Subjects were chosen from the National Collaborative Perinatal Neonatal Network (NCPNN).

Inclusion criteria for the present study were as follows: infants less than 37 weeks of gestation, admitted to the intensive care unit, and having no congenital malformations, sepsis, neurologic disorders, or metabolic and electrolytic disturbances such as hypoglycemia, hypocalcemia, or hypomagnesemia.

Consanguinity was categorized into two levels: first-degree cousin marriages versus nonconsanguineous and distantly related marriages.