YOU should have mentined the age of your wife, anyway you have reasonably ruled out pregnancy
as a cause but their are many other causes- if she is below 35yrs I would advise to look for possibility of PCOS, Even increased stress
of day today life can be a cause, Any new medicine like OCP, Antidepressants, Antiepileptics, antipsychotic
treatments need to be ruled out, Some times a premature menopause
also can manifest like this if the mense continuously absent for more than 6months.