Fundal height, or McDonald'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 uterus to the top of the pubic bone in centimeters. It should match the fetus' gestational age in weeks within 1 to 3 cm, e.g., a pregnant woman's uterus at 26 weeks should measure 23 to 29 cm. This is valid from 24 weeks.

A shorter measure can happen for one of the following reasons:

• Fetus descent into the pelvis, seen normally two to four weeks before delivery
• Error in estimated date of pregnancy based on first day of last menstrual period
• Fetus is healthy but physically small
• Oligohydramnios
• Fetus positioned sideways
• Small for gestational age

On the other side, a longer measure can be caused by:

• Twins, or other types of multiple birth
• Error in estimated date of conception
• Fetus is healthy but physically large
• Polyhydramnios
• Large for gestational age
• Hydatidiform Mole
• Breech birth

As a pregnancy approaches its end, the fundal height will become less accurate.

Most caregivers will record their patient's fundal height on every prenatal visit. Measuring the fundal height can be an indicator of proper fetal growth and amniotic fluid development.

Illustration of fundal height at various points during pregancy