In suspected infanticide, the 'hydrostatic lung test' (docimasia pulmonaris hydrostatica) determines whether a newborn breathed after birth. A POSITIVE test (lungs float) indicates:

• A The infant was stillborn but was immersed in water postmortem
• B The infant was born alive and breathed air, introducing buoyancy-conferring air into the alveoli ✓
• C Putrefaction has produced gases that cause the lungs to float regardless of live birth
• D The lungs have undergone hyaline membrane disease causing surfactant-mediated floatation

Explanation

A positive hydrostatic lung test (lungs or lung fragments float in water) indicates that air entered the alveoli from breathing, proving live birth. Unexpanded foetal lung tissue sinks (negative test = stillbirth or no breathing). However, the test is not infallible: false positives occur from putrefactive gas formation (recognised by the foul smell and green discolouration of lung tissue — frothy gas from decomposition). False negatives occur if the newborn breathed but lung tissue has been compressed. The test is now interpreted cautiously alongside histological evidence of alveolar expansion and surfactant production.

Reference: The Essentials of Forensic Medicine and Toxicology (Narayan Reddy), 34th ed.

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Written and medically reviewed by the StethoPrep medical team.