Let's think about the uterus in a way a child can understand. Begin with explaining how a baby grows in the uterus. It starts out very tiny, smaller than a sunflower seed (or pea; something your child has experience with). It keeps growing and getting bigger, just like your child grows and gets bigger. What does it do in the uterus? Well, it eats and wiggles, just like your child. It also hiccups and sleeps. It can hear, but not see. I love the book, "What to Expect When Mommy's Having a Baby" by Heidi Murkoff. I finally bought a copy because they are really cheap on amazon.com and I was constantly checking it out from the library. I skip the pages on how the baby gets started and the birth process during the first few reads. Our daughter was 3 years old when I was in the last trimester of pregnancy with our son. We began reading this book with her and skipped those two sections. When she was about 4 1/2 years old, we finally read through and discussed the birth process. She was fascinated and enjoyed the book. She loved learning about the birth process. There was nothing scary or gross or intimidating about our discussion. It was actually a wonderful, rewarding experience. We've read the book many times. She is 5 now. I don't actually skip the page on how the baby got started, but make up my own text about how families look forward to growing and having new siblings come into it. I may add text on moms and dads loving each other and wanting to add to their love by bringing a baby into the family. There are lots of good children's books you can find that can help you begin a discussion.
I use a simple analogy when talking about the uterus. It's like a balloon. Children will understand a balloon. The uterus is very small and as the baby grows, it grows with the baby (like blowing into a balloon will make it get bigger. Then, when the baby is born, the uterus will get smaller again (like letting the air out of a balloon). When you get to the part about how babies get started and are born, the balloon analogy helps them understand that more clearly as well. There is an opening where air enters the balloon (an opening for receiving necessary pieces of cells for starting a baby) and an opening for the air (baby) to come out. This will also help them have a better visual understanding of what happens without actually having to show them an anatomy book. Anatomy book pictures can sometimes be a bit abstract for children, although children can gain an understanding from them.
For mothers that are pregnant, now is a great time to talk with your children about pregnancy, even if they are older and understand this information, you can still use this time to review how babies grow and are born. For mothers that aren't pregnant, you can point out pregnant women and begin a discussion about how babies grow. I used a friend that was pregnant that my daughter knew to stimulate discussions to review what she already knew (since her baby brother was already born).
FYI- Ages 6-8 are generally the appropriate ages for talking about how a baby got started. We'll address that topic later ;)