Hip dysplasia in babies happens when the hip socket doesn’t form deeply enough to hold the ball of the thighbone securely in place. It’s not caused by a single factor but by a combination of mechanical forces in the womb, hormonal influences, and genetic predisposition. About 1 in 100 infants are affected to some degree, and understanding the causes can help explain why some babies are at higher risk than others.
How the Hip Joint Develops Before Birth
A baby’s hip is not a solid bone at birth. It’s made of soft, flexible cartilage that gradually hardens over the first years of life. For the socket to develop its proper cup shape, the ball of the thighbone needs to sit snugly inside it during fetal development, applying gentle, consistent pressure as the baby kicks and moves. Anything that disrupts that pressure, whether it’s reduced movement, cramped positioning, or loose ligaments, can leave the socket too shallow. The result is a hip that’s unstable, partially displaced, or fully dislocated.
Breech Position Is the Strongest Risk Factor
Breech presentation, where the baby is positioned bottom-down rather than head-down, is considered a particularly strong risk factor for hip dysplasia. When a baby sits in a breech position after 32 weeks of gestation, the legs are often folded up against the body with the knees extended. This posture pushes the thighbone out of its natural alignment with the socket during a critical window of hip development. The risk is significant enough that screening guidelines from the American Academy of Pediatrics recommend hip imaging for female infants born breech, even when the physical exam appears completely normal.
Maternal Hormones Loosen the Joint
During pregnancy, a mother’s body produces hormones that loosen ligaments in preparation for delivery. One of these hormones works by breaking down collagen, the structural protein that holds joints together. It reduces collagen density, increases water content in the tissue, and makes ligaments more elastic. This is essential for the mother’s pelvis to widen during birth, but these hormones also cross into the baby’s circulation.
Female fetuses appear to be more responsive to these hormones, which is a major reason girls are four to eight times more likely to develop hip dysplasia than boys. Females account for roughly 75% of all cases. In some infants, the response may be amplified by a genetic variation that makes them less efficient at clearing these hormones from their system. A study of 90 newborns with hip dysplasia found that a majority also had measurable looseness in their own pelvic joints, suggesting the same hormone-driven laxity affecting the mother’s pelvis was also acting on the baby’s hips.
Mothers who experience unusual pelvic joint looseness during pregnancy may carry a five-fold increased risk of having an infant with hip dysplasia. Researchers believe this reflects a shared susceptibility: elevated hormone levels that destabilize the mother’s pelvis are simultaneously destabilizing the baby’s developing hip joints.
Crowded Conditions in the Uterus
Firstborn babies have a higher risk of hip dysplasia, and the reason is straightforward. A first-time mother’s uterus hasn’t been stretched by a previous pregnancy, so the baby has less room to move freely. That restricted movement means less kicking and repositioning, which reduces the mechanical stimulation the hip socket needs to form properly.
Low amniotic fluid, a condition called oligohydramnios, works through the same mechanism. The fluid cushion that normally gives the baby space to move is reduced, and research using animal models has shown that this restriction leads to measurably smaller hip sockets and shallower joint surfaces. The more fluid that’s missing, the worse the restriction on fetal movement and the more pronounced the changes to hip development. Encouragingly, the same research found that restoring fluid levels during the fetal period led to significant improvements in hip measurements, suggesting these effects can be partially reversed when caught in time.
Any condition that limits space in the womb increases risk. Large babies, twins, and pregnancies with reduced amniotic fluid all create the kind of crowding that can interfere with normal hip formation.
Genetics and Family History
Hip dysplasia runs in families. If a parent or sibling was treated for the condition, a new baby’s risk increases substantially. The genetic component likely involves multiple genes that influence how the hip socket forms, how much collagen the body produces, and how sensitive the baby’s tissues are to circulating hormones. No single gene has been identified as the cause, but the family pattern is strong enough that a positive family history is used as a screening criterion alongside breech position and female sex.
Swaddling and Positioning After Birth
The hip joint continues developing rapidly after birth, and how a baby’s legs are positioned matters. Swaddling with the legs straightened and pressed together forces the thighbone away from the socket, and this sustained pressure can loosen the soft cartilage that’s still forming. The International Hip Dysplasia Institute warns that sudden straightening of a newborn’s legs to a standing position can damage the socket and destabilize the joint.
Hip-healthy swaddling keeps the arms snug but leaves the legs free to bend and spread naturally into a frog-like position. If you use a commercial swaddle, look for designs with a loose pouch for the legs and feet. Even products marketed as hip-safe can be problematic if they’re tightened around the thighs. The key principle for the first few months of life is to avoid anything that forces or holds the hips in a straightened, legs-together position for extended periods. This includes baby carriers that let the legs dangle straight down rather than supporting them in a spread position.
How Hip Dysplasia Is Detected
Pediatricians check for hip dysplasia at every well-baby visit during the first year. The two main physical tests involve gently maneuvering the hip to feel for a “clunk” as the ball slides in or out of the socket. These tests are most reliable in the first few weeks of life, before the joint tightens up.
One physical sign that often triggers referrals is uneven skin folds on the inner thighs. But this turns out to be a poor predictor on its own. In a study of 584 infants referred for suspicious findings, 78% were sent in for asymmetric thigh folds alone, and none of those babies turned out to have hip dysplasia. The infants who were referred for other reasons, like an abnormal physical exam or known risk factors, were diagnosed with the condition about 18% of the time. Uneven thigh folds are common in healthy babies and aren’t cause for alarm by themselves.
For babies with risk factors like breech presentation, female sex, or family history, an ultrasound of the hips is typically recommended around 6 weeks of age. Ultrasound can visualize the cartilage that X-rays miss in young infants, making it the preferred imaging tool for early detection. When hip dysplasia is caught in the first few months, treatment with a soft brace that holds the hips in the correct position is successful in the vast majority of cases, often resolving the problem within 6 to 12 weeks.