What the alpha angle on a hip ultrasound means
Written by Dr. Mutasem Aldhoon, pediatric orthopedic consultant · Medically reviewed by the Baby Hip Center pediatric orthopedic team · Last reviewed September 1, 2026
The alpha angle measures how deep and well formed your baby's hip socket is. 60° or more is normal at any age. 50-59° in a baby under three months is usually an immature hip that catches up on its own. Below 50°, or 50-59° after three months, normally needs treatment.
The alpha angle is the number parents fixate on, and rightly — it is the most useful single figure on an infant hip ultrasound. But it only means something alongside your baby's age and whether the hip is centred.
How it is measured
On the standard coronal view, one line runs down the straight outer wall of the pelvis and a second runs along the bony roof of the socket. The angle between them is the alpha angle. A deep socket makes a steep roof and a large angle. A shallow socket makes a flat roof and a small angle.
What each range means
| Alpha angle | Under 12 weeks | Over 12 weeks |
|---|---|---|
| 60° or more | Normal (Graf I) | Normal (Graf I) |
| 50-59° | Immature — repeat scan in 4-6 weeks (IIa) | Dysplastic — treatment (IIb) |
| 43-49° | Severely shallow — treatment (IIc / D) | Treatment (IIc / D) |
| Below 43° | Dislocated — specialist treatment (III / IV) | Specialist treatment |
Three things that distort the number
- Positioning. If the probe is not exactly in the standard plane, the angle reads lower than the hip really is. A good report says the standard landmarks were seen.
- Age in weeks, not months. A 5-week-old and a 13-week-old with identical angles get very different advice.
- Who measured it. Alpha angle measurement has genuine variability between observers of a few degrees, which is exactly why a borderline result is repeated rather than acted on immediately.
A single borderline scan is not a trend
One reading of 56° tells you far less than two readings six weeks apart. Improving means the hip is maturing. Static or falling means it is not, and that is the finding that changes the plan.
What about the beta angle?
The beta angle describes the cartilage rim of the socket rather than the bone. A high beta angle with a borderline alpha angle suggests the rim is being pushed upwards by a head that is sitting too far out — a more worrying picture than the alpha angle alone implies. If your report gives both, both matter.
After about six months, this number disappears
Once the femoral head begins to ossify, ultrasound can no longer see through it and the alpha angle is no longer measurable. From then on hips are followed on X-ray, using the acetabular index and Perkin's lines instead.
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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.
- 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.
- Hip ultrasound or X-ray: which one does your baby need?
Ultrasound is used up to about 4-6 months, X-ray after that. Why the cut-off exists, what each test shows, and what radiation exposure really means here.
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.