Before birth, your baby spent months tucked in your womb, knees bent and held high against their body. The hip joints take time to stretch out once a baby is born, but eventually, they grow strong enough to allow for walking. While a baby is still very young, they have flexible hips that can become displaced or even dislocated if stretched before they’re ready. By six months of age, the ligaments are stronger, making hip dysplasia less likely.
The best position for your baby is a natural “froggy position,” where the hips are somewhat spread, and the knees are bent, sitting higher than their rear, and are well supported. Having legs straightened, with the hips and knees parallel, is less healthy for babies and may lead to dysplasia and even a heightened risk of arthritis in adulthood (source).
So, whether a baby carrier causes hip dysplasia depends on its design and the age of the baby:
For soft structured carriers: Look for a wide base that positions legs in a straddling “M” position. Keep in mind how your baby curled up in the womb, and try to mimic that. Avoid narrow seats that let legs hang down straight.
For slings: Avoid having your baby scooped up or cradled sideways in such a way that their legs are pressed together and extended. Instead, have them straddle you with a leg on either side of your torso.
For backpacks: Look for a wide seat that keeps your baby’s legs spread apart.