My DD was 4 when her brother was born. A simple, "He's going to come out the birth canal,' may suffice.
But if yours is like mine, he's want more detail: 'when your brother is ready to be born, mommy's body will let her know. Right now your brother is in mommy's womb, a soft special place where he can grow and be protected. In fact he is floating in some special water (you can even add: 'called amniotic fluid,' it won't be too much for him). When he is ready to be born, the womb will start to push him into the birth canal using my muscles. My tummy will get really tight, maybe you will be able to feel it sometime! It's called a contraction. In fact, the special water might come first, and when it does, mommy is going to go right to the hospital, because brother will be on his way! When you and your brother were born, it was the best day of my life! And you can let him in on the family joke: 'I was really surprised when your little brother was born and my water came at WalMart! Remember hearing about that? Normally, a mommy wants to be resting when she is so close to giving birth. But not with your little brother. No sir! He was in a hurry to come. So he wasn't going to wait til mommy got home . . . . You know, he hasn't slowed down since . . ." etc.
For my C-section birth, we said: "Mommy's birth canal wasn't working, so the doctors had to do surgery to get you out. It didn't hurt too bad, because I had anesthesia to cut the pain, and I had a beautiful baby to look forward to. But it isn't easy either. That's why when mommy's friend Sarah had her baby, we took her dinner. Remember that? We like to help mommies when they have a new baby, because it can be a big job . . .'
As for conception, We said, "When a mommy and daddy decide to have a baby, they lie together in a very special way, so a baby can start growing in mommy's womb.' That seemed to hold her.
Saying baby is in your 'tummy' is really confusing, I think. "Womb" sounds biblical, but 'uterus' is pretty biological. Why not let kids know there is a special place for babies to grow?
Also, just be aware, your child will tell everyone what you told her. She would proudly say, "My mommy has a baby in her womb." My c-section story was reported multiple times to friends and strangers alike. ANd the topper was, in the grocery line, "Daddy, when are you and mommy going to lie together in that special way so I can have a baby sister?"
Good luck with the birth!