Babies gain roughly half a pound per week during the last month of pregnancy, adding about two pounds between week 36 and a typical delivery around week 40. But weight is only part of the story. The final weeks are when the lungs finish preparing to breathe air, the brain undergoes a dramatic growth spurt, and fat stores accumulate at a pace that dwarfs everything that came before. That late-pregnancy hustle explains why even a week or two of extra time in the womb can make a measurable difference in newborn health.
How Fast the Weight Adds Up
One large study that tracked fetal weight gain found an average weekly gain of about 177 grams, or just under 6.3 ounces per week, across the third trimester.1PubMed Central. Average fetal weekly weight gain: a novel measure of fetal growth velocity That rate is not perfectly steady throughout the month. Growth tends to be at its peak somewhere around weeks 33 to 36, then gradually tapers as the placenta approaches its functional limits. Still, the final four weeks deliver a substantial portion of total birth weight. A baby who weighs about five and a half pounds at 36 weeks will often weigh seven to seven and a half pounds by 40 weeks.
Boys tend to run slightly heavier than girls. The same study found that female fetuses gained about 9 grams per week less than males, a gap that compounds over months and helps explain the consistent sex difference in average birth weight. Women who had given birth before also tended to grow slightly larger babies, with about a 7-gram-per-week bump compared to first pregnancies.
Most of It Is Fat
A big share of the weight gained in the last month is body fat, and the timing is lopsided. Over 90 percent of fetal fat deposition happens in the last ten weeks of pregnancy, with the rate climbing steeply to reach about 7 grams of fat per day near term.2PubMed. Placental regulation of fatty acid delivery and its effect on fetal growth–a review That fat is not just padding. It serves as insulation, an energy reserve for the first days after birth when feeding is still getting established, and a cushion for internal organs. Human newborns are unusually fat compared to other mammals, carrying roughly 12 to 15 percent body fat at birth.3Diabetes/Metabolism Research and Reviews. Lipid metabolism in the fetus and the newborn
This fat surge is fueled by the mother’s body shifting into a catabolic state during the third trimester. Her fat stores stop growing and instead begin breaking down, releasing fatty acids and glucose that get routed preferentially toward the fetus.4PubMed Central. Nutrition and Metabolic Adaptations in Physiological and Complicated Pregnancy: Focus on Obesity and Gestational Diabetes The placenta actively shuttles fatty acids across, while glucose gets converted into fat by the baby’s own liver. The result is that even if the mother is not eating more in those final weeks, her metabolism has been reprogrammed to direct resources to the baby.
The Brain’s Growth Spurt
The brain is one of the fastest-growing organs during the last month. Total brain volume increases substantially between weeks 36 and 40, with the cortex folding into increasingly complex ridges and grooves. A study comparing third-trimester brain development in fetuses and preterm infants found that brain growth trajectories for the cerebrum, cerebellum, and total brain volume were all significantly faster when development happened inside the womb rather than outside it.5PubMed Central. Third Trimester Brain Growth in Preterm Infants Compared With In Utero Healthy Fetuses The uterine environment, with its constant temperature, nutrient supply, and sensory insulation, appears to offer conditions that a neonatal intensive care unit simply cannot replicate.
Gestational age at birth is linked with more cortical surface area and more gyrification, the folding process that packs a larger surface area into a fixed skull volume.6JAMA Pediatrics. Association of Gestational Age at Birth With Brain Morphometry Each additional week in utero during the late third trimester contributes to more surface-level brain complexity. This is one reason that even “early term” births at 37 weeks carry slightly different neurological profiles than births at 39 or 40 weeks.
Lungs Finishing the Job
Lung maturation is perhaps the most critical development happening in the final month, because a baby needs functional lungs from the very first breath. The key milestone is surfactant production. Surfactant is a mix of fats and proteins that coats the inside of the tiny air sacs in the lungs and keeps them from collapsing each time the baby exhales. Without enough surfactant, a newborn struggles to breathe, which is why respiratory distress is one of the leading complications in premature births.
Surfactant production by the lung’s specialized cells begins after about 80 percent of gestation is complete (roughly week 32) and ramps up steadily through the final weeks.7PubMed Central. Fetal-to-Maternal Signaling in the Timing of Birth The surfactant system has two components with different jobs. Some proteins work with the lipid layer to reduce surface tension in the air sacs. Others, called collectins, are part of the baby’s earliest immune defense, helping clear bacteria and other pathogens from the lungs after birth. Both sides of this system are still maturing in the final four weeks, which is part of why the timing of delivery matters so much for respiratory outcomes.
Hormones control the pace of surfactant production. Cortisol and prolactin together can increase the rate at which lung cells manufacture the key lipid component by two to three times.8The Journal of Clinical Endocrinology & Metabolism. Multihormonal Regulation of Surfactant Synthesis by Human Fetal Lung in Vitro This is one reason that doctors sometimes give corticosteroid injections to women at risk of early delivery: the goal is to push the baby’s lungs through this maturation window faster.
