The new walker with the wide stance and curved-out legs. The four-year-old whose knees touch when she stands with her feet apart. Both look striking to a parent, and both are usually completely normal stages of how children's legs develop.
The shape of a child's legs at age two doesn't look much like the shape at five, and neither resembles where things settle in adolescence. Most parents who bring a child to the GP about bow legs or knock knees leave reassured: the legs are doing exactly what they're meant to be doing for the age. But there is a small proportion of cases that aren't physiological — and knowing what those look like is the useful bit.
Healthbooq (healthbooq.com) covers normal and atypical physical development in young children, helping parents tell expected variation from the things that need a medical look.
The Normal Developmental Sequence
Newborns are typically born with some degree of bow-leggedness (genu varum), partly from being folded up in the uterus for nine months. In the first two years, that bow-legged appearance is normal and often becomes a bit more obvious once the child starts walking and the legs start bearing weight.
Between roughly two and four years, the legs straighten and then keep going — swinging past neutral into knock-kneed alignment (genu valgum). This is the stage that surprises parents most, because no one warns them it's coming. The knock-kneed appearance tends to peak around three to four years, and looks most pronounced when the child stands with their feet together and you can see the gap between the ankles.
From around five to seven years, the alignment moves back toward the slight valgus angle that's normal in adults. (Most adults aren't perfectly straight-legged — there's a mild knock-kneed alignment in nearly everyone.) By age seven, most children's legs have settled into the alignment they'll keep into adulthood.
The sequence is consistent across children, even if the exact timing and degree vary.
When the Appearance Is Normal
Bow legs are normal from birth to around age two. They tend to look more pronounced in children who walked early, probably because weight-bearing kicks in before the natural remodelling has fully straightened the leg.
Knock knees are normal from roughly two to seven years, peaking around three to four. A practical way to assess: have the child stand with knees touching and measure the distance between the inner ankles. A gap of up to about 8cm is generally within the normal range.
In both cases, you can be reassured if the alignment is symmetric (both legs the same), the gait is normal without pain or limp, the child can do age-appropriate physical activity without difficulty, and the pattern fits the age-related sequence above.
Features That Need Medical Assessment
Not all bow legs and knock knees are physiological. A handful of features signal that something else may be going on:
Asymmetry. If one leg is significantly more bowed or knock-kneed than the other, that is unlikely to be normal development and needs assessment.
Persistence outside the expected window. Bow legs that are still significant after age three or four, or that are getting worse rather than better, need evaluation. Knock knees that are severe or persisting past age seven also warrant a look.
Very severe angulation. An interankle gap consistently greater than 8–10cm for knock knees, or a clearly large gap between the knees when the feet are touching for bow legs, is outside the range of physiological variation.
Pain, limp, or unwillingness to weight-bear. Physiological bow legs and knock knees don't hurt. Pain or a limp is always worth investigating.
The conditions that produce abnormal alignment include Blount's disease (a growth disorder of the inner part of the tibial growth plate, usually presenting as worsening bow legs after age two, more often in children who are overweight or who walked very early), rickets (vitamin D deficiency causing softening of the bones, with bowing through the shaft of the bone rather than just at the knee), and various skeletal dysplasias.
Rickets is worth knowing about specifically because it isn't only a historical condition. Vitamin D deficiency is common in the UK, and clinical rickets, while uncommon, does still occur — particularly in children with darker skin, those with limited sun exposure, and exclusively breastfed infants who haven't been given supplements. The legs in rickets look subtly different from physiological bow legs: the bowing is in the shaft of the tibia and femur rather than just the joint angle, the growth plates may be visible as widened "knobs" at the wrists and ankles, and there can be other signs including poor growth, dental enamel problems, and bone tenderness. The NHS recommends a daily vitamin D supplement (8.5–10 micrograms) for all babies under one year who are breastfed or having less than 500ml of formula a day, and 10 micrograms daily for all children aged one to four.
What Treatment Involves
Physiological bow legs and knock knees do not need treatment. Special shoes, orthotic insoles, braces, and exercises have all been studied and none of them alter the course of normal developmental alignment. They aren't recommended.
For children whose alignment is at the outer edge of normal or where the pattern doesn't quite fit, monitoring with periodic clinical review is appropriate. Serial photographs taken at home from the same angle every few months are a simple way to see whether things are improving, stable, or getting worse.
Pathological causes do require treatment, tailored to the diagnosis. Blount's disease may need bracing or, in more severe cases, surgical correction to realign the growth plate. Rickets is treated with vitamin D and calcium supplementation, usually with rapid improvement in the bone changes once levels are restored.
Key Takeaways
Bow legs and knock knees are normal developmental variations in the alignment of children's legs that follow a predictable pattern: most children are mildly bow-legged until age two to three, then gradually become knock-kneed until age five to six, and then align toward adult leg angle. These variations are physiological and do not cause pain or require treatment in the vast majority of children. Features that distinguish normal from pathological alignment include severity, asymmetry between legs, and the developmental stage at which they are present.
