The legs and head extend while the arms jerk up and out with the palms up and thumbs flexed. Shortly afterward the arms are brought together and the hands clench into fists, and the infant cries loudly.
It is likely to occur if the infant's head suddenly shifts position, the temperature changes abruptly, or they are startled by a sudden noise.
The Moro reflex (also known as the startle reaction or embrace reflex) is present at birth, peaks in the first month of life and begins to disappear around 2 months of age. The reflex normally disappears by 3 to 4 months of age, but it may last up to 6 months.
Bilateral absence of the reflex may mean damage to the infant's central nervous system while a unilateral absence could mean an injury due to birth trauma.
In human evolutionary history, the Moro reflex may have helped the infant cling to his mother while she carried him around all day. If the infant lost its balance, the reflex caused the infant to embrace its mother and regain its hold on the mother’s body.
Definition source: http://en.wikipedia.org/wiki/Primitive_reflexes