Final answer:
Primitive reflexes like the Moro and grasping reflex are present from birth and aid in a newborn's survival, disappearing as the infant grows older. By four months, infants can lift their head when held against a shoulder, reflecting their advancing motor development. These reflexes signify normal neurological development in infants.
Step-by-step explanation:
Primitive Reflexes in Infancy
Primitive reflexes, such as the Moro reflex and the grasping reflex, are present from birth and play a crucial role in an infant's early development. These reflexes are automatic responses to specific stimuli that aid in a newborn's survival. The Moro reflex is characterized by a baby suddenly spreading out the arms and then bringing them back in, often accompanied by crying, when they experience a sensation of falling. This reflex begins at birth and typically fades around three to four months of age. The grasping reflex allows an infant to instinctively grasp a finger or object, supporting the survival by assisting the infant to hold on to the caregiver. Present before birth, the grasping reflex persists until approximately five to six months old.
In terms of motor development milestones, by the end of the fourth month, an infant is expected to lift their head 90 degrees while lying prone and keep their head steady when held against a shoulder. This development indicates the growing strength and control over their body, as they progress from reflexive reactions to more advanced motor functions like sitting with assistance and eventually crawling and walking.
All these reflexive behaviors are indicative of a normally developing brain in infancy. However, they begin to disappear as the infant grows and gains more voluntary control over motor functions. Monitoring these reflexes and motor milestones helps health professionals assess the neurological development of the infant.