester developments isn’t an organ at all. It’s fat. Starting around week 36, the baby accumulates fat rapidly beneath the skin, which serves two purposes: it provides insulation for regulating body temperature after birth, and it stores energy for the transition to feeding outside the womb. By week 39, fat is being added across the entire body.
This fat layer also transforms the baby’s appearance. Earlier in pregnancy, the skin is wrinkled and somewhat translucent. As fat fills in underneath, the skin becomes smooth and opaque. The baby essentially goes from looking fragile and lean to round and ready for the outside world in just a few weeks.
Red Blood Cell Production Shifts Location
Around week 30, the site of red blood cell production moves permanently to the bone marrow. Earlier in development, the liver and spleen handle this job. The shift to bone marrow is a sign that the skeletal system has matured enough to take over this lifelong function. It’s a quiet milestone, but it marks the baby’s blood-forming system reaching its adult configuration months before birth.
Why the Third Trimester Matters So Much
By week 31, most major structural development is complete. From that point forward, the focus shifts to weight gain and functional maturation. The difference between a 28-week baby and a 40-week baby isn’t which organs are present. It’s whether those organs can work well enough without medical support. Lungs need surfactant. The brain needs its folds. Bones need calcium. The immune system needs maternal antibodies. All of these processes converge in the third trimester, which is why each additional week of pregnancy significantly improves a newborn’s chances of thriving independently.