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The heart is forming during the first eight weeks of fetal development. Ventricular development is influenced by blood flowing through it, and since no blood is able to pass through the tricuspid or mitral valve, the right or left ventricle remains small.
Some congenital heart defects may have a genetic link, either occurring due to a defect in a gene, a chromosome abnormality, or environmental exposure, causing heart problems to occur more often in certain families. Most of the time, this heart defect occurs sporadically (by chance), with no clear reason for its development.