RE: finasteride and pregnancies
You do not have to come off if you dont want/need to.
Finasteride in animal studies has been linked to external genitalia abnormalities (hypospadias) in the male baby only. So already any risk is halved depending on if its a girl or boy (ie no risk if a girl).
The experiments that gave abnormalities in animals, they were using drug exposure 750 times (and giving it direct through a vein) more than the amount you would get in semen from using 1mg finsteride day. Thus the risk is theoretical.
Your partner should avoid handling the drug itself (another theoretical risk).
Dutasteride and finasteride are both 5-alpha-reductase inhibitors, the enzyme responsible for converting testosterone to DHT.
There are 2 types of this enzyme 5-alpha-reductase, type 1 and type 2.
Dutasteride is a competitive inhibitor of both type-1 and type-2 isoenzymes while Finasteride is a competitive inhibitor of type-2 isoenzyme. Thus Dutasteride is more potent/stronger.