Building the Skeleton
Bone mineralization accelerates in the last month. The fetal skeleton needs calcium, phosphorus, and magnesium to harden from soft cartilage into functional bone, and the placenta works hard to deliver those minerals. It actively pumps calcium from the mother’s blood into the baby’s circulation, maintaining mineral levels in the fetus that are actually higher than the mother’s own levels.9PubMed. Bone development and mineral homeostasis in the fetus and neonate: roles of the calciotropic and phosphotropic hormones Those elevated concentrations appear to be necessary for the skeleton to reach a normal mineral density by the time the baby is born. Babies who arrive significantly early often have lower bone mineral content, which usually catches up in the first year of life but represents one more system that benefits from the final weeks of pregnancy.
The Placenta’s Limits
All of this growth depends on the placenta, and the placenta does not last forever. As pregnancy extends past 40 weeks, placental function begins to decline. Prolonged pregnancies show reduced placental growth, leading to a rising ratio of baby weight to placenta weight. Microscopic changes include clumping of certain cell nuclei, reduced blood vessel density, and impaired nutrient transport, along with increased signs of oxidative stress and cellular aging.10PubMed. Morphological and functional changes in placentas from prolonged pregnancies Many of these changes resemble what happens in fetal growth restriction and preeclampsia, suggesting that the placenta is essentially wearing out.
Even in pregnancies that do not extend past their due date, some placentas show signs of premature aging. Lower levels of certain protective proteins in cord blood have been linked with accelerated maturation of placental tissue, which can restrict fetal growth.11PubMed Central. Evidence of Placental Aging in Late SGA, Fetal Growth Restriction and Stillbirth—A Systematic Review This is one reason that monitoring fetal growth in the last month is standard practice. A baby whose growth curve flattens unexpectedly may not be getting what it needs from its placenta.
Mothers sometimes worry that reduced fetal movement near the end of pregnancy means something is wrong. The character of movements does change as the baby grows and space becomes cramped, but the frequency and intensity do not normally decrease.12PubMed Central. Fetal movement in late pregnancy – a content analysis of women’s experiences of how their unborn baby moved less or differently Kicks may feel more like pushes or rolls, and the dramatic somersaults of earlier months become impossible, but an actual drop in how often the baby moves is worth reporting to a provider.
When Growth Runs Too Fast
Gestational diabetes is one of the most common reasons a baby grows larger than expected in the last month. The excess weight gain becomes statistically apparent around week 28 and continues widening through delivery. By week 37, fetuses of mothers with gestational diabetes weigh an average of about 6.7 pounds compared to about 6.5 pounds in mothers with normal glucose, and that gap keeps growing.13PubMed Central. Science Update: Fetal weight gain may begin long before gestational diabetes is commonly diagnosed A separate study found that gestational diabetes increased the weekly growth rate from week 25 through week 38, resulting in an average weight difference of about 195 grams (roughly seven ounces) at 40 weeks.14The Journal of Clinical Endocrinology & Metabolism. Effect of Gestational Diabetes on Fetal Growth Rate and Later Overweight in the Offspring
The extra weight is disproportionately fat rather than lean tissue, and it carries longer-term consequences. The same study found that faster fetal growth at week 28 partially explained the link between gestational diabetes and childhood overweight in the offspring at ages five to nine. Modern management of gestational diabetes, including tighter glucose control, has narrowed this gap. More recent cohorts of babies born to mothers with gestational diabetes actually had similar birth weights to control infants, though their body composition still differed, with slightly lower skinfold thickness measurements.15PubMed Central. Reduced size at birth and persisting reductions in adiposity in recent, compared with earlier, cohorts of infants born to mothers with gestational diabetes mellitus
When Growth Runs Too Slow
Late-onset fetal growth restriction, where a baby falls behind in the final weeks, is harder to detect than the early-onset form but may affect a larger number of pregnancies. The placental problems underlying it tend to be milder, with fewer of the severe vascular lesions seen in early growth restriction.16PubMed. Diagnosis and surveillance of late-onset fetal growth restriction Because the baby’s growth was normal for most of the pregnancy, the slowdown can be subtle and easy to miss on a single measurement. Serial ultrasounds that track growth velocity are more reliable than one-time size estimates.
Maternal obesity can also constrain nutrient delivery in ways that are not immediately obvious. Overweight pregnant women in one study absorbed 43 percent less iron during the third trimester and transferred about 17 percent less iron to the fetus compared to normal-weight women.17The American Journal of Clinical Nutrition. Overweight and obesity in pregnant women impair maternal–fetal iron transfer and reduce infant iron stores Their infants had lower iron stores at six months of age. Iron is critical for brain development, so this is a hidden cost of maternal obesity that does not necessarily show up on the scale at birth.
