Funal height, or MacDonald's rule, is a measure of the size of the uterus used to assess fetal growth and development. It is measured from the top of the pubic bone to the top of the uterus in centimeters. It should match the fetus' gestational age in weeks within 1 to 3 cm, e.g., a pregnant woman's uterus at 22 weeks should measure 19 to 25 cm.