Babies kicking one leg more than the other is surprisingly common and, in most cases, completely normal. At 3 to 4 months of age, roughly 76% of an infant’s kicks are unilateral, meaning they naturally favor one leg at a time rather than kicking both together. This is a well-documented phase of motor development, not a sign that something is wrong. That said, there are a few situations where persistent one-sided movement deserves a closer look.
Why One-Sided Kicking Is Normal
Newborns start out kicking in an alternating pattern, left-right-left-right, much like a bicycle motion. By around one month of age, they shift to a dominant unilateral kicking pattern, where one leg kicks independently while the other stays relatively still. This isn’t a glitch. It’s a sign that your baby’s nervous system is learning to control each leg separately rather than moving them as a unit.
Synchronous kicking, where both legs move together at the same time, doesn’t become prominent until after five months. So if your baby is younger than that and seems to favor one leg, they’re following a typical developmental path. Their joints are also learning to work independently during this period. Around two months, the hip and ankle start moving separately from each other, and by four to six months, the knee begins to decouple from the hip and ankle as well. All of this creates periods where leg movements look uneven or lopsided.
Research on spontaneous kicking in healthy infants consistently describes high variability and flexibility in kicking patterns. Babies experiment. They may kick one leg ten times in a row, then switch to the other, then go back. On any given day, one leg might seem far more active. This variability is actually a marker of healthy motor development, not a concern.
The “Fencer” Reflex
If your baby tends to kick one leg while extending the arm on the same side, you’re likely seeing the asymmetric tonic neck reflex, sometimes called the fencer reflex. This reflex appears as early as 18 weeks in the womb and is most prominent between 1 and 4 months of age. When your baby turns their head to one side, the arm and leg on that side extend while the opposite arm and leg bend.
The result looks one-sided: the leg facing the direction your baby is looking straightens and kicks, while the other leg pulls in. If your baby tends to turn their head the same direction most of the time (which about 12% of newborns do in the first six months), you’ll see the same leg kicking over and over. This reflex typically fades between 3 and 9 months as your baby’s voluntary movements take over.
When Side Preference Develops
True limb dominance, the kind of preference that leads to being right-handed or left-handed, develops slowly over the first two years. Nearly half of infants show a consistent early preference for reaching and grabbing objects, but this relates to hand use, not leg kicking. The other half show a variable trajectory, settling into a preference closer to age two. A baby under six months who kicks one leg more is not showing “leg dominance.” They’re going through normal motor experimentation.
Hip Dysplasia
Developmental hip dysplasia, where one hip socket doesn’t fully cover the ball of the thighbone, can make one leg move differently than the other. The signs to watch for during diaper changes are straightforward: one hip may be noticeably less flexible than the other, or one leg may appear longer. You might also notice uneven skin folds on the thighs or a clicking sensation when moving your baby’s legs.
Hip dysplasia is typically caught during routine newborn exams, but milder cases can be missed early on. If your baby consistently resists spreading one leg outward during diaper changes, or if one leg seems stiffer when you move it, bring this up at your next pediatric visit. The condition is very treatable when caught early, usually with a soft brace worn for several weeks.
Nerve Injury
In rare cases, a baby who won’t straighten one leg may have femoral nerve palsy, where the nerve supplying the front of the thigh is affected. The muscle on the front of the thigh essentially “falls asleep,” and the baby can’t extend the knee on that side. This can happen from birth positioning or, in some cases, from a hip brace used to treat dysplasia.
The key sign is specific: your baby doesn’t try to kick one leg out straight at all. This is different from simply kicking one leg less often. A baby with femoral nerve palsy will bend the affected leg but won’t extend it. If you notice this pattern, it’s worth having your pediatrician evaluate the leg directly.
Neurological Causes
Persistent, consistent asymmetry in movement can occasionally be an early indicator of hemiplegia, a form of cerebral palsy affecting one side of the body. Research on infants who experienced brain injury around birth found that early asymmetries visible at both 3 to 6 weeks and 9 to 16 weeks were significantly associated with later signs of hemiplegia. However, many of these infants had known risk factors such as neonatal stroke, and some infants with early asymmetric movements developed normally.
The important distinction is between a baby who kicks one leg more often (normal variability) and a baby whose movements on one side look qualitatively different: stiffer, jerkier, or consistently less coordinated compared to the other side. If you notice that one side of your baby’s body, not just one leg but the arm on the same side too, moves differently in quality rather than just frequency, that pattern deserves medical attention.
What to Watch For
Most parents searching this question have a baby between one and four months old who seems to kick one leg far more than the other. In that age range, this is the most common kicking pattern in healthy babies. Here’s a quick way to sort what’s typical from what isn’t:
- Normal: Your baby kicks one leg more often but can and does move both legs freely. Both legs look similar in tone and flexibility when you move them during diaper changes.
- Worth mentioning at your next visit: One hip seems stiffer or less flexible than the other. One leg appears slightly longer. Uneven thigh skin folds.
- Worth calling about sooner: Your baby never straightens one leg. One side of the body (arm and leg together) moves with noticeably different quality. The asymmetry is getting more pronounced over weeks rather than varying day to day.
The variability itself is reassuring. If your baby’s kicking patterns shift around, favoring one leg some days and the other leg on different days, that flexibility is a hallmark of healthy motor development. It’s the rigid, unchanging patterns that clinicians pay attention to.