At 37 weeks pregnant, your baby is about 19 inches long and weighs roughly 6.3 pounds, or around 2,860 grams. That’s close to the size of a small watermelon or a bunch of romaine lettuce. Your baby is nearly at birth size, though important development is still happening in these final weeks.
Average Weight and Length at 37 Weeks
The average 37-week fetus measures about 48.6 centimeters (just over 19 inches) from head to heel and weighs around 6.3 pounds. But “average” covers a wide range. Birth weight data shows that a healthy boy at 37 weeks can weigh anywhere from about 5 pounds 10 ounces (10th percentile) to 8 pounds 1 ounce (90th percentile), with the middle landing around 6 pounds 12 ounces. Girls tend to run slightly smaller: the 10th percentile sits near 5 pounds 6 ounces, the middle around 6 pounds 9 ounces, and the 90th percentile at about 7 pounds 13 ounces.
If your provider has given you an estimated fetal weight from an ultrasound, keep in mind those estimates can be off by as much as 15 percent in either direction. A baby measuring on the smaller or larger end of the range isn’t necessarily a concern. Your provider looks at your baby’s growth trend over several weeks, not a single snapshot.
Why 37 Weeks Is “Early Term,” Not Full Term
You might have heard that 37 weeks counts as full term. That definition changed. The American College of Obstetricians and Gynecologists now classifies 37 weeks through 38 weeks and 6 days as “early term.” Full term starts at 39 weeks.
The distinction matters because your baby’s lungs, liver, and brain go through a crucial growth period between weeks 37 and 39. Babies born before 39 weeks have higher rates of breathing difficulties, feeding problems, and trouble regulating their body temperature. They’re also more likely to need time in the neonatal intensive care unit and are at slightly higher risk for infections and learning difficulties later on. This is why most providers avoid scheduling elective deliveries before 39 weeks unless there’s a medical reason.
What’s Developing at 37 Weeks
Your baby looks like a newborn at this point, but the finishing work is still underway. The lungs are close to fully developed, though they continue producing a substance that keeps the tiny air sacs from collapsing after birth. The brain is rapidly building new connections and adding a layer of protective fat around nerve fibers, a process that sharpens coordination and reflexes. These last few weeks of brain growth are significant: the brain at 35 weeks weighs only about two-thirds of what it will weigh at 39 or 40 weeks.
Your baby’s toenails have reached the tips of the toes, and fat continues to fill out beneath the skin, giving your baby the rounder appearance you’ll see at birth. That fat layer also helps with temperature regulation once your baby is outside the womb.
What You Might Notice in Your Body
Around 37 weeks, many babies drop lower into the pelvis in preparation for labor. You may feel this as a sudden ability to breathe more easily, since there’s less pressure on your diaphragm. At the same time, the lower position puts more weight on your bladder, so more frequent bathroom trips are common.
Your provider has likely been measuring your fundal height (the distance from your pubic bone to the top of your uterus) at each appointment. That measurement typically tracks with your week of pregnancy, so around 36 to 37 centimeters at 36 weeks. After 36 weeks, the measurement can actually decrease as the baby descends into the pelvis. A drop in fundal height at this stage is normal and doesn’t mean your baby has stopped growing.
Amniotic fluid volume also begins to gradually decline in the final weeks. At 37 weeks, a normal amniotic fluid index ranges from about 6 to 15 centimeters, with the midpoint around 11 centimeters. Your provider may monitor fluid levels if there’s reason to, but a modest decrease from earlier in pregnancy is expected.
Factors That Affect Your Baby’s Size
Several things influence how big your baby measures at 37 weeks. Genetics is the largest factor: taller parents tend to have longer babies, and birth weight often runs in families. First babies tend to be slightly smaller than siblings born later. Gestational diabetes can lead to larger-than-average babies because extra glucose crosses the placenta and stimulates the baby to store more fat. High blood pressure, on the other hand, can restrict blood flow to the placenta and result in a smaller baby.
If you’re carrying twins or multiples, each baby will almost certainly be smaller than a singleton at the same gestational age. The averages above apply specifically to single pregnancies. Your provider will use growth charts designed for multiples if that’s your situation.
What “Small” or “Large” for Gestational Age Means
If your baby’s estimated weight falls below the 10th percentile (roughly under 5.5 pounds at 37 weeks), your provider may describe them as small for gestational age. This doesn’t always signal a problem. Some babies are constitutionally small, meaning they’re healthy but genetically predisposed to be on the petite side. Other times, it can indicate the placenta isn’t delivering enough nutrients, and your provider will want closer monitoring with additional ultrasounds or fetal heart rate checks.
On the other end, a baby above the 90th percentile (over about 8 pounds at 37 weeks) is considered large for gestational age. This is more common with gestational diabetes or significant weight gain during pregnancy. Your provider may discuss delivery timing and method if a very large baby is expected, though ultrasound weight estimates become less precise at higher weights.