How is the baby's length measured?

Fetal sizes, both weight and length, are always estimations and can vary from person to person, depending on factors such as someone’s natural body structure.

There are two ways to measure a baby’s length:

• Crown-to-rump length (from head to bottom) can be thought of as the “sitting height.”
• Crown-to-heel length (from head to toe) can be thought of as the “standing height.”

Until 13 weeks, fetal size is always measured by crown-to-rump length because it’s the most accurate way to measure the size early in pregnancy. At this stage, the fetus is curled up, and measuring from the head to the bottom gives a reliable estimate of growth and development.

From 14 weeks onward, as the fetus grows and develops, different measurements are used to estimate their size from head to toe, known as crown-to-heel length, which gives a more accurate idea of their overall size at this stage.