Fundal Height Gestational Age Calculator
From roughly 20 to 36 weeks of pregnancy, the fundal height in centimeters, measured from the pubic bone to the top of the uterus, tends to roughly equal the number of weeks of pregnancy. This calculator applies that rule of thumb to estimate gestational age from a fundal height, and compares it to your known weeks by dates, flagging the difference against a threshold you can set (commonly 2 to 3 centimeters). It is a teaching-rule screen for general information, not a diagnosis; clinicians interpret fundal height alongside other findings.
Fundal height rule
Estimated weeks = fundal height in cm (about 20 to 36 weeks)
Difference = fundal height - known weeks
Flag = within range if absolute difference <= threshold, else outside
Expected height range = known weeks minus threshold to known weeks plus threshold
The one-to-one rule (McDonald's rule) is most reliable from about 20 to 36 weeks. A difference beyond the threshold you set may prompt closer assessment by a clinician.
Fundal height context
- Fundal height in centimeters roughly equals weeks from about 20 to 36 weeks.
- A difference of more than about 2 to 3 cm may prompt further assessment.
- Body habitus, fibroids, bladder fullness, and twins all affect the measurement.
- It is a screening tool, not a precise dating or growth method.
- This tool applies a teaching rule only and is not medical advice.
Fundal height: frequently asked questions
What is fundal height?
Fundal height is the distance in centimeters from the top of the pubic bone to the top of the uterus (the fundus), measured along the abdomen. From roughly 20 to 36 weeks of pregnancy, it tends to track gestational age closely and is used as a quick screen for fetal growth.
How does fundal height relate to gestational age?
A widely taught rule of thumb (McDonald's rule) is that between about 20 and 36 weeks, the fundal height in centimeters is roughly equal to the number of weeks of pregnancy. So a 28-centimeter measurement suggests about 28 weeks. This calculator uses that one-to-one relationship.
How big a difference matters?
A common teaching is that a fundal height differing from the expected weeks by more than about 2 to 3 centimeters may prompt closer assessment, such as an ultrasound. The exact cut-off varies by clinician. This calculator flags the difference and lets you set your own threshold.
Is the rule accurate at all gestational ages?
No. The one-to-one relationship holds best from about 20 to 36 weeks. Earlier and later in pregnancy it is less reliable, and factors such as body habitus, fibroids, bladder fullness, and multiple pregnancy all affect the measurement. It is a screen, not a diagnosis.
Is this calculator medical advice?
No. It applies a teaching rule of thumb for general information. Fundal height is interpreted by clinicians alongside other findings. Always rely on your obstetric provider for assessment of fetal growth and dating.
Official sources
- U.S. National Library of Medicine, MedlinePlus: fundal height and prenatal care.
- U.S. Centers for Disease Control and Prevention: Maternal and Infant Health.
Reviewed by the CalculatorHub team, edited by James Graham, 17 June 2026. See our methodology.