The startle reflex, or the Moro reflex, is an involuntary and completely natural reaction a baby can have towards excessive stimulation. Such stimulation can include sudden loud noises and the sensation of falling into space. Austrian pediatrician Ernst Moro first recognized and documented this reflex when he observed its frequent occurrence in babies.
Babies begin developing the startle reflex as early as nine to 12 weeks while they are still in the womb. The reflex becomes fully mature at birth and remains a part of their system until they are about six months old. In newborn babies, reaction towards excessive sounds or noise is called an acoustic startle reflex. For instance, the shrill ringing of a telephone or the loud hum of the vacuum cleaner can perturb babies, surprising them into an automatic response.
When this happens, babies extend their heads, fling both their arms out to their sides, spread open their palms and fingers, and flex their thumbs. They may also arch their backs and cry in surprise. Their heartbeat, blood pressure, and breathing increase during the reflex. The reflex comes to an end after a brief moment. They bring their arms back towards their chest and their bodies curl back into a relaxed position. It may take a few extra seconds for their breathing to even out, and their heart rate and blood pressure to return to normal.
The startle reflex does not necessarily occur only as a response to external stimulation. Sometimes babies startle when they experience a profound sensation of falling while they are still asleep. Since babies are no longer in the protective environment of a womb, this sensation conjures up feelings of insecurity and results in the babies' startle reflex. Physically, it appears as if they are trying to grab on to something as they "fall," although in reality, their eyes are still closed in deep slumber. The possibility that they may wake up depends on how strong their reflex was, but startled babies often go right back to sleep again rather quickly.
Parents may worry needlessly when they see their babies going through a startle reflex. This natural reaction is actually evidence of their babies' acoustic, physical, and neurological health. This reflex should appear on both sides of the body, showing identical and symmetrical reactions in both arms. Absence of it on one side can indicate an injury in the shoulder and arm, or a more serious nerve problem in the area between the lower neck and upper shoulder. No startle reflex at all could point to damage in the brain or spinal cord.
The startle reflex usually disappears when babies are about half a year old. Until then, pediatricians perform regular checkups to ensure that babies respond normally to outside stimulation. Concerned parents can also try reproducing a safe, womb-like environment by swaddling their babies in a soft blanket. Doing this reduces the babies' feelings of vulnerability and insecurity, therefore minimizing their reflex to startle.