About

In the realm of prenatal care, accuracy is not just a preference; it is a necessity for monitoring fetal development. This Fetal Weight & Growth Calculator utilizes the Hadlock IV regression formula, widely regarded as the gold standard in obstetric sonography for estimating fetal weight (EFW). By integrating four key biometrics—Biparietal Diameter (BPD), Head Circumference (HC), Abdominal Circumference (AC), and Femur Length (FL)—this tool provides a precision estimate that accounts for the varying proportions of fetal anatomy.

Understanding EFW is critical for identifying growth trends such as Intrauterine Growth Restriction (IUGR) or Macrosomia (large for gestational age). While ultrasound estimates can have a margin of error (typically ±10-15%), they remain the primary clinical method for assessing fetal well-being before birth. This tool also correlates the estimated weight against gestational age to provide a percentile ranking, helping parents and professionals understand where the baby sits compared to a standard growth curve.

Formulas

The calculation relies on the logarithmic regression model developed by Hadlock et al. (1985). This formula accounts for head shape, body density, and limb length simultaneously.

Log₁₀(EFW) = 1.3596 − 0.00386(AC × FL) + 0.0064(HC) + 0.00061(BPD × AC) + 0.0424(AC) + 0.174(FL)

Where all measurements are in centimeters (cm) and the resulting weight is in grams.

Reference Data

Gestational Age	10th Percentile (g)	50th Percentile (g)	90th Percentile (g)	Weekly Gain (Avg)
20 Weeks	286 g	331 g	384 g	-
22 Weeks	398 g	468 g	551 g	~65 g
24 Weeks	534 g	640 g	768 g	~85 g
26 Weeks	703 g	859 g	1049 g	~110 g
28 Weeks	912 g	1133 g	1404 g	~135 g
30 Weeks	1172 g	1474 g	1849 g	~170 g
32 Weeks	1489 g	1882 g	2369 g	~200 g
34 Weeks	1859 g	2353 g	2956 g	~230 g
36 Weeks	2269 g	2859 g	3563 g	~250 g
38 Weeks	2688 g	3351 g	4111 g	~240 g
40 Weeks	3074 g	3780 g	4555 g	~210 g

Frequently Asked Questions

Ultrasound algorithms like Hadlock generally have a margin of error of roughly ±15%. For a baby estimated at 2000g, the actual weight could realistically fall between 1700g and 2300g. Accuracy decreases in late pregnancy due to fluid reduction and fetal positioning.

Percentiles compare your baby's size to a reference population of the same gestational age. The 50th percentile is the exact average. The 10th percentile means 10% of babies are smaller and 90% are larger. Falling below the 10th or above the 90th usually prompts further monitoring but does not strictly indicate a problem.

There are over 30 valid formulas (Shepard, Campbell, Warsof, etc.). Some emphasize the abdomen (AC) more than the head. Most modern hospitals use Hadlock III or IV because they use 3-4 parameters, reducing the risk of skewing data if one measurement (like the head shape) is unusual.

The AC is the most variable parameter and the most sensitive indicator of fetal nutrition. A disproportionately small AC compared to the head size can be a sign of asymmetrical growth restriction, often linked to placental efficiency.