In the last month of pregnancy (weeks 36 through 40), babies gain roughly half a pound per week, adding about 2 to 2.5 pounds total before birth. Most full-term babies weigh between 6 and 9 pounds at delivery, meaning a significant chunk of their final weight is packed on in these last few weeks. Length increases more modestly, with babies growing about half an inch to an inch during this period, reaching an average of 19 to 21 inches at birth.
Weight Gain Week by Week
At 36 weeks, the average baby weighs around 5.5 to 6 pounds. By 37 weeks, that climbs to roughly 6 to 6.5 pounds. At 38 weeks, most babies are around 6.5 to 7 pounds, and by 39 to 40 weeks, they typically land between 7 and 8 pounds. The rate of gain does slow slightly as the due date approaches, but babies are still putting on meaningful weight right up until delivery.
This weight gain isn’t evenly distributed across body systems. The biggest contributor is fat. At 36 weeks, fat is being added under the skin across the entire body, which smooths out the wrinkled appearance babies have earlier in the third trimester. By 39 weeks, this fat layer serves a critical purpose: it helps the baby regulate body temperature after birth. A full-term newborn is roughly 15% body fat, and much of that accumulates in these final weeks.
What’s Happening Beyond Weight
Weight is the most obvious change, but the last month is packed with developmental finishing touches that don’t show up on a scale. The lungs are one of the last organs to fully mature. In weeks 36 through 38, the lungs ramp up production of surfactant, a substance that keeps the tiny air sacs from collapsing and allows the baby to breathe independently. This is one of the key reasons why even “late preterm” babies born at 36 or 37 weeks can have breathing difficulties that full-term babies don’t.
The brain also undergoes rapid growth during this window. It roughly doubles in size between weeks 35 and 40, forming the deep grooves and folds that increase surface area for processing. This brain development continues well after birth, but the foundation laid in the final month affects everything from feeding reflexes to temperature regulation. The liver is also building up iron stores during these last weeks, which the baby will rely on for the first several months of life.
Babies also practice skills they’ll need outside the womb. They swallow amniotic fluid, cycle through sleep and wake patterns, and develop stronger sucking reflexes. Their digestive system produces meconium, the dark, sticky first stool that typically passes within the first day or two after birth.
Why Some Babies Grow More (or Less) Than Expected
Not every baby follows the textbook growth curve. Several factors on the mother’s side can slow growth in the final month, including high blood pressure, diabetes, smoking, poor nutrition, very low or very high maternal weight, and chronic conditions like kidney disease or autoimmune disorders. Problems with the placenta or restricted blood flow through the umbilical cord can also limit how much nutrition reaches the baby, a condition called fetal growth restriction.
On the other end, gestational diabetes often leads to larger-than-average babies because excess blood sugar crosses the placenta and causes the baby to store more fat. Genetic factors play a role too. Parents who are taller or heavier tend to have bigger babies, and second or third babies are often larger than firstborns. Carrying multiples also changes the equation: twins and triplets typically grow more slowly in the final weeks simply because there’s less room and the placenta is supporting more than one baby.
How Growth Is Tracked
Your provider monitors growth in a few ways during the last month. Fundal height, the distance from your pubic bone to the top of your uterus measured in centimeters, generally matches your week of pregnancy (plus or minus 2 centimeters) up through about 36 weeks. After that point, the measurement can actually decrease because the baby drops lower into the pelvis in preparation for labor. So a smaller measurement at 38 weeks doesn’t necessarily mean the baby has stopped growing.
Fundal height is a rough screening tool, not a precise measurement. Factors like your body type, abdominal muscle tone, the baby’s position, and the amount of amniotic fluid all affect the number. If your provider suspects the baby is measuring unusually large or small, they’ll typically order an ultrasound. Ultrasound estimates of fetal weight in the third trimester can be off by 10 to 15% in either direction, so even that comes with some uncertainty. A baby estimated at 7 pounds could realistically weigh anywhere from about 6 to 8 pounds.
What This Means if You Deliver Early or Late
Because so much development happens in the last month, the difference between 36 weeks and 40 weeks is more significant than it might seem. A baby born at 36 weeks may weigh 2 pounds less than the same baby would at 40 weeks and could have immature lungs, difficulty maintaining body temperature, and trouble feeding. This is why elective deliveries are generally not scheduled before 39 weeks unless there’s a medical reason.
Babies born after 40 weeks don’t keep gaining at the same rate. The placenta becomes less efficient as it ages, and growth tends to plateau or slow. Babies born at 41 or 42 weeks are often only slightly heavier than those born at 40 weeks, and in some cases the placenta’s declining function can actually cause the baby to lose weight. This is one reason providers monitor more closely and discuss induction options as pregnancies stretch past the due date.