In the final month of pregnancy, roughly weeks 36 through 40, a baby typically gains about half a pound per week, adding around 2 to 2.5 pounds total before birth. That means a baby who weighs about 5.5 pounds at 36 weeks will usually arrive somewhere between 7 and 8 pounds at full term. This late-pregnancy weight gain isn’t just padding. It serves critical functions that prepare the baby to survive outside the womb.
Week-by-Week Growth in the Final Month
At 36 weeks, the average fetus weighs roughly 5.5 to 6 pounds and measures about 18 to 19 inches long. From this point, growth follows a fairly predictable pattern:
- Week 37: About 6 to 6.5 pounds. The lungs are reaching maturity around this point, which is one reason 37 weeks is considered “early term.”
- Week 38: About 6.5 to 7 pounds. Fat continues accumulating under the skin.
- Week 39: About 7 to 7.5 pounds. Most organ systems are fully developed.
- Week 40: About 7.5 to 8 pounds on average, though healthy birth weights range widely from about 5.5 to 10 pounds.
The rate of weight gain does slow slightly in the final week or two compared to earlier in the third trimester. Before week 36, babies can gain closer to 8 ounces per week. In the very last week, growth may taper as the placenta becomes less efficient at delivering nutrients.
What the Weight Gain Is Actually Made Of
Much of the weight a baby adds in the final month is body fat. Babies go from roughly 5% body fat at the start of the third trimester to about 15% at birth. This fat serves two essential purposes: it provides an energy reserve for the first days of life, when feeding is just getting established, and it helps regulate body temperature after birth.
A specialized type of fat called brown fat is especially important. Unlike regular fat, brown fat generates heat. It can produce up to 300 times more heat per unit of mass than any other tissue in the body. Brown fat activates for the first time right at birth, when the baby is suddenly exposed to air temperatures far cooler than the womb. Without adequate brown fat stores, newborns struggle to maintain their body temperature.
Beyond fat, the final month also brings the last stages of lung development. The lungs are among the last organs to fully mature, typically reaching readiness around 37 weeks. This is why babies born even a few weeks early sometimes need breathing support, while those born at 39 or 40 weeks rarely do.
Why Some Babies Gain More or Less
The half-pound-per-week average is just that: an average. Several factors push individual babies above or below that number.
Genetics play a large role. Taller parents tend to have longer, heavier babies. Babies who will eventually be larger adults are often already bigger in the womb. Gestational diabetes is another major factor. When a mother’s blood sugar runs high, the baby receives more glucose than it needs and stores the excess as fat. This can produce a condition called macrosomia, defined as a birth weight above 8 pounds 13 ounces (4,000 grams), though many researchers use a stricter threshold of 9 pounds 4 ounces (4,500 grams).
On the other side, conditions that restrict blood flow through the placenta can slow growth. The placenta shows measurable signs of aging as pregnancy progresses, with increasing oxidative damage to its cells in the final weeks. In most pregnancies this aging doesn’t cause problems, but in some cases the placenta becomes less effective at delivering oxygen and nutrients, which can limit how much weight the baby gains. This is one reason providers monitor fetal growth closely past the due date.
First babies also tend to weigh slightly less at birth than subsequent siblings, typically by a few ounces. And male babies average about 5 ounces heavier than females at birth.
How Accurately Ultrasound Predicts Weight
If you’ve had a late-pregnancy ultrasound and were told your baby’s estimated weight, it’s worth knowing how precise that number really is. Ultrasound weight estimates at term have a margin of error of about 10 to 15%. In a large study of scans done at 37 weeks or later, about 73% of estimates fell within 10% of the actual birth weight, and roughly 90% were within 15%.
In practical terms, if an ultrasound estimates your baby at 7 pounds, the actual weight could reasonably be anywhere from about 6 pounds 5 ounces to 7 pounds 11 ounces. Estimates tend to skew slightly high, with an average overestimation of about 2 ounces. This matters because an ultrasound showing a “big baby” in the final weeks isn’t always as alarming as it sounds. The prediction could easily be off by a pound in either direction.
Why the Last Few Weeks Matter
It can be tempting to think that a baby who has reached a healthy weight by 37 or 38 weeks is “done” growing, but the final two weeks of a full-term pregnancy contribute more than just extra weight. Brain development accelerates dramatically between 37 and 40 weeks. The brain grows by roughly a third of its volume during the last few weeks alone. Fat deposition under the skin continues, giving the baby a rounder appearance and better ability to stay warm. Liver function matures, building up iron stores that the baby will rely on for the first several months of life.
The distinction between “early term” (37 to 38 weeks) and “full term” (39 to 40 weeks) exists precisely because of this late-stage development. Babies born at 39 weeks have measurably fewer breathing difficulties, feeding problems, and temperature regulation issues than those born at 37 weeks, even though both are considered term pregnancies. Those extra two weeks of weight gain and organ maturation make a meaningful difference in how smoothly the transition to life outside the womb goes.