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Estimate fetal growth and detect potential complications using Hadlock standards
Based on ultrasound measurements
0.0th
For gestational age
0 g
50th percentile for 28 weeks
SGA (Small for Gestational Age):
Below 10th percentile - may indicate IUGR (intrauterine growth restriction)
AGA (Appropriate for Gestational Age):
10th to 90th percentile - normal fetal growth pattern
LGA (Large for Gestational Age):
Above 90th percentile - may indicate macrosomia or gestational diabetes
A fetal weight percentile calculator is an obstetric tool used to assess whether a developing baby's weight is appropriate for their gestational age. During prenatal ultrasounds, healthcare providers measure various fetal biometric parameters including biparietal diameter (head width), head circumference, abdominal circumference, and femur length. These measurements are entered into formulas, most commonly the Hadlock formula, to estimate fetal weight. The calculator then compares this estimated weight to standardized growth charts to determine the percentile. This percentile indicates what percentage of fetuses at the same gestational age weigh less than the baby being assessed. Fetal weight percentiles are crucial for identifying growth abnormalities early in pregnancy. Babies measuring below the 10th percentile are classified as small for gestational age (SGA) and may have intrauterine growth restriction (IUGR), which can result from placental insufficiency, maternal health conditions, or genetic factors. Fetuses above the 90th percentile are considered large for gestational age (LGA), which may indicate maternal diabetes or genetic predisposition to larger size. Regular monitoring of fetal weight percentiles throughout the third trimester helps obstetricians make informed decisions about delivery timing and method, ensuring the best outcomes for both mother and baby.
Calculating fetal weight percentiles begins with a detailed ultrasound examination performed by a trained sonographer or perinatologist. During the scan, four key measurements are obtained: biparietal diameter (BPD), head circumference (HC), abdominal circumference (AC), and femur length (FL). These measurements are taken according to standardized protocols to ensure accuracy. The most widely used formula for estimating fetal weight is the Hadlock formula, which incorporates all four measurements: log10(EFW) = 1.335 - 0.0034(AC)(FL) + 0.0316(BPD) + 0.0457(AC) + 0.1623(FL), where measurements are in centimeters and estimated fetal weight (EFW) is in grams. Alternative formulas include Shepard and Warsof methods. Once the estimated weight is calculated, it's compared to gestational age-specific reference curves, typically Hadlock growth charts or WHO fetal growth standards. The gestational age must be accurately determined, preferably from a first-trimester dating ultrasound. The percentile is calculated using statistical methods that account for the normal distribution of fetal weights at each week of pregnancy. It's important to understand that ultrasound weight estimates have a margin of error of approximately 10-15%, so a single measurement should not cause alarm. Serial ultrasounds tracking growth velocity over time provide more reliable information than isolated percentiles, as the growth trend is more clinically significant than absolute measurements.
Intrauterine growth restriction (IUGR) occurs when a fetus fails to achieve its genetic growth potential, typically defined as estimated fetal weight below the 10th percentile. IUGR can have serious implications for fetal and neonatal health.
| Gestational Age | Weight (grams) | Weight (lbs/oz) | 10th Percentile | 90th Percentile |
|---|---|---|---|---|
| 20 weeks | 300 g | 10.6 oz | 255 g | 345 g |
| 24 weeks | 600 g | 1 lb 5 oz | 510 g | 690 g |
| 28 weeks | 1000 g | 2 lb 3 oz | 850 g | 1150 g |
| 32 weeks | 1700 g | 3 lb 12 oz | 1445 g | 1955 g |
| 36 weeks | 2600 g | 5 lb 12 oz | 2210 g | 2990 g |
| 40 weeks | 3400 g | 7 lb 8 oz | 2890 g | 3910 g |
Ultrasound weight estimates have a typical accuracy of ±10-15% (about ±1 pound at term). This means a baby estimated at 7 pounds could actually weigh anywhere from 6 to 8 pounds. Accuracy decreases with maternal obesity, low amniotic fluid, and fetal position.
A baby measuring below the 10th percentile may be small for gestational age (SGA). This could indicate IUGR requiring monitoring, or the baby may simply be constitutionally small but healthy. Your doctor will use additional tests like Doppler ultrasound to differentiate.
Large for gestational age babies (above 90th percentile) may result from gestational diabetes, maternal obesity, genetic factors, or simply being constitutionally large. LGA babies have increased risks of birth trauma, cesarean delivery, and metabolic issues at birth.
Yes, percentiles can change. Some babies start small but catch up (asymmetric IUGR), while others may develop growth problems later. This is why serial ultrasounds tracking growth velocity over time are more informative than single measurements.
Routine anatomy scans at 18-22 weeks include weight estimation. Additional growth scans are performed if concerns arise, typically every 3-4 weeks. High-risk pregnancies may have more frequent monitoring throughout the third trimester.
Not necessarily. Some babies are constitutionally small due to genetics (small parents often have small babies). True IUGR where the baby fails to reach growth potential due to pathology requires differentiation through Doppler studies and other testing.
Treatment depends on the cause. Ensuring adequate nutrition, managing chronic conditions, stopping smoking, and treating maternal health issues can help. However, placental insufficiency cannot be reversed, so monitoring and timely delivery are key.
It depends on severity and gestational age. Severe IUGR with concerning Doppler studies or non-reactive testing may warrant early delivery. LGA babies with poorly controlled gestational diabetes might also be delivered early. Your perinatologist will weigh risks versus benefits.
This calculator is for educational purposes only and does not replace professional prenatal care. Fetal weight estimates and percentiles must be interpreted by qualified obstetricians or perinatologists in the context of complete clinical assessment. Ultrasound measurements have inherent limitations and margins of error. Always consult your healthcare provider for medical decisions regarding pregnancy management and delivery planning.