The lungs are the last major organ to mature in a fetus. While most organ systems are structurally complete well before the third trimester, the lungs don’t reach functional maturity until roughly 35 weeks of gestation. This is why premature birth carries such significant respiratory risks, and why the final weeks of pregnancy matter more than many people realize.
But lung development is only part of the story. Several critical systems are still finishing up in those last weeks, including the brain, liver, and immune defenses. Here’s what’s happening inside the womb during that final stretch.
Why the Lungs Finish Last
A fetus doesn’t need to breathe in the womb. Oxygen comes through the placenta, so the lungs have the luxury of developing slowly. The key milestone is the production of surfactant, a slippery coating that keeps the tiny air sacs in the lungs from collapsing each time a newborn exhales. The specialized cells that produce surfactant differentiate between 24 and 34 weeks of gestation, but the lungs aren’t considered clinically mature until after about 35 weeks.
This timeline explains why respiratory distress syndrome is the most common complication in babies born early. At 34 weeks, the rate of respiratory distress is around 10.5%. By 38 weeks, it drops to just 0.3%. The difference a few weeks makes is dramatic: a baby born at 34 weeks is roughly 40 times more likely to develop respiratory distress than one born at 39 to 40 weeks. Even babies born at 36 weeks still face about nine times the risk. The gap narrows quickly after that, with 37-week babies at about three times the risk and 38-week babies showing rates comparable to full term.
Babies born before surfactant production is adequate often need breathing support. Those born very early may receive synthetic surfactant directly into their lungs to bridge the gap until their own production catches up.
The Brain Keeps Building Until Birth and Beyond
The brain is another system that’s far from finished in the third trimester. Between 25 and 30 weeks, the surface of the brain undergoes rapid cortical folding, developing the characteristic wrinkles and grooves that dramatically increase its surface area. This folding slows near birth but doesn’t stop entirely.
Deeper in the brain, a process called myelination is underway. Myelin is an insulating layer that forms around nerve fibers, allowing electrical signals to travel faster and more efficiently. During the third trimester, myelination progresses through the brain stem, cerebellum, and thalamus. But the white matter of the cerebral hemispheres, the region responsible for higher-level thinking, language, and complex problem-solving, doesn’t begin myelinating until around six months after birth. In other words, this part of brain development isn’t just unfinished at birth. It hasn’t even started yet.
This extended timeline is one reason late-preterm babies (born at 34 to 36 weeks) face higher rates of neurological complications. Compared to full-term newborns, they’re more likely to experience seizures and related conditions in the newborn period.
The Liver Stocks Up Late
The fetal liver begins storing glycogen, a form of glucose the body can tap into quickly for energy, as early as 13 weeks. But the real buildup happens between 24 and 36 weeks, when glycogen stores increase dramatically. These reserves are essential for maintaining blood sugar in the hours after birth, before regular feeding is established.
Babies born before this stockpiling is complete are at higher risk for dangerously low blood sugar, which is why glucose monitoring is routine in preterm and early-term newborns.
Immune Protection Transfers in the Final Weeks
A fetus doesn’t build its own robust immune system before birth. Instead, it borrows its mother’s. Antibodies begin crossing the placenta as early as 8 to 10 weeks, but only in tiny amounts. By 22 weeks, the fetus has received roughly 10% of the mother’s circulating antibody levels. By 30 weeks, that number reaches about 50%.
The real surge happens in the final stretch. Between 37 and 40 weeks, antibody concentrations in the baby’s blood often exceed the mother’s own levels. This transferred immunity is what protects newborns from infections during their first months of life, before their own immune systems are mature enough to respond effectively. Babies born significantly early miss much of this transfer, leaving them more vulnerable to infections.
Brown Fat Prepares the Body for Cold
Inside the womb, temperature is perfectly regulated. Outside, a newborn needs to generate its own heat immediately. This is the job of brown fat, a specialized type of fat tissue that burns energy to produce warmth rather than storing it for later.
Brown fat makes up only about 2% to 4% of a newborn’s body weight, but its heat-generating capacity peaks at birth. The protein responsible for this heat production is present in only small amounts during fetal life. It activates rapidly at birth in response to a surge of hormones triggered by the sudden exposure to cooler air. Babies born prematurely may not have enough brown fat, or enough hormonal readiness, to regulate their temperature effectively.
Why the Last Few Weeks Matter So Much
The American College of Obstetricians and Gynecologists defines “full term” as 39 weeks through 40 weeks and 6 days. Babies born at 37 or 38 weeks are classified as “early term,” not full term, specifically because of the developmental gaps that still exist at that stage.
The clinical data backs this up across multiple systems. Severe respiratory problems requiring ventilation affect 8.3% of late-preterm newborns compared to 0.28% of full-term babies. Neurological complications, including seizures and brain bleeds, are also more common. Even the seemingly small jump from 38 to 39 weeks shows measurable improvements in outcomes.
This is why scheduled deliveries before 39 weeks are discouraged without a medical reason. Each additional day in the womb during those final weeks gives the lungs more time to produce surfactant, the liver more time to store energy, the brain more time to build connections, and the immune system more time to absorb protective antibodies. The lungs may be the headline answer to what finishes last, but nearly every system is still racing to the finish line right up until birth.