You say your wife gets periods every alternate month- if it's so since begining& is regularly bimonthly , it's her nature. her cycle can be a longer one .she does not need contraceptive
pills for making it monthly.
It is important to make sure of her ovulation in her cycle by sonographic ovulation study. you should start doing itabout 18 days PRIAR TO THE EXPECTED DATE OF COMMING PERIOD with ovulation +ve you can try for pregnancy
on the day or day before.
Contraceptive pills do not hamper conception in future,they just regularise periods in case the periods are hafazard.