In the last month of pregnancy (roughly weeks 36 to 40), a baby gains about half a pound per week, putting on approximately 2 to 3 pounds total before birth. By week 40, the average baby weighs about 7.5 pounds (3,400 grams) and measures around 14 inches from crown to rump, or about 19 to 21 inches from head to toe. That final stretch of growth isn’t just about getting bigger, though. Critical development is happening in the brain, lungs, and fat stores that prepare your baby for life outside the womb.
Weight Gain Week by Week
At around 34 weeks, the average baby weighs just over 4.5 pounds. By 40 weeks, that number climbs to roughly 7.5 pounds. That means your baby nearly doubles in weight over the final six weeks of pregnancy, with most of the gain concentrated in the last four. The rate of weight gain during this period is roughly 200 to 250 grams (about half a pound) per week, making these the heaviest weeks of growth in the entire pregnancy.
Much of that weight comes from fat being laid down beneath your baby’s skin. Earlier in pregnancy, the baby is lean and almost translucent. In the final month, a thick layer of body fat fills out the arms, legs, and torso. This fat serves two purposes: it provides an energy reserve for the first days after birth and it helps your baby regulate body temperature once they’re no longer surrounded by your warmth. By full term, body fat makes up roughly 15% of a baby’s total weight.
How Much Longer Your Baby Gets
Length growth slows down compared to weight gain during the last month. At 34 weeks, crown-to-rump length is about 12 inches (300 millimeters). By 40 weeks, that measurement reaches about 14 inches (360 millimeters). So your baby adds roughly 2 inches in body length over those final weeks. In total head-to-toe length, most full-term babies measure between 19 and 21 inches at birth.
Because length increases more slowly while weight climbs rapidly, your baby looks noticeably different in the final month. The long, skinny limbs fill out, the cheeks become rounder, and the overall appearance shifts from lean to plump.
Brain Growth in the Final Weeks
Some of the most important last-month development happens in the brain. At 35 weeks, a baby’s brain weighs only two-thirds of what it will weigh at 39 to 40 weeks. That remaining one-third of brain growth, packed into just four to five weeks, is a major reason why full-term delivery matters so much. During this window, the brain’s outer surface develops deeper folds and grooves, creating more surface area for the neural connections that will support breathing, feeding, temperature regulation, and sleep cycles after birth.
This is also why the difference between 37 weeks and 39 weeks is more significant than the two-week gap might suggest. A baby born at 37 weeks, while considered “early term,” still has meaningful brain maturation ahead compared to one born at 39 or 40 weeks.
Lung and Organ Maturation
Your baby’s lungs are among the last organs to fully mature. Throughout the final month, the lungs ramp up production of surfactant, a slippery substance that coats the air sacs and prevents them from collapsing with each breath. Without enough surfactant, newborns struggle to breathe on their own. By 39 to 40 weeks, most babies have enough surfactant for smooth, independent breathing.
The liver is also finishing its work during this period, building up iron stores that your baby will rely on for the first several months of life. The digestive system practices by processing amniotic fluid, and the immune system receives a boost of antibodies transferred from your bloodstream through the placenta.
What You Feel as Your Baby Grows
As your baby fills the available space, the type of movement you feel changes. Earlier in pregnancy, you may have noticed flips, rolls, and sweeping kicks. In the last month, the movements tend to feel more like pushing, stretching, and squirming rather than dramatic somersaults. This shift is normal and simply reflects the fact that there’s less room to maneuver. Your baby should still be active, just in a different way.
Many babies settle into a head-down position during the last month, which can change where you feel kicks (often under your ribs) and may create a sensation of increased pressure low in your pelvis, especially if the baby “drops” lower in preparation for labor.
Changes in Your Body
Your body reflects your baby’s rapid growth. Fundal height, the distance from your pubic bone to the top of your uterus, generally matches your week of pregnancy in centimeters (give or take 3 centimeters) after week 24. So at 36 weeks, you’d expect a measurement of roughly 33 to 39 centimeters. After 36 weeks, though, fundal height measurements become less reliable as the baby drops lower into the pelvis and the relationship between uterine size and gestational age becomes less predictable.
Amniotic fluid also shifts during this time. Fluid levels peak between 34 and 36 weeks, then slowly decrease as you approach your due date. This gradual decline is normal and partly explains why your baby’s movements may feel sharper or more pronounced in the final weeks, since there’s less cushioning fluid between you and those elbows and knees.
Why the Last Weeks Matter
It can be tempting to think of the last month as just a waiting game, but the growth happening between weeks 36 and 40 is some of the most consequential of the entire pregnancy. Your baby is gaining the fat they need to stay warm, the brain mass they need to feed and breathe effectively, and the lung maturity they need to take their first breaths without assistance. Each week in the final month adds measurable readiness for the transition to life outside the womb.