Hip dysplasia happens when the hip socket doesn’t form deeply enough to hold the ball of the thighbone securely in place. It affects roughly 11.5 per 1,000 live births, and the causes are a mix of genetics, hormones, physical crowding in the womb, and even how a baby is positioned after birth. No single factor is responsible. Instead, several risk factors stack on top of each other to determine whether a baby’s hip develops normally or ends up unstable.
How the Hip Socket Develops (and What Goes Wrong)
At birth, the hip joint is made mostly of soft cartilage that gradually hardens into bone over the first years of life. The ball at the top of the thighbone and the cup-shaped socket in the pelvis act as molds for each other. When the ball sits firmly in the socket, the socket deepens and forms a snug fit. When the ball slips partially or fully out of position, the socket stays too shallow. That shallow socket is what doctors mean by “dysplasia.”
This is why early detection matters so much. The longer the ball sits outside its proper position, the less stimulus the socket gets to develop correctly. Babies identified early can be treated with a simple harness that holds the hips in a bent, spread position, giving the socket time to deepen around the ball as the cartilage hardens into bone.
Genetics and Family History
Hip dysplasia runs in families. If a first-degree relative (parent or sibling) had the condition, a baby’s risk goes up significantly. Researchers have identified associations with specific chromosomes, gene locations, and genetic variations that influence how the hip socket forms and how loose or tight the surrounding ligaments are. The exact inheritance pattern isn’t straightforward, though. It’s not a single-gene condition like cystic fibrosis. Multiple genes interact with environmental and mechanical factors, which is why some babies with a strong family history develop perfectly normal hips while others with no family history do not.
Breech Position in the Womb
Breech presentation, where the baby’s bottom or feet point downward instead of the head, is one of the strongest known risk factors. Babies in breech at the time of birth have roughly four times the risk of hip dysplasia compared to babies in a head-down position. The risk increases the longer the baby stays breech: those in breech for 5 to 8 weeks before delivery face about 2.7 times the risk, while those in breech for 9 to 13 weeks face 3.6 times the risk.
The mechanism is mechanical. In a frank breech position (bottom down, legs extended with feet near the face), the hamstring muscles pull continuously on the hip joint while the walls of the uterus press against the baby’s hips from the outside. Combined with the rapid growth rate of the fetus in the final trimester, these sustained forces can push the ball of the hip out of its developing socket. Breech presentation that occurs only early in pregnancy and resolves before 30 weeks carries much less risk, because the forces are lower and the hip has time to recover normal positioning.
Why First-Born Babies Are at Higher Risk
First-born children are more likely to develop hip dysplasia than later siblings. The reason is straightforward: the uterus hasn’t been stretched by a previous pregnancy, so the space is tighter. Less room means the baby’s hips are more constrained, and that physical restriction can prevent the hip joint from moving freely enough to develop normally. Any condition that reduces space in the womb, including low amniotic fluid or carrying a large baby, has a similar effect.
Hormonal Factors and the Female-to-Male Gap
Girls are far more likely to be affected than boys. For every boy diagnosed with hip dysplasia, roughly five to six girls receive the same diagnosis. The incidence is about 19 per 1,000 for girls compared to 4.1 per 1,000 for boys.
A key reason is a hormone called relaxin. During pregnancy, the mother’s body produces relaxin to loosen the pelvic ligaments in preparation for delivery. This hormone crosses the placenta and reaches the baby. In the fetus, relaxin breaks down collagen in ligament tissue and reduces the body’s production of new collagen, making the ligaments around the hip joint looser than normal. Female infants appear to have more receptors for relaxin in their hip ligaments, which means the hormone has a stronger loosening effect on their joints. That extra laxity makes it easier for the ball to slip out of the socket during the critical window when the hip is forming.
There’s also an indirect effect. If relaxin levels drop in the mother’s blood, her own pelvic ligaments may not soften enough, which can contribute to a tighter birth canal and fetal malpositioning, adding yet another mechanical stress to the baby’s hips.
Swaddling and Postnatal Positioning
What happens after birth matters too. Cultures that practice traditional swaddling, where a baby’s legs are wrapped straight and pressed together, have consistently higher rates of hip dysplasia. Forcing the hips into a straight, legs-together position mimics the same mechanical problem as a tight uterus: the ball gets pushed toward the edge of the socket instead of being held deep inside it.
Hip-safe swaddling keeps the legs in a natural “froggy” position, with the hips slightly bent and spread apart and enough room inside the wrap for the baby to kick and move. Baby carriers that hold the infant’s legs in an “M” shape (knees higher than the bottom, hips spread) are considered the safest option and actually replicate the position used in medical treatment for the condition.
If you swaddle your baby, the key rule is simple: wrap the upper body snugly but leave the legs loose enough to bend and splay naturally. A swaddle that pins the legs straight is working against healthy hip development.
Is Adult Hip Dysplasia a Separate Condition?
Most adults diagnosed with hip dysplasia were born with it. In many cases, a mild form went undetected in infancy because it didn’t cause obvious symptoms. The shallow socket functions well enough during childhood and early adulthood, but the abnormal fit gradually wears down the cartilage. Pain, stiffness, and a limp typically show up in the 20s, 30s, or 40s as the joint surface deteriorates. Hip dysplasia is actually one of the leading causes of early hip arthritis in young adults.
True adult-onset dysplasia from trauma or repetitive stress is rare. Almost always, the underlying anatomy was abnormal from the start, and the symptoms simply took decades to appear.
How It’s Detected Early
Newborns are screened during their first physical exams with simple hands-on tests. A pediatrician gently moves the baby’s hips through specific positions, feeling for a clunk or shift that signals the ball is slipping in or out of the socket. These tests are most reliable in the first few months of life, before the joint tightens up naturally.
For babies who have a normal exam but carry risk factors (girls with a family history, or any baby in breech during the third trimester), current guidelines recommend a hip ultrasound at six weeks of age. Waiting until six weeks is deliberate: it gives the normal looseness of a newborn’s joints time to resolve on its own, reducing false alarms. After four months, limited hip movement, where one leg doesn’t spread as far as the other, becomes the most important sign to watch for.