The backward flopping of the head when an infant is placed in a sitting position. Weak neck muscles make the baby unable to hold up his or her head until age 4 months. After that, head lag is a sign of possible developmental delay, a failure to develop physical and intellectual skills at the normal time for the child’s age and sex. Head lag can be the sign of a serious problem such as cerebral palsy.
Head lag refers to the backward flopping of an infant’s head when placed in a sitting position. In newborns, head lag is evident due to the underdeveloped neck muscles. However, by the age of four months, most babies can hold their heads upright with greater control and strength.