Tone and reflex development before term.

The evolution of tone and reflexes from 25 weeks postmenstrual age (gestational age plus chronologic age) to term in a population of 42 surviving infants is described. The infants were born in 1983 at the Johns Hopkins Hospital, had birth weights less than 1300 g, were examined weekly until neonatal intensive care unit discharge, and did not develop cerebral palsy. Lower-extremity flexor tone was first detectable at 29 weeks post-menstrual age by the popliteal angle and heel to ear maneuvers. Flexor tone, recoil, and hyperreflexia were all noted 2 to 3 weeks earlier in the lower extremities (33 to 35 weeks) than in the upper extremities (35 to 37 weeks). Hip tone (35 to 37 weeks) followed knee flexor tone, but preceded shoulder tone (37 to 38 weeks). Trunk tone on ventral suspension emerged closer to term (36 to 40 weeks), and more than half of infants evaluated at term continued to demonstrate head lag when pulled to sitting position. The emergence of the primitive and pathologic reflexes reflects (both in timing and pattern) the evolution of tone: development of the reflexes in the lower extremities precedes that of those in the upper extremities, and development of the distal reflexes precedes that of the proximal. Maturation of tone, deep tendon reflexes, pathologic reflexes, and primitive reflexes occurs in an orderly, sequential manner, with a well-defined pattern: caudocephalad (lower extremities to upper extremities) and centripetal (distal to proximal).