Methods
This study was approved by an Institutional Review Board, and all mothers pro-vided written informed consent. This study was a prospective longitudinal study.
Families were recruited into the study when their infant was 3, 6, or 9 months of
age and followed every 3 months for 12 to 18 months. Families were recruited
from the community and asked to participate in a longitudinal study about infant
development.