Fetal size by week of pregnancy
See your baby's estimated length and weight for each week, or calculate size from the femur length.
How the size is calculated
The table uses the length and weight reference values by gestational week published by Hadlock and colleagues in 1991, the same values used by most ultrasound software. In the first half of pregnancy length is measured head to bottom (crown-rump); afterwards, head to heel.
Femur mode estimates length with the Vintzileos (1984) formula: length in centimetres equals 6.18 plus 0.59 times the femur length in millimetres. It is a useful approximation for understanding the report, but the official calculation in your scan considers other measurements.
Model and references
- Length: Vintzileos AM, Campbell WA, Neckles S, Pike CL, Nochimson DJ. The ultrasound femur length as a predictor of fetal length. Obstet Gynecol. 1984;64(6):779-82. Formula: 6.18 + 0.59 × femur length (mm).
- Weight: Hadlock FP, Harrist RB, Martinez-Poyer J. In utero analysis of fetal growth: a sonographic weight standard. Radiology. 1991;181(1):129-33.
Reference calculator: fetalmed.net/calculadoras/calculadora-do-tamanho-do-bebe
Independent reproduction of the published models, for educational purposes.
Fetal size and weight week by week
| Week | Length | Weight |
|---|---|---|
| 8 | 1.6 cm | 1 g |
| 9 | 2.3 cm | 2 g |
| 10 | 3.1 cm | 4 g |
| 11 | 4.1 cm | 7 g |
| 12 | 5.4 cm | 14 g |
| 13 | 6.5 cm | 23 g |
| 14 | 8.7 cm | 43 g |
| 15 | 10.0 cm | 70 g |
| 16 | 11.6 cm | 100 g |
| 17 | 13.0 cm | 140 g |
| 18 | 14.2 cm | 190 g |
| 19 | 15.2 cm | 240 g |
| 20 | 25.6 cm | 300 g |
| 21 | 26.7 cm | 360 g |
| 22 | 27.7 cm | 430 g |
| 23 | 28.8 cm | 501 g |
| 24 | 29.8 cm | 600 g |
| 25 | 33.7 cm | 660 g |
| 26 | 35.6 cm | 760 g |
| 27 | 36.8 cm | 875 g |
| 28 | 38.0 cm | 1.0 kg |
| 29 | 39.2 cm | 1.1 kg |
| 30 | 40.4 cm | 1.3 kg |
| 31 | 41.6 cm | 1.5 kg |
| 32 | 42.8 cm | 1.7 kg |
| 33 | 44.0 cm | 1.9 kg |
| 34 | 45.2 cm | 2.1 kg |
| 35 | 46.4 cm | 2.4 kg |
| 36 | 47.4 cm | 2.6 kg |
| 37 | 48.6 cm | 2.9 kg |
| 38 | 49.6 cm | 3.1 kg |
| 39 | 50.5 cm | 3.3 kg |
| 40 | 51.5 cm | 3.4 kg |
| 41 | 52.0 cm | 3.5 kg |
| 42 | 52.5 cm | 3.6 kg |
Reference values (Hadlock et al., 1991). Every baby grows at its own pace: differences of one or two weeks are common and should be assessed by your doctor.
Frequently asked questions
Important notice
This calculator is an educational tool and does not replace a medical consultation. Results are estimates based on published models and may not apply to your case. Never make decisions about your health or your pregnancy without advice from your doctor.
Reviewed by: Dr. Peterson Fernandes — CRM-MG 42452 · RQE 37814
Last reviewed: 11/3/2026