Your baby’s percentile is determined by ultrasound measurements that compare your baby’s estimated size to thousands of other babies at the same gestational age. If your baby is in the 40th percentile, for example, that means 40% of babies at the same week of pregnancy are smaller and 60% are larger. You won’t find your baby’s percentile from a home calculator alone. It comes from specific measurements your ultrasound technician takes during a growth scan, which your provider then plots on a standardized growth chart.
How Your Baby’s Percentile Is Calculated
During an ultrasound, the technician takes several measurements of your baby’s body. In the first trimester, the most accurate measurement is crown-rump length, which is the distance from the top of your baby’s head to their bottom. After about 12 weeks, four other measurements become more useful: the diameter of the head, the circumference of the head, the circumference of the belly, and the length of the thigh bone.
These measurements get plugged into a formula that produces an estimated fetal weight. That weight is then compared to a reference chart for your exact week of pregnancy, giving you a percentile. The key word is “estimated.” Ultrasound weight estimates can be off by 10% to 15% in either direction, so a baby estimated at the 30th percentile could actually be closer to the 40th or 20th.
What Counts as a Normal Percentile
In routine obstetric care, a baby measuring at or above the 10th percentile is generally considered normal, while anything below the 10th percentile is flagged as small for gestational age. On the other end, a baby above the 90th percentile is considered large for gestational age. The range most commonly used as the reference for normal growth is between the 20th and 90th percentiles.
That said, percentile cutoffs are not rigid boundaries. Research increasingly shows that the relationship between fetal size and health risk follows a continuum rather than flipping at a single threshold. Babies measuring between the 10th and 20th percentiles, for instance, are technically in the “normal” range but may still carry a slightly higher risk of complications compared to babies in the 30th to 70th range. There is no magic number where risk disappears entirely.
The 50th percentile is not a goal. A healthy baby can sit at the 25th percentile their entire pregnancy and be perfectly fine. What matters more than any single number is whether your baby’s growth is consistent over time. A baby who has been tracking along the 30th percentile and suddenly drops to the 10th is more concerning than a baby who has measured at the 12th percentile all along.
Which Growth Chart Your Provider Uses
Your baby’s percentile can actually shift depending on which growth chart is used. The two most widely used international standards are the WHO fetal growth chart and the INTERGROWTH-21st chart. A study from China found that the WHO chart identified 50% more small-for-gestational-age babies than the INTERGROWTH-21st chart did, meaning the same baby could be flagged as small on one chart but not the other.
Recent research suggests the WHO chart may be slightly better at distinguishing babies who are truly small from those who are not. Some countries and hospitals also use older charts like the Hadlock formula (which the WHO chart is based on) or customized charts that adjust for your height, weight, ethnicity, and whether this is your first pregnancy. If you’re comparing percentile numbers from two different appointments or two different providers, it’s worth asking whether they’re using the same chart.
Why Some Babies Measure Small
About 40% of babies who measure below the 10th percentile have no underlying health problem at all. They’re constitutionally small, meaning their parents are smaller or their family tends to produce smaller babies. This is especially common among babies who are small at full term rather than preterm.
When there is a medical cause, the most common one is a problem with blood flow through the placenta, which limits how much oxygen and nutrients reach the baby. Maternal health conditions can also play a role, including high blood pressure, diabetes, severe anemia, and autoimmune disorders. Smoking, alcohol use, cocaine, and low maternal weight gain during pregnancy are linked to smaller babies as well. Genetic conditions account for about 5% of cases, and infections another 5% to 10%.
Mothers who were themselves growth-restricted as babies carry roughly double the risk of having a growth-restricted baby.
Why Some Babies Measure Large
A baby above the 90th percentile is classified as large for gestational age. The most common cause is maternal diabetes, whether it existed before pregnancy or developed during pregnancy as gestational diabetes. When blood sugar runs high, the baby receives more glucose than it needs and stores the extra as fat, particularly around the shoulders and trunk.
This pattern of growth matters for delivery. Babies who are large because of diabetes tend to have disproportionately broad shoulders, which increases the risk of shoulder dystocia, a complication where one or both shoulders get stuck during vaginal birth. Other potential complications include a longer pushing phase, vaginal tearing, and heavier postpartum bleeding. Not every large baby causes problems during delivery, but your provider will factor the baby’s estimated size into your birth plan.
What Happens If Your Baby’s Percentile Is Concerning
If your baby measures below the 10th percentile, your provider will typically schedule follow-up growth scans every two to four weeks to track whether the baby continues growing at a steady pace. Babies between the 3rd and 10th percentile with no other warning signs, like normal blood flow patterns on Doppler ultrasound, are often healthy but small. These pregnancies are usually monitored with serial scans and delivery is commonly offered around 39 weeks.
For higher-risk situations, scans can happen as frequently as every two weeks starting as early as 24 weeks. The minimum monitoring schedule for babies with growth concerns is typically three scans: around 28 weeks, 32 weeks, and 36 to 37 weeks. Scans are never done more frequently than every 14 days because the baby needs enough time between measurements for detectable growth to occur.
Babies below the 3rd percentile carry a meaningfully higher risk of complications. Research on term infants found that neonatal health problems and mortality were significantly elevated at or below the 3rd percentile compared to babies in the middle of the growth curve. The lower the percentile and the earlier in pregnancy it’s detected, the more closely your care team will monitor.
What You Can Take Away From the Number
Your baby’s percentile is a screening tool, not a diagnosis. It tells your provider whether to look more closely, not whether something is definitively wrong. A single percentile reading is less informative than the trend across multiple scans. If your provider hasn’t expressed concern, your baby is likely growing appropriately for their gestational age.
If you want to know your baby’s specific percentile, ask at your next ultrasound appointment. Most providers will share the estimated fetal weight and the corresponding percentile, and they can tell you which growth chart they’re using. The number on its own is neutral information. What gives it meaning is context: your health history, your baby’s growth pattern over time, and whether other markers like blood flow and amniotic fluid volume look normal.