In the last month of pregnancy, your baby gains roughly half a pound to three-quarters of a pound every week. That adds up to about 2 to 3 pounds of total weight gain between weeks 36 and 40, bringing most full-term babies to somewhere between 6 and 9 pounds at birth. But weight is only part of the story. The final four weeks are when your baby’s brain, lungs, and fat stores undergo critical finishing touches that prepare them for life outside the womb.
Weight Gain Week by Week
Starting around week 35, your baby enters the most rapid period of weight gain in the entire pregnancy, putting on about 8 to 12 ounces (roughly half a pound to three-quarters of a pound) each week. At 36 weeks, most babies weigh around 6 pounds. By 40 weeks, the average is closer to 7.5 pounds, though anywhere from 6 to 9 pounds is normal.
Much of this weight comes from fat being laid down under the skin. Earlier in pregnancy, your baby was lean and almost translucent. In these final weeks, fat fills out the face, arms, and legs, giving your baby the rounder look you’ll see at birth. This fat layer isn’t just cosmetic. It’s what allows a newborn to regulate body temperature after leaving the warm, constant environment of the uterus. Babies born even a few weeks early often struggle with temperature control precisely because they missed out on this late fat accumulation.
Brain Growth in the Final Weeks
At 35 weeks, your baby’s brain weighs only about two-thirds of what it will at birth. That means a full third of your baby’s brain weight is added in roughly the last five weeks of pregnancy. During this period, the brain’s surface develops more of the deep folds and grooves that increase its surface area, and the connections between nerve cells multiply rapidly.
This is one of the key reasons doctors are cautious about elective early deliveries. A baby born at 37 weeks may look fully formed, but their brain is measurably less developed than one born at 39 or 40 weeks. The difference shows up in subtle ways: earlier-born babies are slightly more likely to have trouble with feeding, sleep regulation, and attention in the first months of life.
Lung Maturation
Your baby’s lungs start producing surfactant, the slippery substance that keeps the tiny air sacs from collapsing, as early as 26 weeks. But production ramps up significantly in the final month. By 36 to 37 weeks, most babies have enough surfactant to breathe independently, though the lungs continue maturing right up to delivery. Babies born before 36 weeks sometimes need help breathing because their surfactant levels aren’t quite sufficient.
Along with surfactant, the lungs are also practicing. Your baby “breathes” amniotic fluid in rhythmic movements that strengthen the muscles of the diaphragm and chest wall. These practice breaths become more coordinated as the due date approaches.
How Movement Changes
If you’ve noticed that your baby’s movements feel different in the last month, that’s expected. Earlier in the third trimester, movements tend to be strong, varied, and often sudden, with lots of kicking and jerky jolts. By the final weeks, those sharp kicks give way to pushing, rolling, and shuffling movements. The motions feel smoother and more sustained, with longer gaps between bouts of activity.
This shift happens because your baby is running out of room. A 7-pound baby in a confined space simply can’t wind up for the dramatic kicks you felt at 28 weeks. What matters is that you’re still feeling regular movement throughout the day. The type of movement changes, but the overall level of activity should not drop off dramatically. If you notice a significant decrease in movement, that’s worth a phone call to your provider.
What Your Body Provides
Supporting this rapid growth requires a significant transfer of nutrients across the placenta. Your body sends about 200 to 250 milligrams of calcium to the fetus daily, primarily during this final stretch. That calcium goes straight into hardening your baby’s bones and developing their teeth beneath the gums. Your body adapts to meet this demand by absorbing calcium from food more efficiently, which is why calcium requirements don’t technically increase in the third trimester, even though transfer to the baby does.
Iron transfer also peaks in the last month, as your baby builds up iron stores that will last through the first several months after birth. Newborns rely heavily on these stores because breast milk is relatively low in iron. This is another reason babies born early sometimes need iron supplementation sooner than full-term babies.
When Growth Falls Outside the Norm
Not every baby follows the average growth curve. Fetal growth restriction is diagnosed when a baby’s estimated weight falls below the 10th percentile for their gestational age, meaning they’re smaller than 90% of babies at the same stage. Severe growth restriction is defined as weight below the 3rd percentile. These measurements are taken by ultrasound, and your provider may order additional monitoring if your belly is measuring small or if there are risk factors like high blood pressure or placental problems.
On the other end, some babies grow larger than average in the final month, particularly if the mother has gestational diabetes. Higher blood sugar levels cross the placenta and cause the baby to produce more insulin, which acts as a growth hormone and can lead to a larger-than-expected baby. This is one reason glucose screening happens in the late second or early third trimester.
Length and Other Dimensions
While weight gets most of the attention, your baby also grows in length during the final month, though not as dramatically. Most babies gain about an inch to an inch and a half, going from around 18 to 19 inches at 36 weeks to 19 to 21 inches at birth. Head circumference increases too, as the skull expands to accommodate that rapidly growing brain. The bones of the skull remain soft and slightly separated at the suture lines, which allows them to overlap slightly during delivery. This is why many newborns have a temporarily elongated head shape after a vaginal birth.
By 40 weeks, your baby’s fingernails have grown past the fingertips, their skin has smoothed out as fat filled in beneath it, and most of the fine body hair that covered them earlier has shed. The waxy coating that protected their skin from amniotic fluid has mostly been reabsorbed, though some may remain in the creases of the skin. These are all signs of a baby who used the last month to put on the finishing details.