Gripping and moro responses: differences between small-for-gestational age and normal weight term newborns.

Small-for-gestational age and normal weight term newborns were compared on the grip and the Moro reflex. Responses were elicited by a rapid downward tilt of a board upon which the baby rested. Under the grip condition the baby held a bar with his left hand, while the baby's hands were free during the Moro condition. Two phases of the grip (finger flexion and relaxation time) and the Moro (arm extension and flexion) were measured. Small babies differed from normal weight term babies only on the second phase of each response. They took longer to relax their grips but showed weaker arm flexions during the Moro. Magnitude of Moro flexion was not related to grip amplitude, but was negatively correlated with grip relaxation time. Two explanations for these findings, related to the activity of the antagonist or of the agonist muscles involved in the two responses, were considered.