Nareau
Explorer
My thinking with questions like these is "What's the most interesting thing that might happen?" I agree with others who have stated that real-world science shouldn't enter the equation.
So if a gnome and an elf fell madly in love and wanted to conceive a child, I'd let polymorph help them out.
If a dragon polymorphed into a human female in order to get it on with her human boyfriend, I'd let her remain pregnant when she returned to her dragon form (and the child would eventually be laid in an egg, and hatch out).
If a pregnant rat were to be polymorphed into an ogre, we would see a scene of incredible horror.
If a pregnant woman were to be polymorphed into an asian man, this would happen.
Spider
So if a gnome and an elf fell madly in love and wanted to conceive a child, I'd let polymorph help them out.
If a dragon polymorphed into a human female in order to get it on with her human boyfriend, I'd let her remain pregnant when she returned to her dragon form (and the child would eventually be laid in an egg, and hatch out).
If a pregnant rat were to be polymorphed into an ogre, we would see a scene of incredible horror.
If a pregnant woman were to be polymorphed into an asian man, this would happen.

Spider