You have to pick:

By what means?
Magic? Me? Random guys?
As long as it's not the middle one, I'm fine with this.

I'm thinking its magic but you would show up as the father in the paternity test.

Do I have to sit on the egg for nine months or can I buy an incubator?

Is this like an aura of pregnancy or a one time thing?

I see no reason why you couldn't buy an incubator. With modern technology we'd probably be able to build incubators that do far better than natural care.

Permanent aura

If it's by magic, then would they still get pregnant if they physically can't?

awkward when you visit your family

would you spawn like a fish or lay eggs like a chicken?

also if chicken how many unfertalized eggs would women lay a day? Also would be tasty/taboo to eat

It occurs to me that this doesn't limit the effect to *human* females.

Those paternity tests could potentially become extremely embarrassing. And many interesting hybrids would ensue.

I think (and hope) it means human females.