Fundal Height Calculator

Think of the uterus as a balloon inflating at a fairly consistent rate. Between 20 and 36 weeks of pregnancy, the top of that balloon rises approximately 1 centimeter each week. Because the rise is so consistent during this window, the distance from the pubic bone to the top of the uterus in centimeters ends up roughly equal to the number of weeks along the pregnancy is. That coincidence is what makes fundal height useful as a screening tool — no equipment needed, no radiation, and it takes under a minute.

The plus-or-minus 2 cm rule accounts for normal biological variation. Two women both 28 weeks pregnant might measure 26 and 30 cm and both be carrying perfectly healthy babies. Fetal position, the angle of the uterus, how the provider holds the tape, and how full the bladder is can all shift the number by a centimeter or two without meaning anything clinically. The rule is not diagnostic — it is a trigger for closer inspection.

After 36 weeks the rule breaks down because the baby begins to engage into the pelvis. The fundus no longer rises week by week, so comparing the centimeter reading to gestational age becomes less meaningful. Before 20 weeks, the uterus is still largely below the umbilicus and the measurement varies too much to be reliable. The sweet spot for this tool is the middle third of pregnancy.

Use this tool when you have a fundal height measurement from a prenatal visit and want to understand where it falls relative to the expected range before discussing it with your provider. It is also useful for tracking your own growth trend across multiple visits if you have been recording measurements.

Do not rely on this tool as a substitute for a clinical assessment. A measurement outside the normal range is a reason to ask a question, not a reason to conclude that something is wrong. Your provider has context this tool does not — your prior growth trajectory, your body composition, fetal presentation, and any ultrasound findings on file.

This tool is not appropriate for multiple pregnancies, pregnancies with known placenta previa or significant fibroids, or pregnancies where growth is already being monitored with serial ultrasounds. In those situations, fundal height screening adds little and ultrasound biometry is the standard of care.

The most common mistake is treating a single out-of-range reading as a diagnosis. A fundal height of 26 cm at 30 weeks is 2 cm below the lower bound — but that same measurement made at the next visit as 31 cm at 32 weeks would show appropriate growth. One data point does not make a pattern. Providers follow the trend, not any single number.

A second frequent error is forgetting that the rule only applies to singleton pregnancies. With twins or higher-order multiples, the uterus grows faster and fundal height norms derived from singletons do not apply. Entering a twin pregnancy into this calculator will produce a misleading result — the expected range shown will be too low and the measurement will appear falsely elevated.

A third mistake is measuring after eating a large meal or with a full bladder, both of which can displace the uterus slightly upward and add a centimeter or two to the reading. Clinics that standardize the measurement — same provider, same technique, bladder emptied beforehand — get more consistent serial readings than clinics where the measurement varies by who does it. If your fundal height seems to jump around between appointments, ask whether the same clinician has been measuring each time.

The expected fundal height for a given gestational age is simply the gestational age in weeks. So at 30 weeks, the expected measurement is 30 cm. The acceptable range is the gestational age minus 2 cm as the lower bound and the gestational age plus 2 cm as the upper bound.

The difference displayed in this tool is the actual measurement minus the expected midpoint. A result of plus 2 means the measurement is 2 cm above the expected midpoint — still within range. A result of minus 3 is 1 cm outside the lower bound and would prompt a clinical note.

Growth rate between visits is calculated by dividing the change in fundal height in centimeters by the number of weeks between measurements. Expected growth is approximately 1 cm per week from 20 to 36 weeks. A rate significantly above or below that pace — particularly if consistent across two or three visits — is more informative than any single measurement. That is why tracking the trend matters as much as the absolute value.

The plus-or-minus 2 cm rule assumes a linear relationship between gestational age and fundal height that holds reasonably well from 20 to 36 weeks but overstates precision at the extremes. The formula also assumes the examiner is experienced and consistent — studies have shown inter-observer variation of up to 3 cm on the same patient measured by different providers on the same day. This means a marginal finding of 3 cm below expected could be measurement noise rather than a growth concern. Serial measurements by the same provider, graphed over time against a customized growth chart, are more predictive than a single comparison against the population mean. Customized charts that account for maternal height, weight, parity, and ethnicity reduce false-positive referral rates compared to the simple age-equals-centimeters rule used here.