Fundal Height Calculator: SFH by Week, Fetal Weight Estimate
Enter your gestational age and measured fundal height to instantly check whether your symphysis-fundal height (SFH) is within the expected range for your week of pregnancy. The calculator applies McDonald's rule, shows your result against 10th to 90th percentile reference bands, flags small or large for gestational age findings, and estimates fetal weight using Johnson's formula. Switch between centimetres and inches. Results update as you type.
What is fundal height and why does it matter?
Fundal height (FH) is the distance in centimetres from the top of the pubic symphysis bone to the top of the uterus (the fundus). It is measured with a non-elastic tape during antenatal visits and, between about 20 and 36 weeks of pregnancy, it roughly equals the number of weeks of gestation. This simple rule, called McDonald's rule, gives midwives and obstetricians a fast, low-cost way to check that the baby is growing at the expected rate without needing an ultrasound at every visit. A measurement that is consistently smaller or larger than expected may prompt further investigation with a growth scan.
How to measure fundal height correctly
The measurement should be taken with the pregnant person lying flat, bladder emptied, and a non-elastic tape running from the highest point of the uterine fundus to the midpoint of the upper edge of the pubic symphysis. The tape is placed with the centimetre markings facing down (to reduce bias) and the result is read after the tape is in position. The same technique should be used at each visit for consistency. Measurements taken by different clinicians, or with the woman in a different position, can vary by 1-2 cm even for the same pregnancy, so trends over multiple visits matter more than a single reading.
Small for gestational age vs. large for gestational age
A fundal height below the 10th centile for gestational age is described as small for gestational age (SGA). This may indicate the baby is not growing as fast as expected, possibly due to placental insufficiency, intrauterine growth restriction (IUGR), or oligohydramnios (low amniotic fluid). A fundal height above the 90th centile is described as large for gestational age (LGA) and may reflect rapid fetal growth, polyhydramnios (excess fluid), gestational diabetes, or a multiple pregnancy. Both findings are screening signals, not diagnoses: a single abnormal measurement is much less meaningful than a consistent pattern, and an ultrasound growth scan is the gold standard for assessing fetal size and fluid levels.
Johnson's formula for estimated fetal weight
Johnson's formula estimates the weight of a baby in a vertex (head-down) position from the fundal height: estimated fetal weight (grams) = (SFH in cm - maternal weight factor - head position factor) x 155. The maternal weight factor is 1 if the mother weighs more than 91 kg, otherwise 0. The head position factor is 12 if the fetal head is at or above the level of the ischial spines and 11 if it has descended below them. The formula is most accurate after about 33-36 weeks and only in vertex presentations. It carries an error margin of roughly 15-20%, so it is used as a guide rather than a precise measurement.
Factors that affect fundal height accuracy
Several factors can make fundal height measurements misleading. Maternal obesity increases the tissue between the tape and the fundus, often causing measurements to read high. Strong abdominal muscles can push the fundus down, causing measurements to read low. Incorrect dating of the pregnancy, fibroids, full bladder, and the baby's position can all shift the measurement. McDonald's rule is most reliable from 20 to 36 weeks: before 20 weeks the fundus has not risen high enough, and after 36 weeks the baby's head may begin to descend into the pelvis, causing the fundus to drop even as the baby continues to grow.
Fundal height reference ranges by gestational week (10th, 50th, 90th centile)
| Gestational week | 10th centile (cm) | 50th centile / median (cm) | 90th centile (cm) | Assessment |
|---|---|---|---|---|
| 16 weeks | 14.0 | 15.8 | 17.6 | McDonald rule applies |
| 18 weeks | 15.9 | 17.8 | 19.6 | McDonald rule applies |
| 20 weeks | 17.8 | 19.8 | 21.7 | McDonald rule applies |
| 22 weeks | 19.8 | 21.8 | 23.8 | McDonald rule applies |
| 24 weeks | 21.7 | 23.8 | 25.8 | McDonald rule applies |
| 26 weeks | 23.6 | 25.7 | 27.9 | McDonald rule applies |
| 28 weeks | 25.5 | 27.7 | 29.9 | McDonald rule applies |
| 30 weeks | 27.3 | 29.6 | 31.8 | McDonald rule applies |
| 32 weeks | 29.1 | 31.4 | 33.8 | McDonald rule applies |
| 34 weeks | 30.8 | 33.2 | 35.6 | McDonald rule applies |
| 36 weeks | 32.4 | 34.9 | 37.3 | McDonald rule applies |
| 38 weeks | 33.9 | 36.5 | 39.0 | Late pregnancy - may descend |
| 40 weeks | 35.4 | 38.0 | 40.5 | Late pregnancy - may descend |
INTERGROWTH-21st International Symphysis-Fundal Height Standards (Papageorghiou et al, BMJ 2016;355:i5662). Values in centimetres, for singleton pregnancies dated by ultrasound. Measurements below the 10th centile or above the 90th centile warrant further evaluation.
