Free Fetal Weight Percentile Calculator

Fetal Weight Percentile: Understanding Baby Growth Through Ultrasound

Assessing fetal size is a core part of prenatal care, and the fetal growth percentile offers a meaningful way to interpret ultrasound measurements. This baby weight percentile tool relies on standard sonographic parameters — biparietal diameter (BPD), head circumference (HC), abdominal circumference (AC), and femur length (FL) — to estimate fetal weight at a given gestational week. By comparing the estimated weight against reference data from the WHO multinational growth charts (using the Hadlock equation), the EFW percentile helps clinicians quickly identify whether a fetus falls within an expected range.

Percentiles are widely used for variables that vary naturally across a population, such as height or weight in children. Instead of a single “normal” value, a percentile tells you what proportion of same‑aged fetuses are smaller or larger. For example, a fetus at the 60th percentile is heavier than 60% of its peers and lighter than 40%. This approach provides context for individual growth without forcing a rigid threshold, making it invaluable for detecting potential deviations.

Key Ultrasound Markers and the Hadlock Formula

The ultrasound fetal weight estimation integrates four primary measurements:

• Biparietal Diameter (BPD): The distance between the two parietal bones of the skull, taken in a plane that includes the thalami and cavum septum pellucidum.
• Head Circumference (HC): Measured at the same level as BPD, reflecting overall skull growth.
• Abdominal Circumference (AC): Taken around the fetal abdomen at the level of the umbilical portion of the left portal vein.
• Femur Length (FL): The length of the thigh bone, a strong indicator of linear growth.

The Hadlock formula combines these parameters to produce an estimated fetal weight (EFW) in grams. The equation used in this fetal weight percentile calculator is:

$$\log_{10}(\text{EFW}) = 1.3596 - (0.00386 \times AC \times FL) + (0.0064 \times HC) + (0.00061 \times BPD \times AC) + (0.0424 \times AC) + (0.174 \times FL)$$

All lengths are in centimeters. Once the EFW is calculated, it is compared to the WHO percentile tables to determine the fetal weight chart by week position.

Small for Gestational Age (SGA) and Large for Gestational Age (LGA)

When the EFW percentile falls outside typical boundaries, specific definitions apply:

• SGA (Small for Gestational Age): EFW below the 10th percentile. This means the fetus is smaller than 90% of fetuses the same age.
• LGA (Large for Gestational Age): EFW above the 90th percentile. Only 10% of the population is larger.

These cutoffs help guide further monitoring and clinical decisions. Note that some providers also use the 3rd and 97th percentiles for more extreme deviations.

How to Use the Fetal Weight Percentile Calculator

The tool is designed for straightforward input:

1. Select the fetal gender (if known; results for unknown gender are also available).
2. Enter gestational age (weeks; the calculator is validated for pregnancies at or beyond 14 weeks).
3. Provide the ultrasound measurements (BPD, HC, AC, FL) in either metric or imperial units.

The calculator then returns the EFW along with its percentile rank and a brief interpretation (e.g., “above the 90th percentile – LGA”). The same results can be obtained manually by computing EFW with the Hadlock formula and then consulting a reference table.

Reference Table: Fetal Weight by Gestational Week (Unknown Gender)

The values below represent smoothed percentile limits for fetuses of unknown sex, based on WHO multinational data. To find the percentile, locate the row for the gestational age and compare the EFW (in grams) with the numbers in each percentile column.

Gestational Age (weeks)	2.5% (g)	5% (g)	10% (g)	25% (g)	50% (g)	75% (g)	90% (g)	95% (g)	97.5% (g)
14	70	73	78	83	90	98	104	109	113
15	89	93	99	106	114	124	132	138	144
16	113	117	124	133	144	155	166	174	181
17	141	146	155	166	179	193	207	217	225
18	174	181	192	206	222	239	255	268	278
19	214	223	235	252	272	292	313	328	340
20	260	271	286	307	330	355	380	399	413
21	314	327	345	370	398	428	458	481	497
22	375	392	412	443	476	512	548	575	595
23	445	465	489	525	565	608	650	682	705
24	523	548	576	618	665	715	765	803	830
25	611	641	673	723	778	836	894	938	970
26	707	743	780	838	902	971	1,038	1,087	1,125
27	813	855	898	964	1,039	1,118	1,196	1,251	1,295
28	929	977	1,026	1,102	1,189	1,279	1,368	1,429	1,481
29	1,053	1,108	1,165	1,251	1,350	1,453	1,554	1,622	1,682
30	1,185	1,247	1,313	1,410	1,523	1,640	1,753	1,828	1,897
31	1,326	1,394	1,470	1,579	1,707	1,838	1,964	2,046	2,126
32	1,473	1,548	1,635	1,757	1,901	2,047	2,187	2,276	2,367
33	1,626	1,708	1,807	1,942	2,103	2,266	2,419	2,516	2,619
34	1,785	1,872	1,985	2,134	2,312	2,492	2,659	2,764	2,880
35	1,948	2,038	2,167	2,330	2,527	2,723	2,904	3,018	3,148
36	2,113	2,205	2,352	2,531	2,745	2,959	3,153	3,277	3,422
37	2,280	2,372	2,537	2,733	2,966	3,195	3,403	3,538	3,697
38	2,446	2,536	2,723	2,935	3,186	3,432	3,652	3,799	3,973
39	2,612	2,696	2,905	3,135	3,403	3,664	3,897	4,058	4,247
40	2,775	2,849	3,084	3,333	3,617	3,892	4,135	4,312	4,515

How to read the table: For example, a fetus at 20 weeks with an EFW of 330 g lies exactly at the 50th percentile. An EFW of 380 g at the same week puts it at the 90th percentile. Interpolate between columns if the value falls between two lines.

Manual Calculation of Fetal Weight Percentile

If a calculator is not available, you can determine the percentile manually:

1. Estimate fetal weight using the Hadlock formula above (or a standard ultrasound machine’s output).
2. Locate the gestational age on the reference table (or a growth chart).
3. Find the weight value in the row for that week, and note which percentile column it falls closest to (e.g., between the 75th and 90th lines).

Example: EFW = 250 g at 18 weeks. In the 18‑week row, 250 g lies between the 50th (222 g) and 75th (239 g) percentile values. The exact position is above the 75th line (closer to 75th than 90th), so the percentile is approximately 75th. This means the baby is heavier than about 75% of other fetuses the same age.

Important Considerations

Percentile values are population‑based references; they are not absolute diagnoses. A single measurement below the 10th or above the 90th percentile warrants further evaluation but does not automatically indicate pathology. Always consult a healthcare provider for clinical interpretation. Additionally, the fundal height method can be a complementary screening tool, though it is less accurate than ultrasound‑based EFW.

This baby weight percentile tool is designed for educational and reference purposes. When used alongside clinical judgment, it provides valuable insight into fetal growth and helps track the EFW percentile across successive scans.

Note: The formulas and data apply to singleton pregnancies after 14 weeks. For early gestations, crown‑rump length (CRL) is the standard parameter.