Super-Mutants aren't born. In fact it's believed that most, if not all Super-Mutants are in fact sterile if not completely genderless. They're created by exposing humans to the FEV virus in extreme quantities. A small injection of the FEV virus isn't sufficient to create a super-mutant. They must either be exposed to the FEV by being 'dipped' in a vat of the viral fluid or by being exposed by a continuous aerosol spray.