A newborn's early leg movements are largely due to reflex but soon develop into stronger, more intentional kicks. Kicking and leg movement help the baby prepare for rolling over, crawling and eventually walking.
If a baby was born at or near his due date, he probably had little room to move in utero. "While we have no way of knowing exactly what's going on, I suspect the first thing we see is release from the constraint of the uterus," says Dr. Charles Shubin, director of paediatrics at Mercy FamilyCare, a division of Family Health Centers of Baltimore. "The baby suddenly has a lot of freedom."
According to the Healthy Children website by the American Academy of Pediatrics, it's normal for a baby's movements to be quite jerky during her first few weeks of life. Many movements are caused by reflexes. For instance, in the Moro reflex, a newborn who is startled or suddenly shifted downward will react by flinging her legs and arms out and arching her neck back, then bringing her arms in and crying. A newborn's legs probably curve inward slightly during her first month.
Months One Through Three
As a baby moves into his second month, his legs become straighter. He may be able to move his legs briefly in a smooth, nearly bicycling motion when placed on his tummy, and he may even push with his legs as if attempting to crawl. According to Dr. Shubin, the baby is developing his ability to move with intent. This is made possible by the continuing development of his nervous system, something that happens in a cephalocaudal pattern. "Cephalocaudal" means "from head to tail." A baby gains control of his head and upper body before gaining control of his lower trunk and legs.
A newborn's kicking isn't just a side effect of development. It also plays an important part in that development, says Dr. Shubin. "Kicking helps to develop your baby's muscles, joints and bones so he can move on, eventually, to walking."
By the time a baby is 3 to 4 months old, she may be able to kick herself from a front-lying position to her back (back-to-front rolling usually comes later, at about 6 months of age). On the Healthy Children website, however, the American Academy of Pediatrics warns that exactly when a baby will start rolling over can't be known. So it's important to be especially cautious when she's on any elevated surface.