Fundal height is a measurement which can be taken during pregnancy to assess the growth of the fetus and the development of the pregnancy. It is done by measuring from the pubic bone to the top of the uterus, and is usually recorded in an expecting mother's chart. Historically, fundal height was one of the few ways to gauge the development of the pregnancy. Today, other measures are available and they tend to be more accurate, but they aren't as fast, and they can be costly.
The only tool needed for a fundal height measurement is a tape measure. The height is usually measured in centimeters, and at around the 20th week of pregnancy, the measurement should start to correspond with the number of weeks in the pregnancy. A woman at 27 weeks, for example, should have a fundal height of 27 centimeters (about 10.6 inches). As the pregnancy progresses and the uterus expands, the top of the uterus will continue to move up to make room for the growing baby.
A number of things can interfere with a fundal height measurement. Sometimes it is done incorrectly, causing disparities in week to week measurements. If a woman has a full bladder, the measurement may be off. Likewise if she has thick uterine tissue, has had a C-section, or is carrying twins. Not all babies follow a growth chart perfectly either, so being slightly off is not a cause for concern. Finally, in the last few weeks of pregnancy, the fetus starts to descend, causing the fundal height to shrink.
However, if the fundal height is extremely off from the estimated age of the pregnancy or it fails to progress, it can be a sign that there is a problem. The fetus may be getting too big, or may not be growing enough as a result of medical complications. Twins or multiples can also cause a skew in the fundal height. If a woman's pregnancy appears to be failing to progress as expected, a doctor may recommend diagnostic tests and additional measurements to learn more about what is going on.
McDonald's Rule, as fundal height is sometimes known, can be a valuable tool for quickly assessing the development of a pregnancy at no cost. Experienced providers can even do measurements without a tape, using their fingers as a guideline to see how much the uterus has grown. For more accurate information, ultrasound needs to be used to image the uterus and see how it has grown.