If your wife has regular 28-30 day cycles, the time of accident occurred during the safe period and therefore, chances of failure are almost nil. The cause of delay in the period could be the usual once-in-a-while disturbance, weight changes, stress
, other hormonal
imbalances, any acute illnesses and so on. Yet, you can get her test for pregnancy
either by a home test or a blood test
and if not conclusive, get a trans-vaginal sonogram and see a gynecologist for further support. Take care.