I always assumed it was ~handwavy gods and magic stuff~ like joyfulfeather mentioned, although, if you think about it, Asgardians *aren't* human. Who says they have pregnancies like we do? Maybe after sex if they want a baby the fertilized egg is placed in an incubator to grow? Or if, instead of copulation as we think of it, two consenting adults submerge themselves in a special pool, kiss under water and when they emerge to take a breath a baby is in their arms?

It's mythology, anything goes!