Microcephaly

A healthcare provider can sometimes diagnose microcephaly before birth by prenatal ultrasound. This diagnosis, with the fetus still in the uterus, happens late in the second trimester or the third trimester.

A diagnosis most often happens within 24 hours after your baby is born. A healthcare provider will measure your baby’s head circumference (all the way around). Then, they’ll compare your baby’s measurement to growth standards for their age, which takes into account your baby’s length and weight.

If your baby has acquired microcephaly, a healthcare provider will diagnose the condition in infancy when symptoms become apparent. During a physical exam, your baby’s healthcare provider will measure your child’s head circumference. They may ask questions about your child’s progress in reaching developmental milestones for their age, like crawling or walking.

What is a head circumference percentile?

A percentile is a number on a scale of 1 to 100 that shows how a person compares to others. Percentiles help healthcare providers diagnose certain conditions. To receive a microcephaly diagnosis, your baby’s head circumference must be in the 3rd percentile or lower. A head circumference in the 3rd percentile means 3% of all infants have a smaller head size, and 97% have a larger head size.

What tests diagnose microcephaly?

If your child’s healthcare provider suspects microcephaly, they’ll measure your child’s head circumference and run tests to determine the cause. Tests could include:

• Imaging tests like a head ultrasound or brain MRI.
• Blood tests to detect changes to their genetic code or any underlying conditions.