Free Fundal Height Calculator

Defining Symphysis‑Fundal Height and Its Clinical Role

Fundal height, also referred to as symphysis‑fundal height (SFH), is a standard obstetric measurement obtained by placing a flexible tape from the upper margin of the pubic symphysis to the top of the uterine fundus. This simple, non‑invasive technique provides insight into fetal growth and the progression of pregnancy. A dedicated fundal height calculator can process the SFH value in three principal ways: estimating the expected fundal height for a given gestational age, back‑calculating the gestational age from a measured SFH, or predicting fetal weight when the baby has assumed the vertex position. By integrating these functions, the tool supports both clinical assessments and self‑monitoring during pregnancy.

The INTERGROWTH‑21st Formula for Median SFH

To determine the median symphysis‑fundal height for any week of gestation, the calculator relies on the equation developed by the INTERGROWTH‑21st Project:

$$\text{Median SFH (cm)} = 5.1333 + 0.1058 \times \text{GA}^2 - 0.02312 \times \text{GA}^2 \times \ln(\text{GA})$$

Here, $\text{GA}$ denotes the gestational age expressed in weeks (with the fractional part representing days, e.g., 22 weeks 3 days = 22.43 weeks). Once a woman’s actual SFH is measured, the tool compares it to this median and returns the corresponding percentile, helping to identify whether uterine size is appropriate for the stage of pregnancy.

How the Calculator Works: Three Modes

The SFH calculator offers three distinct calculation modes, each tailored to a different clinical question:

1. SFH Percentile – Enter your precise gestational age (weeks + days) along with your measured SFH. The tool uses the INTERGROWTH‑21st reference to compute the centile (e.g., 10th, 50th, 90th). This percentile can flag values that are unusually low or high, prompting further evaluation.

2. SFH to Gestational Age – When the exact week of pregnancy is uncertain but a fundal height measurement is available, the calculator estimates the gestational age. Two complementary approaches are provided: the quick McDonald’s rule and a more precise regression equation.

3. SFH to Fetal Weight – For pregnancies that have reached 33–36 weeks and where the fetus is in vertex presentation, Johnson’s formula gives an estimated birth weight. This mode requires the mother’s weight category and the position of the fetal vertex relative to the ischial spine.

Estimating Gestational Age from Fundal Height

McDonald’s Rule offers a simple anatomical landmark check:

• At 12 weeks the uterine fundus is barely palpable just above the pubic bone.
• By 22 weeks it lies at the level of the umbilicus.
• At 36 weeks it reaches the lower end of the sternum.
• After 37 weeks the fundus gradually descends as the fetal head engages in the pelvis.

For a numerical estimate, the following regression equation can be used:

$$\text{GA (weeks)} = 6.5858 - 2.7073 \times \sqrt{\text{SFH}} + 1.2952 \times \text{SFH}$$

where $\text{SFH}$ is measured in centimeters. This formula provides a continuous prediction that is especially useful when the McDonald landmarks are ambiguous.

Johnson’s Formula for Fetal Weight

Johnson’s method estimates fetal weight solely from SFH, but it is valid only after the fetus has turned to a vertex position (typically from 33–36 weeks onward). The formula is:

$$\text{Fetal weight (g)} = (\text{SFH} - \text{W} - \text{V}) \times 155$$

where:

• W = 1 if the mother’s weight exceeds 91 kg (200 lb), otherwise 0.
• V = 12 if the fetal vertex is above the ischial spine, or 11 if it is below.

The result is a grams‑based approximation that should be interpreted alongside ultrasound and clinical examination.

Fundal Height Reference Charts

Alongside the simple landmark chart described above, a detailed centile table (derived from the INTERGROWTH‑21st study) provides SFH values in millimeters for each completed week from 16 to 42. The table below reproduces the 3rd, 5th, 10th, 50th, 90th, 95th, and 97th percentiles:

GA (completed weeks)	3rd	5th	10th	50th	90th	95th	97th
16	132	135	140	158	176	181	185
17	141	144	149	168	186	191	195
18	150	154	159	178	196	202	205
19	160	163	169	188	207	212	215
20	169	173	178	198	217	222	226
21	179	182	188	208	227	233	236
22	188	192	198	218	238	243	247
23	198	201	207	228	248	254	257
24	207	211	217	238	258	264	268
25	217	221	227	247	268	274	278
26	226	230	236	257	279	285	289
27	235	239	246	267	289	295	299
28	245	249	255	277	299	305	309
29	254	258	264	286	309	315	319
30	263	267	273	296	318	325	329
31	271	276	282	305	328	335	339
32	280	284	291	314	338	344	348
33	289	293	300	323	347	354	358
34	297	301	308	332	356	363	367
35	305	309	316	340	365	372	376
36	313	317	324	349	373	380	385
37	320	325	332	357	382	389	393
38	328	332	339	365	390	397	402
39	335	339	347	372	398	405	410
40	341	346	354	380	405	413	418
41	348	353	360	387	413	420	425
42	354	359	369	393	420	427	432

These values serve as a reference to gauge whether an individual SFH measurement deviates from the expected range. A value persistently below the 10th centile may signal growth restriction, while a value above the 90th centile could suggest macrosomia. Clinicians typically evaluate serial measurements rather than a single reading.

Important Caveat

The calculations provided by this tool are intended for educational and screening purposes only. They do not replace a thorough medical evaluation by a qualified healthcare professional. If you have concerns about your pregnancy or the results obtained from this calculator, please consult your doctor or midwife for personalized advice.