In the final month of pregnancy, a baby typically gains between 8 and 12 ounces per week, or roughly 2 to 3 pounds total over those last four weeks. This period, starting around week 35, is the fastest stretch of weight gain in the entire pregnancy. Most babies weigh somewhere between 6 and 9 pounds at birth, and a significant chunk of that weight accumulates in these final weeks.
Weekly Weight Gain After Week 35
Around week 35, a baby enters its most rapid period of weight gain. At half a pound to three-quarters of a pound each week, the baby is putting on weight faster than at any other point in pregnancy. To put that in perspective, at the start of the third trimester (around week 28), the fetus weighs roughly 2 to 2.5 pounds. By the end of the last month, that number has climbed to an average of about 7.5 pounds.
Much of this weight comes from fat deposits building up under the baby’s skin. Earlier in pregnancy, a fetus is lean and translucent. In the final weeks, that layer of fat fills out the baby’s body, smoothing the skin and providing insulation that will help regulate body temperature after birth. The brain also grows rapidly during this stretch, roughly doubling in weight between weeks 35 and 39. Lungs continue maturing too, producing the substance that keeps the tiny air sacs from collapsing once the baby starts breathing on its own.
What Drives the Final Growth Spurt
The dramatic weight gain in the last month isn’t random. Your body actively shifts its metabolism to funnel more energy to the baby. In the final 12 weeks of pregnancy, your sensitivity to insulin naturally decreases, which keeps more glucose circulating in your bloodstream and available for the baby. During this window, the average fetus grows from about 2.2 pounds to 7.7 pounds.
The placenta plays a central role as well. It produces large quantities of a hormone called leptin, which regulates energy balance and correlates with both maternal body fat and metabolic rate. Think of the placenta as a logistics hub: it manages the flow of nutrients, oxygen, and hormones that determine how efficiently your baby can convert calories into growth.
Several maternal factors influence whether a baby lands on the higher or lower end of that 8-to-12-ounce weekly range:
- Pre-pregnancy weight. Women who start pregnancy at a higher BMI tend to gain less total pregnancy weight themselves, but their babies may still grow larger due to increased nutrient availability.
- Nutrition. The balance of carbohydrates, protein, and fat in your diet becomes a more significant predictor of weight gain later in pregnancy, particularly for women who were overweight before conceiving.
- Physical activity. Women who continue exercising at least three times per week through pregnancy tend to gain less body weight and subcutaneous fat during the third trimester compared to women who stop exercising, though fetal growth generally remains healthy in both groups.
- Placental function. A placenta that isn’t delivering nutrients efficiently can slow fetal growth, while conditions like gestational diabetes can accelerate it.
How Weight Gain Differs With Twins
If you’re carrying twins, the math changes substantially. Each twin individually tends to be smaller than a singleton baby at birth, but the combined weight gain is greater. Mothers of twins gain an average of 4 to 9 extra pounds compared to mothers carrying one baby, and total pregnancy weight gain averages around 44 pounds for twin pregnancies with good outcomes.
During the second and third trimesters, a woman carrying twins should expect to gain roughly 1.5 pounds per week (total maternal weight gain, not just baby weight). Twin pregnancies also tend to deliver earlier, often around 36 to 37 weeks, which means the babies have less time in that final rapid-gain phase. This is one reason twins are frequently smaller at birth, typically 5 to 6 pounds each rather than the 7-plus pounds common in singletons.
How Your Provider Tracks Growth
At your late-pregnancy appointments, your provider likely measures from your pubic bone to the top of your uterus with a tape measure. This is called fundal height, and it’s a quick screening tool, but it’s not especially accurate. Research shows it catches fewer than half of babies who are measuring small for their gestational age. For larger-than-expected babies, it performs slightly better but still misses about half.
Maternal obesity makes these measurements even less reliable, because the tape is measuring through more tissue. When there’s a concern about growth, your provider will typically order an ultrasound, which can estimate fetal weight more precisely by measuring the baby’s head, abdomen, and thigh bone. Even ultrasound estimates carry a margin of error of about 10 to 15 percent, so a baby estimated at 7 pounds could reasonably be anywhere from about 6 to 8 pounds.
When Growth Falls Outside the Range
Not every baby follows the textbook pattern, and that’s not always a problem. Genetics play a large role: smaller parents tend to have smaller babies. But growth that falls below the 10th percentile or above the 90th percentile for gestational age gets closer attention because it can signal issues with placental function, blood sugar regulation, or nutrient delivery.
A baby that isn’t gaining enough in the final weeks may not be getting adequate blood flow through the placenta. Your provider might recommend more frequent monitoring, including ultrasounds to check amniotic fluid levels and blood flow patterns in the umbilical cord. On the other end, a baby gaining too quickly could indicate uncontrolled gestational diabetes, which increases the risk of delivery complications.
If your baby’s growth pattern changes suddenly in the last month, or if you notice a significant decrease in fetal movement, those are signals worth raising with your provider promptly. Movement patterns are one of the most reliable indicators you have at home that your baby is doing well in those final, fast-growing weeks.