In the last month of pregnancy (roughly weeks 36 through 40), a baby typically gains about 700 to 750 grams, or around 1.5 pounds. That takes the average baby from about 5.7 pounds at week 36 to roughly 7.4 pounds at week 40, based on WHO growth standards. While that number might seem modest compared to earlier growth spurts, this final stretch is about far more than just adding weight.
Week-by-Week Weight Gain
Fetal growth doesn’t happen at a steady pace. Babies hit their maximum growth rate around week 35, putting on nearly 220 grams (about 7.8 ounces) per week. After that peak, the rate gradually slows. By week 40, average weekly gain drops to about 185 grams (6.5 ounces). So while your baby is still growing in the final month, the pace is tapering compared to just a few weeks earlier.
At week 38, babies are packing on roughly half a pound per week as they approach their final birth size. This is a consistent enough pattern that providers use it to estimate expected birth weight when scheduling deliveries or monitoring growth on ultrasound.
What’s Happening Beyond Weight
The last month isn’t really about getting bigger. It’s about getting ready. The brain is the standout story here. At week 35, a baby’s brain weighs only about two-thirds of what it will at birth. That means a full third of brain growth by weight happens in the final five weeks. Month nine and the early days of month ten are largely dedicated to finishing brain development and fine-tuning the systems that will need to work immediately after birth.
Fat stores are also building rapidly during this time. These aren’t just padding. The fat layer a baby accumulates in the final weeks helps regulate body temperature after delivery, when the baby suddenly loses the warmth of the womb. Babies born a few weeks early often struggle with temperature regulation precisely because they missed this window of fat accumulation.
Why Growth Slows Near the Due Date
The placenta, which has been delivering oxygen and nutrients throughout pregnancy, begins to show signs of aging as the due date approaches. In prolonged pregnancies especially, the placenta’s ability to transport nutrients becomes less efficient. Blood vessel density within the placenta decreases, and the tissue shows increased oxidative stress and cellular wear. The ratio of baby size to placenta size shifts, meaning the placenta is working harder to support a larger baby with relatively fewer resources.
This is one reason growth naturally plateaus near week 40. It’s also why pregnancies that go well past the due date are monitored more closely. The placenta’s declining function is initially a normal part of the process, but it can become a limiting factor if pregnancy extends significantly beyond 40 weeks.
Factors That Shift the Numbers
Not every baby gains the same amount in the final month. Several maternal health factors can push growth higher or lower than average. High blood pressure, including preeclampsia, can restrict blood flow through the placenta, limiting the nutrients that reach the baby and slowing late-pregnancy weight gain. Preexisting diabetes tends to have the opposite effect, often producing larger-than-average babies because of elevated blood sugar crossing the placenta.
Maternal nutrition plays a straightforward role too. Significant calorie or nutrient deficiency in late pregnancy can contribute to growth restriction, though this is more common in resource-limited settings than in typical pregnancies where someone is eating regularly. Infections and exposure to certain toxins or substances can also impair the placenta’s ability to deliver nutrients efficiently.
Genetics matters as well. Taller parents tend to have larger babies, and birth weight often runs in families. A baby measuring slightly above or below average on ultrasound in the final weeks isn’t necessarily a concern if the growth trend has been consistent throughout pregnancy.
What “Normal” Looks Like at Birth
Full-term babies born between 39 and 40 weeks typically weigh between 6 and 9 pounds, with the average hovering around 7.5 pounds (about 3,400 grams). There’s a wide range of healthy birth weights, and where your baby falls depends on a combination of genetics, placental function, and the factors described above.
The timing of delivery within that final month makes a meaningful difference. A baby born at 37 weeks may weigh a full pound less than one born at 40 weeks, simply because of those three additional weeks of fat and tissue accumulation. This is one practical reason that elective deliveries are generally not recommended before 39 weeks unless medically necessary. Those last two to three weeks contribute both weight and critical organ maturation that affect how smoothly a baby transitions to life outside the womb.