If you aren't up for the whole truth yet and want to avoid explanations which may lead to more questions, what about Mummies have eggs in their tummy that grow into babies. You could go on to explain exactly where the eggs are and about ovulation and the egg travelling down the tube etc and how the baby comes out when it's born. Maybe with out adding a daddy into the conversation she might not ask about his role in the whole thing.
I always like to tell the truth too, but I really worry about other kids because my DS is an absolute parrot and repeats everything, So I do try to keep in mind he does talk to other kids who's parents may not be happy with their kids knowing every detail when they're not comfortable with it, or the kids aren't ready to process the info, and possibly be scarred for life lol IYKWIM.

So that explanation isn't lying even though it is excluding certain things. If she keeps asking maybe she's ready to know if she understands when it's appropriate to talk to other people about things then it's not a bad thing to learn good luck.