How the Fetal Growth Percentile Calculator Works
This calculator compares your baby's estimated ultrasound weight with Hadlock reference curves, the international standard for fetal growth assessment. The result is a percentile indicating whether growth is on track.
The Calculation Method
The calculator uses reference data from the Hadlock et al. (1991) study, which established median fetal weight curves for each gestational week. The entered weight is compared with the corresponding week's median to calculate an approximate percentile.
The Classifications
Based on percentile, the fetus is classified into three categories:
- SGA (Small for Gestational Age): weight below the 10th percentile. Requires monitoring.
- AGA (Appropriate for Gestational Age): weight between 10th-90th percentile. Normal growth.
- LGA (Large for Gestational Age): weight above the 90th percentile. Requires evaluation.
Median Fetal Weight by Week
| Week | Median | Week | Median |
|---|---|---|---|
| 20 | 300 g | 30 | 1,319 g |
| 24 | 600 g | 34 | 2,146 g |
| 28 | 1,005 g | 38 | 3,083 g |
| — | — | 40 | 3,462 g |
Important Limitations
Ultrasound fetal weight is an estimate with a 10-15% margin of error. A single percentile value isn't sufficient for diagnosis: the trend over time (growth trajectory) is far more significant. Always consult your OB/GYN for interpretation.