Best way to explain the facts of life to ds

[startrek90]

Posting for advice. I am pregnant with baby number 3 and we are telling our two sons (5 and 3) the older one is in a why and how phase and I know he is going to ask me how the baby got in there. He already knows that babies come from mummies tummies but we have approached the how part now. What's the best fact based way to explain without overcomplicating it? Also should I explain or will other parents hate me if my son blabs? Should I tell him not to discuss it with friends?

I would read this book to him and then answer any questions he might have. all the best x

Simply stay matter of fact and answer their questions in an age appropriate way, rather than give them information they haven’t asked for.
You could quite truthfully tell them that women are born with a set number of eggs inside them, and each month one egg has the chance to grow into a baby, if you want one.
If they persist and asks what causes that process, you can then say that daddy provides a sperm that is the other half of the baby, which it needs in order to grow.
It’s only if they ask specifically how the sperm gets inside you that you need to broach the subject of sex.
Again, if necessary just keep it simple and mechanical - explain that a penis has two jobs, one is weeing and the other is to be put inside mummy’s vagina to squirt the sperm up to where the baby will grow. You can explain that sperm are made inside testes, and that when they are grown up they will make them themselves and can give their wives a baby.
At your DC age, this info is usually received as simple facts, like the names of dinosaurs or how electricity works - they are unlikely to be embarrassed unless you are!