That's Not a Baby!
1. Giving birth to animals. For many women, the closest they've come to motherhood is having a pet. Taking care of puppy or kitten is relatively simple, in contrast to nurturing a baby, which is unknown territory. Giving birth to a pet is a "practicing" dream, in which the animal stands in for the baby. In other words, it's your mind's way of rehearsing for the real thing, but on a more manageable level. These dreams may speak to an underlying fear of being completely responsible for a helpless being.
2. Giving birth to an alien. While unsettling, this dream is quite normal. After all, at times it feels as if an alien has taken over your body. And the only hint you have of what your baby looks like is from the sonogram image -- which looks like an alien. This dream is similar in meaning to giving birth to an animal: you don't know who this little person is going to be, or how you're going to take care of it.