In a 4‑month‑old infant, should I order a hip ultrasound or an X‑ray to assess the hips?

Hip Imaging for a 4-Month-Old Infant

At 4 months of age, either hip ultrasound or X-ray is appropriate, but you should strongly consider ordering an X-ray (anteroposterior pelvis radiograph) as the preferred initial imaging modality at this transitional age.

Rationale for X-ray at 4 Months

The American College of Radiology (ACR) specifically designates radiograph of the pelvis as "usually appropriate" for initial imaging of children between 4 to 6 months of age with concern for DDH. 1 This recommendation reflects the transitional nature of this age window:

• The femoral head ossific nucleus typically appears between 4-6 months of age (range 1.5-8 months), making radiographic assessment increasingly reliable at 4 months 1
• A normal pelvic radiograph at 4 months can reliably exclude DDH in children with risk factors 1
• X-ray decreases the need for treatment in infants 4-6 months old with positive ultrasound by 40%, eliminating unnecessary serial follow-ups and potential iatrogenic treatment-related complications 1

Why Ultrasound Becomes Less Reliable After 4 Months

The AAP and AAOS do not advocate ultrasound screening for DDH after 4-5 months of age 1:

• Ultrasound overdiagnoses DDH in 40% of patients aged 4-6 months when compared to radiographic findings 1
• Limited evidence supports ultrasound use for screening beyond 4 months 1

Clinical Context Matters

If the infant has known DDH and is undergoing Pavlik harness treatment:

• Ultrasound remains the preferred modality for monitoring treatment response, confirming concentric hip reduction, and identifying signs of therapy failure 1
• Radiography is not preferred during active harness treatment due to suboptimal patient positioning within the harness 1

If this is initial diagnostic imaging (screening or concern for DDH):

• X-ray is preferred at 4 months and beyond 1
• The ACR guidelines clearly state ultrasound is "usually appropriate" for children younger than 4 months, while radiograph becomes "usually appropriate" for the 4-6 month age group 1

Practical Algorithm

For a 4-month-old infant:

1. If already in harness treatment → Order ultrasound 1
2. If initial evaluation for suspected DDH → Order anteroposterior pelvis X-ray 1
3. If the ossific nucleus has not yet appeared (visible on initial X-ray) → Ultrasound may still provide complementary information, though X-ray remains appropriate 1

Key Caveats

• The timing of ossific nucleus appearance varies widely (1.5-8 months), and in dysplastic hips it is often delayed and eccentric when it does appear 1
• At exactly 4 months, you are at the transition point between modalities, so either study is defensible, but radiography is increasingly preferred as you approach 4-6 months 1
• Research demonstrates ultrasound remains technically feasible at 6 months despite ossific nucleus presence in 66% of hips, with good correlation to X-ray findings 2, but guideline recommendations still favor radiography in the 4-6 month window 1