Fundal Height Calculator

What is Fundal Height?

Fundal height, also known as symphysis-fundal height (SFH), is the distance measured in centimeters from the top of the pubic bone (symphysis pubis) to the top of the uterus (fundus). It is a simple, inexpensive, and non-invasive clinical measurement used to monitor fetal growth during the second and third trimesters of pregnancy.

The measurement follows a well-established rule of thumb: from approximately 20 weeks of gestation onward, the fundal height in centimeters roughly corresponds to the gestational age in weeks, with an acceptable variation of plus or minus 2-3 centimeters. For example, at 30 weeks of gestation, the expected fundal height is approximately 28-32 cm.

Fundal height measurement has been part of standard antenatal care for decades. While it has been partially supplemented by ultrasound examination in many settings, it remains an essential screening tool, particularly in resource-limited settings where access to ultrasound may be restricted. Serial measurements plotted on a growth chart are more informative than a single measurement, as they reveal the trajectory of uterine growth over time.

The World Health Organization includes fundal height measurement in its recommended package of antenatal care interventions, and most national obstetric guidelines recommend it at every prenatal visit from 24 weeks onward.

How to Measure Fundal Height

Accurate measurement of fundal height requires proper technique to minimize inter-observer variability:

1. Patient positioning: The pregnant woman should lie supine (on her back) on a firm examination surface with her legs slightly bent. The bladder should be emptied before measurement, as a full bladder can elevate the fundal height by 2-3 cm.
2. Identify the symphysis pubis: Palpate the upper border of the pubic bone at the midline. This is the lower fixed point of the measurement.
3. Identify the fundus: Using the ulnar border of your hand, palpate downward from the xiphoid process until you feel the firm, rounded top of the uterus. This is the upper point of the measurement.
4. Measure: Using a non-elastic measuring tape, measure the distance in centimeters from the upper border of the symphysis pubis to the highest point of the fundus. The tape should follow the contour of the abdomen and be held between the fingers (not stretched taut). Some clinicians recommend measuring with the tape's markings facing downward (away from the examiner) to reduce bias.
5. Record: Document the measurement in centimeters and plot it on a fundal height growth chart appropriate for the population. Compare with previous measurements to assess the growth trajectory.

Consistency is key: ideally, the same examiner should perform serial measurements to reduce variability. Studies have shown that inter-observer variation can be 2-3 cm between different examiners, which is significant relative to the expected growth rate of approximately 1 cm per week.

Normal Values by Week

The table below shows the expected fundal height at each gestational age from 20 to 42 weeks. The general rule is that fundal height in centimeters approximates the gestational age in weeks, with a normal range of plus or minus 2 cm.

Gestational Age (weeks)	Expected SFH (cm)	Normal Range (cm)
20	20	18 – 22
22	22	20 – 24
24	24	22 – 26
26	26	24 – 28
28	28	26 – 30
30	30	28 – 32
32	32	30 – 34
34	34	32 – 36
36	36	34 – 38
38	37	35 – 39
40	37	35 – 39
42	37	35 – 39

Note that after 36-38 weeks, the fundal height may plateau or even decrease slightly as the fetal head descends into the pelvis (lightening or engagement). This is a normal phenomenon, particularly in first pregnancies, and should not be mistaken for growth restriction.

Before 20 weeks, fundal height is less reliable because the uterus has not yet risen above the level of the pubic symphysis consistently. During this period, uterine size is better assessed by bimanual pelvic examination.

When Fundal Height Doesn't Match

A fundal height that is significantly larger or smaller than expected for gestational age warrants further investigation, typically with ultrasound examination.

Fundal Height Larger Than Expected

• Wrong dates: The most common cause of size/date discrepancy. If the gestational age is uncertain, ultrasound dating should be performed.
• Multiple gestation: Twins or higher-order multiples cause more rapid uterine growth.
• Macrosomia: A large-for-gestational-age fetus, often associated with maternal diabetes, obesity, or post-term pregnancy.
• Polyhydramnios: Excess amniotic fluid increases uterine size disproportionate to fetal size.
• Uterine fibroids: Large fibroids can increase the overall uterine size.
• Maternal obesity: Increased abdominal subcutaneous fat can artificially elevate the measurement.

Fundal Height Smaller Than Expected

• Wrong dates: The pregnancy may be earlier than estimated.
• Intrauterine growth restriction (IUGR): The fetus is growing more slowly than expected, which may indicate placental insufficiency or other pathology.
• Oligohydramnios: Reduced amniotic fluid decreases uterine size.
• Transverse lie: If the fetus is lying horizontally, the fundal height may be reduced.
• Fetal demise: In the unfortunate event of intrauterine fetal death, the uterus may not continue to grow.
• Constitutional: Some healthy fetuses are constitutionally small, particularly in petite mothers.

Johnson's Formula for Birth Weight Estimation

Johnson's formula is a simple clinical method for estimating fetal birth weight from the fundal height measurement. It was published by Robert Johnson in 1957 and remains in use as a quick bedside estimate, particularly when ultrasound is not available.

Where:

• SFH = Symphysis-Fundal Height in centimeters
• n = 12 if the vertex (fetal head) is at or below the ischial spines (engaged)
• n = 11 if the vertex is above the ischial spines (not engaged)
• 155 = constant derived from the original study

If the maternal weight exceeds 91 kg (200 lbs), some clinicians subtract an additional 1 cm from the SFH before applying the formula to account for increased abdominal wall thickness.

