Hip ultrasound or X-ray: which one does your baby need?
Written by Dr. Mutasem Aldhoon, pediatric orthopedic consultant · Medically reviewed by the Baby Hip Center pediatric orthopedic team · Last reviewed September 1, 2026
Ultrasound is the right test up to about four to six months old, because the hip is still mostly cartilage and ultrasound sees cartilage well. After the femoral head starts to turn to bone, ultrasound can no longer see into the joint and X-ray becomes the better test. The cut-off is about your baby's age, not about severity.
Why age decides the test
A newborn hip is largely cartilage, which is invisible on X-ray and clearly visible on ultrasound. Ultrasound also shows the hip moving, so instability can be seen directly. Between four and six months the centre of the femoral head ossifies, and that new bone blocks the ultrasound beam — the socket behind it can no longer be assessed. At the same moment the bony landmarks needed for X-ray measurement appear. The tests swap over.
| Ultrasound | X-ray | |
|---|---|---|
| Best age | Birth to about 4-6 months | From about 4-6 months onwards |
| Shows cartilage | Yes | No |
| Shows movement / instability | Yes, in real time | No — a still image only |
| Key measurement | Alpha angle (Graf) | Acetabular index, Perkin's and Hilgenreiner's lines |
| Radiation | None | A very small dose, comparable to a few days of natural background radiation |
| Depends on operator skill | Strongly | Less so, but positioning still matters |
Why very early ultrasound is often avoided
In the first two weeks many hips are mildly lax simply from the mother's hormones and the birth itself, and scanning then produces a crop of abnormal-looking results that resolve without treatment. Screening scans are therefore usually timed at around six weeks — early enough to treat easily, late enough to avoid alarming families about hips that were always going to be fine.
Is the X-ray safe?
A single pelvic X-ray in a baby delivers a very small dose, and it is used only when it will change decisions. The risk of leaving a dislocated hip undetected is far greater than the radiation risk of the film that finds it. Modern departments also shield and collimate to keep the dose to a minimum.
When MRI or CT is used
Not for screening. MRI is normally used after a closed reduction and spica cast to confirm the hip is truly seated, and occasionally before complex surgery. CT is used rarely, for surgical planning.
If you already have the images
Whichever test your baby had, the images can be reviewed independently. Ultrasound stills are only as good as the plane they were taken in, and X-rays are only as good as the positioning — a rotated pelvis can make a normal socket look shallow, and vice versa. That is a large part of what a second reading checks.
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Read next
- Reading your baby's hip ultrasound: Graf types I to IV explained
What Graf type I, IIa, IIb, IIc, III and IV mean on your baby's hip ultrasound report, and what each type usually means for treatment.
- What the alpha angle on a hip ultrasound means
The alpha angle measures how well formed your baby's hip socket is. Here is what a normal value looks like, and what a low number means at each age.
- The Ortolani and Barlow tests, explained for parents
What the doctor is feeling for during the newborn hip examination, what a positive Ortolani or Barlow means, and why the tests stop working after about 3 months.
This page is general information about developmental dysplasia of the hip, not a diagnosis of your own baby. It does not replace examination by your baby's doctor, and it is not for emergencies.