At 24 weeks of pregnancy, a baby typically weighs about 1⅓ pounds (630 grams) and measures roughly 8¼ inches long from head to rump. That’s about the size of an ear of corn. This is an average, and healthy babies can fall above or below this number depending on genetics, maternal health, and other factors.
What 24-Week Size Estimates Actually Mean
The weight your doctor gives you at 24 weeks comes from an ultrasound estimate, not a direct measurement. The technician measures your baby’s head, abdomen, and thigh bone, and software calculates an estimated weight from those numbers. These estimates can be off by 10 to 15 percent in either direction, so a reading of 600 grams or 680 grams at this stage is well within normal range.
Fetal growth charts now use day-specific standards rather than broad weekly averages. A baby measured at 24 weeks and 1 day will have a slightly different expected weight than one measured at 24 weeks and 6 days. Your provider plots your baby’s estimated weight on a percentile curve, similar to the growth charts used for children after birth. A baby between the 10th and 90th percentiles is generally considered appropriately grown.
Why 24 Weeks Is a Significant Milestone
At 24 weeks, the lungs are beginning to produce a substance that will eventually help them inflate after birth, though they’re still far too immature to function on their own. The brain is growing rapidly, and the baby is developing more coordinated movements. You may notice stronger, more regular kicks around this time.
This week also marks an important threshold for survival outside the womb. Babies born at 24 weeks have survival rates between 42 and 59 percent with intensive neonatal care, according to the American College of Obstetricians and Gynecologists. That’s a significant jump from 23 weeks, where survival drops to 23 to 27 percent. By 25 weeks, the odds improve again to 67 to 76 percent. Every additional day in the womb at this stage meaningfully improves outcomes.
What Can Cause a Baby to Measure Small
If your baby is measuring below the 10th percentile for gestational age, your provider may evaluate for fetal growth restriction. This doesn’t always mean something is wrong, as some babies are simply small. But in some cases it signals that the baby isn’t getting enough nutrients or oxygen to grow at the expected rate.
Placental problems are one of the most common causes. If the placenta doesn’t attach well to the uterine wall or blood flow through the umbilical cord is limited, the baby receives fewer resources for growth. Maternal health conditions can also play a role, including high blood pressure, diabetes, anemia, kidney disease, and autoimmune conditions like lupus.
Lifestyle factors matter too. Smoking, alcohol use, drug use, and poor nutrition are all linked to smaller babies. On the flip side, being significantly underweight before pregnancy or gaining very little weight during pregnancy can contribute. Carrying twins or triplets also means each baby typically weighs less than a singleton at the same gestational age.
Some causes originate with the baby itself. Certain infections, heart defects, or chromosomal differences can slow growth. Your doctor will look at the overall pattern of growth across multiple appointments rather than relying on a single measurement.
What Can Cause a Baby to Measure Large
A baby measuring above the 90th percentile may be described as large for gestational age. The most common cause is gestational diabetes or pre-existing diabetes, where elevated blood sugar in the mother leads the baby to store extra fat and grow faster than usual. Genetics also plays a straightforward role: taller or larger parents tend to have bigger babies.
Excessive maternal weight gain during pregnancy can contribute as well. In many cases, though, a baby measuring large at 24 weeks simply reflects normal variation. Your provider will track the growth trend over time before drawing conclusions.
How Growth Is Monitored From Here
Most pregnancies don’t require frequent ultrasounds to track growth. If your baby is measuring within the expected range at your anatomy scan (usually done around 18 to 22 weeks), your provider will monitor growth through fundal height measurements at regular prenatal visits. This is the simple tape measure check of your belly from the pubic bone to the top of the uterus.
If there’s a concern about growth, whether too small or too large, your provider will order additional ultrasounds spaced two to three weeks apart to see how quickly the baby is gaining weight. The trend matters more than any single number. A baby consistently tracking along the 15th percentile is very different from one that drops from the 50th to the 15th over a few weeks.
Between 24 weeks and delivery, your baby will roughly triple in weight. Most of the fat that insulates the body and helps regulate temperature after birth is added in the third trimester, so the most dramatic weight gain is still ahead.