For example, if the fundal height is 35 cm and the vertex is at the ischial spines (n=12): Weight = (35 - 12) x 155 = 23 x 155 = 3,565 grams (approximately 3.6 kg or 7.9 lbs).

Johnson's formula is most applicable at gestational ages of 33 weeks or more, when the fetus is large enough for the measurement to have clinical relevance. Its accuracy is approximately plus or minus 375 grams (about 0.8 lbs) in 75% of cases, which is comparable to ultrasound estimation in the third trimester.

IUGR and Macrosomia

Intrauterine Growth Restriction (IUGR), also called Fetal Growth Restriction (FGR), occurs when the fetus fails to reach its genetically determined growth potential. It affects 3-10% of pregnancies and is a major cause of perinatal morbidity and mortality. IUGR is classified as:

• Symmetric IUGR (Type 1): The entire fetus is proportionally small, usually due to an early insult such as chromosomal abnormality, congenital infection, or severe early-onset placental dysfunction. All measurements (head, abdomen, limbs) are equally reduced.
• Asymmetric IUGR (Type 2): The abdominal circumference is disproportionately small relative to the head, reflecting brain-sparing physiology. This pattern is typical of later-onset placental insufficiency where blood flow is preferentially directed to the brain. It is the more common type, accounting for about 70-80% of IUGR cases.

Fundal height measurement has a sensitivity of approximately 27-76% for detecting small-for-gestational-age babies, depending on the study and the clinician's experience. Serial measurements plotted on a growth chart improve detection rates compared to single measurements.

Macrosomia is generally defined as a birth weight above 4,000 grams (8.8 lbs) or above 4,500 grams (9.9 lbs) depending on the definition used. Risk factors include maternal diabetes (pre-gestational and gestational), maternal obesity, excessive gestational weight gain, post-term pregnancy, male fetal sex, and a history of previous macrosomic babies. Macrosomia increases the risk of shoulder dystocia, birth injuries, cesarean delivery, and postpartum hemorrhage.

Limitations

While fundal height is a valuable screening tool, it has several well-recognized limitations:

• Maternal body habitus: Obesity significantly reduces the accuracy of fundal height measurement. Increased subcutaneous fat can make palpation of the fundus difficult and inflate the measurement.
• Inter-observer variability: Different examiners may measure different values, with variation of 2-3 cm being common. This can be mitigated by having the same provider perform serial measurements.
• Fetal position: A transverse lie, oblique lie, or breech presentation can alter the fundal height measurement.
• Amniotic fluid volume: Both polyhydramnios and oligohydramnios confound the relationship between fundal height and fetal size.
• Fibroids and other uterine pathology: These can increase the uterine size independent of fetal growth.
• Multiple gestation: Standard fundal height charts apply to singleton pregnancies only.
• Moderate sensitivity: Studies consistently show that fundal height measurement misses 30-50% of growth-restricted fetuses. It is a screening tool, not a diagnostic test.

For these reasons, fundal height measurement should be viewed as a first-line screening tool that triggers further evaluation (usually ultrasound) when abnormalities are detected, rather than as a definitive assessment of fetal size.

Frequently Asked Questions

When should fundal height be measured?

Fundal height measurement is typically started at 24 weeks of gestation and repeated at every subsequent prenatal visit. Before 20 weeks, the uterus has not consistently risen above the symphysis pubis, making measurement unreliable. Some guidelines recommend starting at 20 weeks, while others prefer 24 weeks. The measurement is most useful when performed serially so that the growth trajectory can be assessed.

What if my fundal height is 3 cm less than expected?

A discrepancy of more than 3 cm below the expected value warrants an ultrasound examination to evaluate fetal size, amniotic fluid volume, and placental function. Causes include incorrect gestational dating, intrauterine growth restriction, oligohydramnios, and transverse lie. A single measurement may be inaccurate due to examiner technique or maternal factors, so a repeat measurement before ultrasound referral may be appropriate in some cases.

Is fundal height accurate for estimating birth weight?

Johnson's formula provides a rough estimate with an accuracy of approximately plus or minus 375 grams in about 75% of cases. This is similar to ultrasound-based weight estimation, which has an accuracy of plus or minus 10-15% of actual weight. Neither method is precise enough for definitive weight determination, and both are used as clinical estimates to guide management decisions.

Does fundal height decrease near term?

Yes. After 36-38 weeks, particularly in first pregnancies, the fetal head often descends into the pelvis (a process called lightening or engagement). This descent can cause the fundal height to plateau or decrease by 1-2 cm. This is a normal finding and is often accompanied by the mother feeling that she can breathe more easily (less pressure on the diaphragm) but has increased pelvic pressure and urinary frequency.

Can I measure my own fundal height at home?

While it is technically possible to measure your own fundal height with a measuring tape, self-measurement is generally not recommended because it is difficult to accurately palpate the fundus and symphysis pubis while lying on your back, and the results are unlikely to be reliable enough for clinical decision-making. Fundal height measurement is best performed by a trained healthcare provider who can also palpate the fetus, assess its position, and integrate the measurement with other clinical findings.

What is the difference between fundal height and ultrasound for monitoring fetal growth?

Fundal height is a simple, quick, inexpensive screening tool that can be performed at every prenatal visit. Ultrasound provides detailed measurements of fetal biometry (head circumference, abdominal circumference, femur length), estimates fetal weight, assesses amniotic fluid, and evaluates the placenta. Ultrasound is more accurate but more expensive, requires equipment and trained personnel, and is typically used when fundal height screening suggests a problem or when other risk factors warrant closer monitoring. In many countries, a combination of fundal height screening with targeted ultrasound is the standard approach.