Duchenne muscular dystrophy is the most common inherited neuromuscular disorder. The onset of myopathic symptoms is observed at an average age of 2.5 years. However, the definitive diagnosis is not reached until between 3 and 5 years of age, indicating a delay in relation to the onset of first symptoms. The diagnosis is confirmed with genetic testing. This paper presents a case of a 6.5-month-old boy with reduced motor activity already in the foetal period, hypertransaminasaemia, constipation and reduced muscle tone. Extensive differential diagnosis of these abnormalities, including genetic testing, confirmed Duchenne muscular dystrophy. Detection of this disease at an early, poorly symptomatic stage offers a chance to achieve better treatment outcomes and improve the patient’s quality of life. Modern gene therapies implemented before irreversible changes are induced by the disease may in the future give the patient a chance to be completely cured. In the presented case, the symptoms of myopathy were present already in foetal life, and the diagnosis was reached at a younger age than the typical age reported in most of the available literature data.