Estimate fetal growth percentile by gestational age
Enter gestational age and estimated fetal weight (EFW) from ultrasound to compare with a typical reference range.
Educational tool only. It does not diagnose fetal growth restriction or macrosomia.
How this fetal growth calculator works
This fetal growth calculator estimates how your baby’s estimated fetal weight (EFW) compares with a typical reference distribution at the same gestational age. It gives you a practical way to see whether a measurement is likely below average, near average, or above average.
The tool uses gestational age in weeks and days, then interpolates expected weight values across pregnancy. If you provide an EFW value, it estimates percentile and category. If you also provide two scan weights and the interval between scans, it calculates approximate growth velocity in grams per week.
How to use the calculator
Step 1: Enter gestational age
Input the current pregnancy age using completed weeks and extra days. For example, if you are 31 weeks and 4 days, enter 31 and 4.
Step 2: Enter estimated fetal weight
Use the EFW number from your ultrasound report, usually reported in grams. The calculator compares this value with expected values for your gestational age and returns an approximate percentile.
Step 3 (optional): Track interval growth
To estimate growth velocity, enter a prior EFW and the number of days between scans. This helps you understand trend over time, which is often more informative than one isolated value.
Understanding the output
- Median weight (50th percentile): The middle expected value for that gestational age.
- 10th to 90th percentile range: A broad “typical” interval used for screening context.
- Estimated percentile: Where the entered weight sits in the distribution.
- Category: Below 10th, 10th-90th, or above 90th percentile.
Percentiles do not give a diagnosis by themselves. Clinicians combine serial ultrasounds, Doppler studies, placental assessment, maternal health factors, and clinical judgment before reaching conclusions.
Typical growth pattern by trimester
Second trimester
Growth tends to be steady, with increasing lean mass and organ maturation. Small week-to-week differences are common and often normal.
Third trimester
Weight gain accelerates in many pregnancies, then may slow near term. Some babies naturally track lower or higher curves while remaining healthy.
What can influence fetal growth?
- Placental function and uteroplacental blood flow
- Maternal conditions (e.g., hypertension, diabetes, thyroid disease)
- Nutritional status and weight gain pattern
- Smoking, alcohol, or substance exposure
- Multiple gestation and genetic factors
- Normal biological variation between families
When to contact your care team
Reach out promptly if your clinician advises concern about growth, amniotic fluid, Dopplers, or fetal movement. Also seek care for symptoms such as bleeding, fluid leakage, persistent severe headache, vision changes, significant abdominal pain, or decreased fetal movement.
Important limitations
- Ultrasound EFW has a known error margin and can differ from actual birth weight.
- Reference ranges vary by population and method.
- A single percentile is less useful than serial trend plus clinical context.
- This page is informational and not a substitute for medical care.
FAQ
Is below the 10th percentile always dangerous?
No. Some babies are constitutionally small and healthy. Your provider evaluates whether growth is pathologic or simply a normal variant.
Is above the 90th percentile always a problem?
Not always. Some babies are naturally larger. Your care team may monitor for maternal diabetes, delivery planning factors, and birth risk considerations.
How often should growth scans be repeated?
That depends on clinical indication. In higher-risk scenarios, interval scans are often performed every 2-4 weeks, guided by specialist recommendations.