Newborns scrunch because they spent months curled tightly inside the womb, and their bodies haven’t yet adjusted to the open space outside it. This curled posture, sometimes called the “newborn tuck,” is driven by a type of muscle tone that isn’t actually produced by active muscle contraction. It’s a residual effect of being folded up in utero, and it typically fades by around two months of age.
The Womb Left a Physical Imprint
For the last trimester of pregnancy, your baby was packed into a tight space with limbs bent, knees drawn up, and arms folded close. That prolonged positioning creates what’s called flexor tone: a baseline tightness in the muscles that bend the joints inward. EMG studies of newborns found that even though electrical activity in the arm muscles was low during the first 48 hours of life, the elbows still stayed flexed and the arms still pulled inward. In other words, the scrunching isn’t the result of your baby actively tensing their muscles. It’s more like a memory held in the body from months of compression.
This explains why a newborn’s legs naturally curl into a frog-like position, why their fists stay clenched, and why they pull their limbs tight to their torso when you pick them up. The popliteal angle (how far the knee can straighten) varies quite a bit from baby to baby in those first 48 hours, which is why some newborns look more tightly scrunched than others.
Reflexes That Trigger Sudden Scrunching
Beyond the resting curl, you’ll also see moments of sudden, dramatic scrunching. Much of that comes from the Moro reflex, also called the startle reflex. When your baby’s vestibular system detects the sensation of falling, their brainstem fires off an emergency response: arms fling outward, fingers fan open, head tips back, and then the whole body pulls inward again, arms flexing tight against the chest. It can happen when you lay your baby down on their back, when there’s a loud noise, or seemingly out of nowhere.
The Moro reflex is one of several primitive reflexes that newborns arrive with. It’s a normal part of neurological development and not a sign of pain or distress, even though it often comes with crying. The startle passes quickly, and your baby will relax once they feel supported again.
Scrunching During Sleep
If your baby seems to scrunch, squirm, and grunt more while sleeping, that’s partly because newborn sleep cycles are unusually fast. For the first three to four months, babies cycle between REM and non-REM sleep roughly every 45 to 50 minutes, and they don’t always transition smoothly. During those shifts, you’ll see twitching, scrunching, and even brief crying as their nervous system navigates between sleep stages.
Digestion also plays a role. A newborn’s gut doesn’t pause during sleep, and pushing out gas or stool requires real muscle coordination that their bodies are still learning. That can produce a lot of visible scrunching, drawing the knees up, tensing the belly, and squirming, even while your baby stays asleep.
Why the Scrunch Is Good for Hip Development
The natural frog-leg posture your newborn defaults to isn’t just a leftover from the womb. It’s actually the ideal position for healthy hip joint development. Research shows that keeping the hips in flexion and abduction (bent and spread, forming an “M” shape with the legs) creates the joint forces most conducive to normal growth. At roughly 60 degrees of abduction and 120 degrees of flexion, the mechanical load on the hip joint closely matches what’s needed for the socket to develop properly.
This is why pediatric guidelines discourage tight swaddling that forces the legs straight and pressed together. In one study, applying tight swaddling to an unstable hip caused it to completely dislocate, while “hip-safe” swaddling that left the legs free to flex showed no change in stability. When you swaddle, keep the wrap snug around the upper body but loose enough below the waist for your baby’s legs to bend and spread naturally. Baby carriers that hold the legs in that same M position are also considered protective.
Forcibly straightening a newborn’s curled legs works against the body’s own blueprint. The physiological flexion contractures in the hips and knees are there for a reason, and attempting to correct them can actually increase the risk of developmental hip problems.
When Scrunching Fades
The classic newborn scrunch typically becomes less noticeable after about two months. As your baby’s nervous system matures and they spend more time stretching against gravity, the residual flexor tone from the womb gradually loosens. You’ll notice legs extending more during sleep, fists unclenching, and a generally more relaxed posture replacing the tight tuck. Every baby moves through this transition at their own pace, but it’s not something that lingers for long.
When Scrunching Looks Different
Normal scrunching is irregular, brief, and usually tied to a trigger like being startled, moved, or working through digestion. Infantile spasms, a rare but serious seizure type, can look superficially similar but have distinct features. During an infantile spasm, a baby’s arms extend outward suddenly while the body jackknifes forward at the waist with the knees drawn up. These spasms tend to occur in clusters, often right after waking, and repeat in a rhythmic pattern over several minutes.
The key differences: normal scrunching and startle reflexes are random and isolated. Infantile spasms are repetitive, patterned, and tend to happen in groups. If you notice episodes of sudden stiffening that repeat in a series, especially if your baby seems distressed or different afterward, capturing a video of the movement is the most useful thing you can do before contacting your pediatrician.