Enter Pregnancy Details
Provide only the values you know. When ultrasound measurements are entered, the calculator blends them with the gestational age growth baseline.
Formula Used
This calculator combines a gestational growth baseline with optional ultrasound data and small adjustment factors. It is designed for education and planning, not for clinical decision-making.
| Step | Formula | Purpose |
|---|---|---|
| Gestational Baseline | Average size = linear interpolation between weekly anchor values | Finds expected weight and length for the chosen week. |
| Ultrasound Weight | log10(EFW) = 1.326 − 0.00326(AC×FL) + 0.0107HC + 0.0438AC + 0.158FL | Estimates fetal weight when HC, AC, and FL are supplied. |
| Adjusted Weight | Estimated weight = blended baseline and ultrasound value × adjustment factor | Applies sex, BMI, parity, parent height, and plurality modifiers. |
| Percentile | z = (estimated − average) ÷ expected spread | Shows whether the prediction is below, near, or above average. |
| Projected Birth Size | Birth estimate = current growth ratio × week-40 baseline | Projects a possible full-term outcome from the current trend. |
HC = head circumference, AC = abdominal circumference, FL = femur length, EFW = estimated fetal weight.
How to Use This Calculator
- Enter gestational weeks and days first. These fields drive the weekly growth baseline.
- Add maternal and other parent details if you want a more personalized estimate.
- Enter HC, AC, and FL from an ultrasound report to activate the advanced measurement model.
- Select fetal sex, pregnancy type, and parity for small trend adjustments.
- Click Predict Baby Size to show results above the form.
- Review the estimated weight, length, percentile, category, projected birth size, and graph.
- Use the CSV or PDF buttons to save the result summary.
- Always discuss medical concerns or unexpected growth patterns with your clinician.
Example Data Table
This example shows how one completed entry may look.
| Gestation | HC (cm) | AC (cm) | FL (cm) | Maternal Height (cm) | Other Parent Height (cm) | Predicted Weight (g) | Percentile |
|---|---|---|---|---|---|---|---|
| 30w 4d | 28.4 | 25.7 | 5.8 | 162 | 176 | 1,470 | 58th |
| 34w 1d | 31.2 | 29.8 | 6.5 | 158 | 170 | 2,170 | 49th |
| 37w 0d | 33.6 | 32.1 | 7.1 | 167 | 181 | 2,980 | 63rd |
Frequently Asked Questions
1) Is this calculator medically exact?
No. It gives an educational estimate using growth references and optional ultrasound measurements. Medical imaging, clinical history, and professional interpretation are still more important for diagnosis or treatment decisions.
2) Which inputs improve accuracy most?
Gestational age is essential. Adding head circumference, abdominal circumference, and femur length usually improves the estimate because those values directly influence fetal weight modeling.
3) Why does the result show a percentile?
The percentile compares your prediction with a typical weekly average. A higher percentile suggests larger-than-average size, while a lower percentile suggests smaller-than-average size for the same gestational week.
4) Can this predict birth weight exactly?
No birth-weight tool can promise exact accuracy. This calculator projects a possible full-term trend from current growth, but real growth can speed up, slow down, or change near delivery.
5) Why are parent heights included?
Parental height can slightly influence expected fetal length and weight trends. The adjustment here is intentionally small so measurements and gestational age still remain the main drivers.
6) Can I use it for twins?
Yes. Choose the twins option and the model applies a smaller expected size trend. Twin pregnancies are more complex, so results should be treated as rough guidance only.
7) What if I only know gestational age?
That is fine. The calculator will use the weekly growth baseline and still return estimated size, percentile, and projected birth weight. Measurement fields are optional.
8) When should I ask a clinician instead?
Ask a clinician if you notice major differences from scan reports, reduced fetal movement, concerning symptoms, or repeated size estimates that seem far above or below expectations.