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.

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

Permanent aura

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

awkward when you visit your family

THIS WOULD BE AWESOME.

Perhaps. Becoming a unicorn would give you magical powers and becoming a pegasus would give you flight. But an earth pony? All they get is pity. ;)

Either way, you'll be a freak (seeing as talking colorful ponies are not normal in real life). The unicorn might be worth it. The pegasus I'm a bit more dubious. Earth pony - that would suck.

So you're taking a risk.

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.