In the final month of pregnancy (roughly weeks 36 through 40), a baby typically gains about half a pound per week, though that rate gradually slows as the due date approaches. By 40 weeks, growth averages around 185 grams (about 6.5 ounces) per week. In total, most babies put on roughly 2 to 3 pounds during this last stretch, arriving at an average birth weight of about 7.5 pounds.
Week-by-Week Growth in the Final Month
At 36 weeks, a baby weighs roughly 5.5 to 6 pounds. Fat is filling in under the skin, and the limbs start to look noticeably chubby. By week 37, the baby can grasp firmly and may begin moving head-down into the pelvis in preparation for birth.
At 38 weeks, the head and belly circumference are roughly equal. The toenails have reached the tips of the toes, and most of the fine body hair (the downy coating that covered the skin earlier in pregnancy) has disappeared. By week 39, the chest is getting larger and fat continues to accumulate across the body. At 40 weeks, the average baby measures about 14 inches from crown to rump and weighs around 7.5 pounds (3,400 grams).
The rate of gain isn’t constant. Earlier in the third trimester, babies can pack on close to 8 ounces a week. By 40 weeks, that pace drops to about 6.5 ounces per week. This natural slowdown is partly because the placenta has a ceiling on how many nutrients it can deliver, and partly because the baby is running out of room.
Why the Last Month Matters So Much
Most of this late weight gain is body fat, and it serves a specific purpose: keeping the baby warm after birth. Newborns lose heat quickly, and the fat layer built up in the final weeks acts as insulation. It also provides a short-term energy reserve while breastfeeding or bottle-feeding gets established in the first days of life.
Weight isn’t the only thing developing. The brain is growing rapidly during these final weeks, and the lungs are completing their maturation. Babies born in the early-term window (37 to 38 weeks) have a higher risk of breathing problems than those born at 39 or 40 weeks, which is one reason providers avoid elective early deliveries when possible. Each additional week in this final stretch helps the baby prepare for life outside the womb.
What Affects How Much Your Baby Gains
Not every baby follows the same growth curve. Several factors can push weight gain higher or lower in the final month:
- Placental health. The placenta is the baby’s sole supply line for oxygen and nutrients. When blood flow through the placenta is compromised, a condition called placental insufficiency, the baby may gain weight more slowly. Abnormal blood flow patterns in the umbilical artery are one sign providers watch for.
- Gestational diabetes. Higher-than-normal blood sugar in the parent can cause the baby to grow larger than average, sometimes gaining more than expected in the final weeks.
- Preeclampsia. This blood pressure condition is present in about 35% of pregnancies affected by early-onset growth restriction. It can limit nutrient delivery and slow fetal growth.
- Chronic health conditions and substance use. Conditions like high blood pressure or kidney disease, along with smoking or other substance use, can restrict how well the placenta functions.
- Genetics. Parental size plays a role. Taller or larger parents tend to have bigger babies, and this becomes more apparent in the final growth phase.
If a provider suspects a baby isn’t growing on track, they’ll typically order serial growth ultrasounds to monitor weight gain over time rather than relying on a single measurement.
How Accurate Are Late-Pregnancy Weight Estimates
If you’ve had a late ultrasound that estimated your baby’s weight, it’s worth knowing that these numbers come with a margin of error. At 37 weeks or later, about 73% of ultrasound weight estimates fall within 10% of the actual birth weight. Around 90% are within 15%, and about 96% are within 20%.
In practical terms, if your ultrasound says 7 pounds, the actual weight could reasonably be anywhere from about 6.3 to 7.7 pounds, and occasionally further off than that. This means a single scan isn’t precise enough to predict exact birth weight. It’s more useful as a general range, and it’s one reason providers look at trends across multiple scans rather than treating one number as definitive. If you’ve been told your baby is “measuring big” or “measuring small,” the real weight could still be well within the normal range.
What “Normal” Looks Like
Full-term babies range widely in birth weight, from about 5.5 to 10 pounds, and most of that variation is perfectly healthy. A baby who gains 2 pounds in the last month and one who gains 3.5 pounds can both be right on track for their individual growth curve. What matters more than a single number is whether the baby’s growth is consistent over time.
The average total gain over the last four weeks works out to roughly 1.5 to 2.5 pounds for most babies, landing them at that 7- to 8-pound range by 40 weeks. Babies born a week or two early will naturally weigh a bit less, simply because they missed some of that final accumulation of fat and body mass.