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What Is the Moro Reflex? Why Your Sleepy Baby Suddenly Freaks Out

This seriously strange reflex may keep new parents up at night, but according to experts, the absence of it is more concerning.

Here’s one of those things they don’t tell new parents: You will one day put your sleepy newborn down and they will immediately go bugged-eyed, throw their hands in the air, and start crying. Known as the Moro reflex, this natural reaction occurs from birth up until about 5 months of age, peaking around the 2-month mark. What causes it? Most experts think it’s an evolutionary response to prevent abandonment. What stops it? Not putting down your baby.

No one ever said this parenting thing was easy.

The reflex occurs in three parts. First, the baby spreads out their arms, then they pull their arms in as if they’re clutching a tiny set of pearls, and finally, typically, they start crying. The response occurs in the so-called reptilian brain, made of the brain stem and cerebellum, and is even present in infants born with anencephaly who are missing large portions of their brains. Doctors test for it — gleefully, we’ve found — by putting their hand under an infant’s head while they lay flat, and let it drop very slightly, but parents might experience it went they rock babies to sleep. To the baby, it just feels like they’re falling.

There are numerous theories as to why the Moro reflex occurs. When Austrian pediatrician Ernst Moro coined the term Moro reflex back in 1918, he believed it was an instinctive reaction young infants have to cling to their caretakers for their own safety, similar to young bats and primates. “Others have argued that it serves as an internal alarm system to ensure baby responds to danger – loud noises, sudden movement, falling,” says Denise Gassner, a professor of Primatology and Biological Anthropology at the University of British Columbia.

While the evolutionary purpose remains up for debate, the medical significance of the reflex is clear. Doctors test for it as a sign of a healthy growing brain, whereas the absence of the response or the presence of it for too long is an indicator that something is wrong. When babies only display the reflex on one side, it might mean they endured an injury to the brachialplexus, peripheral nerve, or clavicle during vaginal birth. That’s fairly common and can be treated surgically. But when infants do not show the reflex on both sides, it raises concerns about neurological development. A persistent reflex beyond the 6 months of age is troublesome as well. 

“Absence of the response may indicate a birth injury, severe birth asphyxia, intracranial hemorrhage, brain malformation, muscular weakness, or cerebral palsy,” warns orthopedic surgeon Dr. Anthony Kouri. “Conversely, a hyperactive Moro reflex is often due to drug withdrawal from maternal drug use during pregnancy, but can be due to other conditions.”

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Kouri notes that the Moro reflex is often confused for the startle reflex, and there is some overlap in these flexes when experts test for them. But the startle reflex can be decreased with repeated stimulation and be found in a variety of developmental ages. Adults might also experience a similar sensation of suddenly falling while drifting off to sleep, which is not the Moro reflex either, but a scientific phenomenon known as hypnic jerks. This is believed to be the central nervous system responding to suddenly drifting off to sleep, and is more likely to occur in sleep-deprived individuals. So both newborns and their parents are prone to feeling like they’re falling, but for very different neurological reasons. Parents and newborns may also get a little more sleep with the help of swaddling. 

“When a baby is impacted by the Moro reflex, swaddling is a very effective way to keep the baby calm,” Kouri says. Swaddling, which is most effective if implemented at birth, helps restrict the arm movement associated with this reflex, making it easier for them to stay asleep. However, it’s important to practice safe swaddling techniques, as wrapping babies too tight and placing them on their sides and stomach have been found to increase SIDS risks.

For parents who do not want to swaddle, simply placing their baby’s head down extra gently can help them avoid the Moro reflex. If you don’t stick the landing and your kid wakes up crying, rest assured that it is temporary and a sign of healthy development — even if you don’t rest easy.