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The first thing you should do is to find the cause of irregular period & get properly treated from a Gynaec.Irregular periods
can be due to hormonal
or any other reason which will require treatment.Once the periods become regular please try to have intercourse every alternate day from 10th to 21st day of the normal 28 day cycle;as it is the fertile period when ovulation takes place.This will increase the chances of getting pregnant.If the periods become regular;then everything should be fine & you can get pregnant in a way without much expense as you wanted.
All the best.