Most babies begin noticeably straightening their legs between 3 and 6 months of age, as the curled-up fetal position they’re born with gradually relaxes. Full leg alignment, though, is a much longer process that continues well into early childhood as bones grow and your child starts bearing weight.
Why Newborns Keep Their Legs Curled
Babies spend months in the womb with their legs folded tightly against their bodies. By 14 to 16 weeks of gestation, a fetus is already practicing large body movements like flexion, extension, and vigorous leg kicks. But the cramped quarters mean that by birth, the muscles, tendons, and ligaments in a newborn’s legs are naturally biased toward a flexed, frog-like position. This is called physiological flexion, and it’s completely normal.
After birth, the real challenge begins: working against gravity. A newborn’s nervous system is still wiring itself, and the muscle responses needed for extending the legs are present but immature. In the first few months, the nervous system is essentially learning how to coordinate muscle tone, cycling through periods of higher and lower excitability as neural circuits organize themselves at both the spinal cord and brain level. You’ll see this play out as your baby kicks, stretches, and gradually uncurls.
The 3-to-6-Month Shift
Between about 2 and 4 months, something interesting happens. Newborns have a built-in “straightening reflex” triggered by touch on the soles of their feet, but this reflex fades between 4 and 7 months as the body’s bias toward flexor tone temporarily increases. At the same time, babies gain leg fat faster than leg muscle during this window, which makes it harder for them to lift or fully extend their legs against gravity when upright. When lying on their backs, though, gravity actually helps pull the legs into a more extended position, which is why you’ll notice your baby’s legs looking straighter during diaper changes well before they look straight while being held.
By around 4 to 6 months, most babies can fully extend their legs when lying down and will push strongly against surfaces when you hold them in a standing position. Some research has shown that with regular supported standing practice, babies as young as 4 months (on average) can bear their own weight for 8 seconds or more. This doesn’t mean they’re ready to stand, but it shows that the extension strength is building.
Standing, Walking, and True Alignment
Pulling to stand with support typically happens between 6 and 12 months, and independent standing follows between 9 and 16 months, with the average around 11 months. These milestones matter for leg straightening because weight-bearing is what drives the final stages of alignment. Standing requires two things working together: the ability to extend the body upright against gravity, and the balance to keep centered over the feet.
Even once your child is standing and walking, their legs won’t look perfectly straight. Nearly all babies are born with some degree of bowing in their legs, and this is a separate (and equally normal) part of development from the fetal curl.
Bowed Legs Are Normal Until About 18 Months
Bowing happens because of how a baby’s bones are positioned in the womb and how they grow afterward. As your baby starts walking, typically between 11 and 18 months, the legs gradually straighten. Most children grow out of noticeable bowing by 18 months.
Then the legs often swing in the other direction. Between ages 2 and 3, many toddlers develop knock knees, where the knees angle inward. This can increase until about age 4 or 5 before slowly correcting. Up to 99 percent of children with knock knees grow out of it by age 7, when the legs reach their adult-like alignment.
So the full timeline looks like this:
- Birth to 3 months: Legs stay mostly curled in the fetal position
- 3 to 6 months: Legs begin extending more fully, especially when lying down
- 6 to 12 months: Legs extend during supported standing and weight-bearing play
- 11 to 18 months: Bowed legs straighten as walking develops
- 2 to 7 years: Knock knees appear, peak, and resolve
When Leg Curling or Bowing Could Signal a Problem
Distinguishing normal bowing from a condition called Blount’s disease (abnormal growth of the shinbone) is genuinely difficult in young children. Research has found that even standard X-ray measurements used by orthopedic specialists aren’t reliable for making the distinction in babies under about 30 months, because so many cases resolve on their own.
That said, there are patterns worth paying attention to. If your child’s legs haven’t straightened noticeably by age 2, or if the bowing is getting worse rather than better after they start walking, those are reasons to bring it up with your pediatrician. Bowing that affects only one leg, or that appears alongside a significant limp, is also worth flagging earlier. For knock knees, the concern point is age 7, or if the inward angle is severe or asymmetric.
In the first few months, the more relevant concern isn’t bowing but whether your baby is moving both legs equally and developing muscle tone on a typical timeline. A baby who keeps one leg consistently more flexed than the other, or whose legs feel unusually stiff or floppy compared to what you’d expect, may benefit from an evaluation. Your pediatrician checks for these patterns at routine well-child visits.