Why 37 Weeks Is Not the Same as 39
The traditional definition of “term” used to be anything from 37 to 42 weeks, as if those five weeks were interchangeable. The evidence shows they are not. Babies born at 37 weeks, now classified as “early term,” have higher rates of complications compared to those born at 39 weeks. One study found that composite neonatal morbidity dropped significantly for babies delivered at 39 weeks compared to 37 weeks across several delivery categories.18PubMed Central. Neonatal Outcomes in Early Term Birth A more recent study confirmed that early-term infants land in an intermediate zone between late-preterm and full-term babies. They need more respiratory support, lose more weight after birth, and get readmitted to the hospital more often than full-term babies.19PubMed Central. Short-Term Neonatal Outcomes Following Early-Term vs. Full-Term and Late-Preterm Births: Insights from a Retrospective Cohort Study
This is the practical upshot of all the growth and maturation described above. Those last two weeks between 37 and 39 deliver meaningful gains in lung readiness, brain complexity, and fat stores. The difference is small for any individual baby, but across a population, it shifts the odds enough that medical guidelines now discourage elective deliveries before 39 weeks unless there is a medical reason.
How Accurately Can Late Growth Be Measured?
If you have had a late-pregnancy ultrasound, you may have noticed a weight estimate printed on the report. Those numbers are useful but imperfect. A systematic review of ultrasound fetal weight estimation found that across multiple formulas and studies, the pooled average error was a slight overestimate of about 0.14 percent, which sounds reassuring until you see that individual formulas ranged from underestimating by nearly 7 percent to overestimating by 22 percent.20PubMed Central. The accuracy of ultrasound estimation of fetal weight in comparison to birth weight: A systematic review The random error was wide too, meaning any single estimate could easily be off by half a pound or more.
Accuracy also varies depending on the baby’s size. A Swedish study found that the most common formulas overestimated weight the most in the largest babies, with errors around 8 to 9 percent for fetuses above the 97th percentile.21PubMed Central. Accuracy and precision of sonographic fetal weight estimation in Sweden This means the babies most likely to trigger concern about size are also the ones whose estimates are least reliable. If a provider tells you your baby is measuring big or small at 37 weeks, the trend across multiple measurements is more informative than any single number.
What Happens After Birth for Smaller Babies
Babies who are born small for gestational age, whether because of growth restriction or other causes, do not simply stay small. The majority experience what pediatricians call catch-up growth, mostly between 6 months and 2 years of age, and about 85 percent reach a normal height range by age two.22PubMed Central. Catch-up growth and catch-up fat in children born small for gestational age The remaining 10 to 15 percent may remain below the third percentile for height into adulthood.
But catch-up growth is not a simple good-news story. A prospective study found that the trajectory matters as much as whether catch-up happens at all. Babies who showed no catch-up growth had higher risks of infections and lower IQ scores at age seven. Those who caught up too aggressively had higher risks of overweight and elevated blood pressure by the same age. Only babies with moderate, steady catch-up growth avoided both sets of problems.23PubMed. The optimal postnatal growth trajectory for term small for gestational age babies: a prospective cohort study The sweet spot, which accounted for about 37 percent of the small-for-gestational-age babies in the study, was characterized by gradual gains that brought the baby into a normal range without overshooting it.
For late preterm infants who are also small, breastfeeding appeared to be protective. In one study, infants who received any human milk were less likely to fail at catching up in both weight and length by 36 months.24Pediatric Research. Postnatal growth of small for gestational age late preterm infants: determinants of catch-up growth
Fetal Sleep in the Final Days
An underappreciated aspect of late-pregnancy development is the maturation of sleep states. By the last week of pregnancy, the fetus cycles between distinct sleep patterns that researchers can identify from electrical activity recordings. These fetal sleep states map onto the quiet sleep and active sleep stages observed in newborns, but the time spent in each state changes substantially at birth. One study measuring neurophysiologic continuity found significant differences between fetal and neonatal sleep: both quiet sleep and active sleep time dropped after delivery, and fetal sleep states correlated strongly with their postnatal equivalents.25PubMed Central. Neurophysiologic Measurement of Continuity in the Sleep of Fetuses during the Last Week of Pregnancy and in Newborns The findings suggest that the neural circuits governing sleep cycles are being tuned during the final days in utero and then recalibrate once the baby encounters the sensory environment of the outside world.
Why Humans Are Born When They Are
Given how much development is still happening at 40 weeks, you might wonder why babies are not simply born later, after even more growth. Part of the answer is a physical constraint: the human pelvis can only accommodate a head so large. This trade-off between large brains and an upright posture has shaped human evolution for millions of years.26PubMed. Evolution of the human birth canal The old theory was that a wider pelvis would make walking inefficient, though that specific claim has not been well supported by biomechanical studies. More recent clinical research points to a different constraint: a wider birth canal would increase the risk of pelvic floor disorders, including incontinence and organ prolapse. The timing of human birth, then, is not a single optimization problem but a tangle of competing pressures. Babies arrive relatively immature compared to many other primates, and the final month of pregnancy represents the tail end of a compromise that has been evolving for a very long time.