Laryngomalacia
The most likely diagnosis is B. Laryngomalacia, which classically presents with positional stridor that improves prone and worsens supine, beginning in the first weeks of life in an otherwise thriving infant.
Clinical Presentation
The key diagnostic features in this case point definitively to laryngomalacia:
Positional stridor that improves in prone position and worsens supine is pathognomonic for laryngomalacia, as the supraglottic structures collapse more readily when gravity pulls them posteriorly in the supine position 1, 2
Onset within first 2 weeks of life is typical, with stridor usually noticed by 14 days of age 1, 2
Normal examination and growth parameters indicate this is uncomplicated laryngomalacia, which occurs in approximately 80-85% of cases 3
High-pitched inspiratory stridor results from turbulent airflow through the partially collapsed supraglottic structures during inspiration 1, 2
Why Other Diagnoses Are Unlikely
Nasal atresia (Option A) would present with immediate respiratory distress at birth, cyanosis relieved by crying, and inability to pass a nasogastric tube—none of which are described here.
Subglottic stenosis (Option C) typically causes biphasic stridor that does NOT improve with positioning changes and often presents with more severe respiratory distress 2.
Vocal cord paralysis (Option D) produces stridor that is not position-dependent and would be evident on examination as a weak cry or voice changes 2.
Natural History and Management
Spontaneous resolution occurs in most cases by 12-24 months of age, even in children with multiple congenital anomalies 3
Conservative management with observation and weight monitoring is appropriate for uncomplicated cases like this one 1, 2
Surgical intervention (supraglottoplasty) is only needed in 15-20% of cases presenting with severe symptoms: poor weight gain, severe dyspnea with intercostal retractions, obstructive sleep apnea, or feeding difficulties 1, 2, 3
Important Clinical Caveat
While prone positioning improves the stridor in laryngomalacia, infants must still be placed supine for all sleep periods to reduce SIDS risk, as the AAP guidelines clearly state that prone positioning can only be considered in rare cases where the risk of death from airway compromise outweighs SIDS risk 4. This infant's normal growth parameters and lack of severe symptoms indicate supine sleeping remains appropriate.