Fetal Weight Calculator (Johnson’s Formula)
Estimate fetal weight using Fundal Height (SFH) and fetal station. A quick clinical tool for midwives and doctors to assess fetal development.
Measure from pubic symphysis to top of uterus.
About
Johnson’s Formula is a widely used clinical method for estimating fetal weight, particularly in environments where ultrasound is not immediately available. By measuring the Symphysis-Fundal Height (SFH)—the distance from the pubic bone to the top of the uterus—practitioners can gain valuable insight into the baby's growth trajectory.
This calculator adjusts for the "station" of the fetal head (how far the baby has descended into the pelvis), which is a critical variable often overlooked in simpler tools. Accurate weight estimation is vital for identifying potential macrosomia (large baby) or intrauterine growth restriction (IUGR), allowing for better birth planning and risk management.
Formulas
The calculator uses the standard Johnson's equation, where the weight depends on the fundal height minus a correction factor determined by the fetal station (descent of the head).
Where:
- W = Fetal Weight in grams
- SFH = Symphysis-Fundal Height (cm)
- n = 13 (if head not engaged), 12 (if engaged), 11 (if below spines)
- 155 = Constant multiplier
Reference Data
| Station of Fetal Head | Position Description | Correction Factor (n) |
|---|---|---|
| Minus Station | Head is above ischial spines (floating). | 13 |
| Zero Station | Head is engaged (at ischial spines). | 12 |
| Plus Station | Head is below ischial spines. | 11 |