During the last month of pregnancy (weeks 36 to 40), babies gain roughly 1.8 to 2 pounds. That works out to about half a pound per week, though the rate slows slightly as the due date approaches. This final stretch of growth is less about getting longer and more about filling out, building fat stores, and preparing for life outside the womb.
Week-by-Week Weight in the Final Month
At 36 weeks, the average baby weighs about 5.8 pounds. From there, growth follows a fairly predictable curve:
- Week 37: 6.3 pounds
- Week 38: 6.8 pounds
- Week 39: 7.25 pounds
- Week 40: 7.6 pounds
The biggest weekly jumps happen around weeks 36 to 38, when babies put on roughly half a pound each week. Growth tapers slightly in weeks 39 and 40, dropping closer to 6 or 7 ounces per week. This slowdown is normal. The placenta’s capacity to deliver nutrients plateaus near the end of pregnancy, and the baby is running out of room to grow.
What All That Weight Actually Is
Most of the weight gained in the last month comes from fat, not from organs or bone. By this point, the baby’s major organ systems are already formed. The final weeks are about insulation and energy reserves.
A significant portion of that new fat is a special type called brown fat, which sits around the shoulders, neck, and kidneys. Unlike regular body fat, brown fat generates heat. At full capacity, it can produce up to 300 times more heat per unit of mass than any other tissue in the body. Babies need this because they’re about to go from a perfectly regulated 98.6°F environment to the much cooler delivery room. Brown fat is fully developed at birth and activates immediately upon exposure to cooler air, helping newborns maintain their body temperature during those critical first hours.
The baby is also building up stores of glycogen (an energy reserve in the liver) and continuing to mature the lungs, which are among the last organs to fully develop. Babies born at 36 weeks sometimes need breathing support precisely because those final weeks of lung maturation hadn’t finished.
Factors That Affect Late-Term Growth
Not every baby follows the average curve. Several factors can push weight gain higher or lower during the final month.
Gestational diabetes is one of the most common reasons babies grow larger than expected. When a mother’s blood sugar runs high, the baby receives extra glucose and converts it to fat, sometimes gaining weight faster than the half-pound-per-week average. Babies weighing over 8 pounds 13 ounces (4,000 grams) at birth are considered large for gestational age, a threshold that carries higher risks for delivery complications.
On the other end, placental insufficiency can restrict how much nutrition reaches the baby. When the placenta isn’t working efficiently, blood flow through the umbilical cord drops and growth slows. Preeclampsia, a condition marked by high blood pressure, is present in about 12% of growth restriction cases diagnosed after 32 weeks. Smoking, chronic high blood pressure, and certain autoimmune conditions can also limit placental function and slow late-term weight gain.
Genetics plays a straightforward role too. Taller parents tend to have longer, heavier babies. First babies tend to weigh slightly less than subsequent siblings, and boys typically weigh a few ounces more than girls at birth.
Twins Gain Weight Differently
If you’re carrying twins, expect each baby to be smaller than a singleton at every point in the final month. Twins share the same blood supply and uterine space, which naturally limits how much each one can grow. Individual twins commonly weigh 5 to 6 pounds at 38 weeks, compared to nearly 7 pounds for a singleton at the same point. Their growth rate also slows earlier, often tapering around weeks 35 to 36 rather than weeks 39 to 40. This is one reason twins are frequently delivered a week or two before the standard due date.
How Accurate Are Late Ultrasound Estimates
If your provider orders an ultrasound to estimate your baby’s size in the final weeks, keep in mind that these measurements come with a meaningful margin of error. About 73% of ultrasound weight estimates at term fall within 10% of the actual birth weight. That means for a baby who ends up weighing 7.5 pounds, the ultrasound estimate could be anywhere from 6.75 to 8.25 pounds and still be considered within normal accuracy. Roughly 90% of estimates land within 15% of actual weight, and about 96% are within 20%.
On average, late-term ultrasounds slightly overestimate weight by about 2 ounces. So if your scan says 8 pounds, the baby may be closer to 7 pounds 14 ounces. This imprecision matters because a baby estimated at just over 9 pounds might prompt a conversation about early delivery or a planned cesarean, when the actual weight could be well under that threshold. It’s worth asking your provider how much the estimate might vary before making decisions based on a single number.
What You Might Notice in the Last Month
As your baby puts on nearly two pounds, you’ll likely feel the difference. Many women notice increased pressure on the bladder and pelvis, especially after the baby “drops” lower into the birth canal (a process called lightening that can happen anywhere from a few weeks to a few hours before labor). Movements may feel different too. Instead of big kicks and rolls, you might feel more pushing, stretching, and squirming as the baby runs out of room. The total number of movements should stay consistent, but the quality changes.
Your own weight gain during this final month typically ranges from 2 to 4 pounds total, which includes the baby’s growth plus additional fluid and blood volume. Some women actually stop gaining or lose a pound or two in the very last week as fluid levels shift before labor begins.