In a premature infant, the pupillary light reaction is present at approximately how many weeks gestation?

• A. 26 weeks gestation
• B. 30 weeks gestation
• C. 34 weeks gestation
• D. 40 weeks gestation

Answer: (B)

Pupillary light reflex reflects the maturation of midbrain pathways controlling the pupil. When light hits the retina, signals travel via the optic nerve to the pretectal area, then bilaterally to the Edinger–Westphal nuclei, and out through the oculomotor nerve to constrict the pupil. In premature infants, these brainstem circuits become reliably functional around 30 weeks gestation. So, a present and consistent pupillary constriction in response to light indicates that the infant’s midbrain and ocular pathways have matured to about this point. If the infant is earlier than that, the reflex may be absent or variable due to immaturity; by the time you’re at around 34 or 40 weeks, the reflex is typically well established, but the milestone used for prematurity assessment is around 30 weeks.