Frequently asked questions
What is a normal fundal height at 28 weeks?
At 28 weeks, the median fundal height in the INTERGROWTH-21st international standard is 27.7 cm. The normal range (10th to 90th centile) runs from 25.5 cm to 29.9 cm. McDonald's rule puts the expected measurement at approximately 28 cm plus or minus 2 cm. A reading between about 25.5 and 30 cm at 28 weeks is generally considered within the normal range, though your provider will interpret the result in context with your full history.
Can fundal height predict fetal weight?
Yes, roughly. Johnson's formula uses fundal height to estimate fetal weight and is reasonably accurate after about 33-36 weeks in a vertex (head-down) pregnancy. However, the error margin is typically 15-20%, so a calculated estimate of 3,000 g could mean the baby weighs anywhere from about 2,400 to 3,600 g. Ultrasound biometry is considerably more accurate, especially for detecting the extremes of fetal size.
What does it mean if my fundal height is measuring 2 weeks ahead?
A fundal height 2 cm ahead of gestational age - which roughly corresponds to 2 weeks ahead on McDonald's rule - falls within normal variation for many pregnancies. Some causes include a well-grown baby, increased amniotic fluid (polyhydramnios), or the baby's position. A single reading 2 cm above the median is not usually a cause for concern, but if measurements are consistently large over several visits, your midwife may request an ultrasound to assess fluid levels and fetal growth.
What happens if fundal height is small for dates?
A fundal height below the 10th centile for gestational age is flagged as small for gestational age. Your provider will usually refer you for an ultrasound growth scan to measure the baby directly, assess the amniotic fluid index, and evaluate placental blood flow with Doppler. Many babies who measure small on fundal height turn out to be perfectly healthy: the scan resolves the uncertainty. If IUGR is confirmed, the management plan depends on how severe it is and how far along you are.
Does fundal height work the same for twins?
No. McDonald's rule and the standard centile charts are derived from singleton pregnancies. In a twin or higher-order pregnancy, the fundus rises faster and the standard reference ranges do not apply. A separate growth assessment protocol, typically with regular ultrasound, is used for multiple pregnancies instead of relying on fundal height measurements.
At what gestational age is fundal height measurement most accurate?
Fundal height correlates best with gestational age between 20 and 36 weeks. Before 20 weeks the fundus has not reached a height that tracks consistently with gestational age. After 36 weeks, fetal head engagement (the baby dropping into the pelvis) can lower the fundal measurement even while the baby continues to grow, making the correspondence less reliable in late pregnancy.
Sources
- Papageorghiou AT, Ohuma EO, Gravett MG, et al. International standards for symphysis-fundal height based on serial measurements from the Fetal Growth Longitudinal Study of the INTERGROWTH-21st Project. BMJ. 2016;355:i5662. (Source of the 10th/50th/90th centile table used here.)
- Johnson RW, Toshach CE. Estimation of fetal weight using longitudinal mensuration. American Journal of Obstetrics and Gynecology. 1954;68(3):891-896. (Original Johnson's formula.)
- Gardosi J, Francis A. Controlled trial of fundal height measurement plotted on customised antenatal growth charts. British Journal of Obstetrics and Gynaecology. 1999;106(4):309-317.
- American College of Obstetricians and Gynecologists. ACOG Practice Bulletin No. 134: Fetal Growth Restriction. Obstetrics and Gynecology. 2013;121(5):1122-1133.
- Cleveland Clinic. Fundal Height: Measurement, What It Means and Accuracy.