The use of Regestrone (norethindrone) is not permitted during pregnancy. Since there are cases of false negative pregnancy tests (i.e. test shows negative but the woman is pregnant), one should do 2-3 tests to exclude pregnancy before prescribing Regestrone.
When Regestrone is given in high doses at the time of the development of external genitalia (from Day 45 of pregnancy onwards), the female fetus may show signs of musculanisation (male characters such as hair on unwanted places) but no problem in male fetus. Apart from this no adverse effects have been reported.
Since in your wife's case, (a) the dose was not high, (b) Regestrone was given before Day 45, it is pretty safe to continue with pregnancy.