We have a 5-year-old son and my wife is pregnant with a second child. Our son asked us how the baby got inside of her. Should I give him the talk? If so how much detail should I go into?
You are already giving it more importance than it has by calling it The Talk. It's such a big deal to you (and many more parents) just because you want it to be so. Treating sexuality as something Top Secret or Super Important while it is just another thing, it's a mistake as, what will happen in the future to your kids, is the same happening to you right now, you don't feel comfortable talking about it. And in most cases, this is the main reason why children are sexually uneducated, because their parents are afraid to talk about it.
If you want to make a change here and let your child be different and more mature, go ahead and talk with him about it without fear, don't give it the importance it hasn't and let him speak aswel, let him ask and be curious as far as he wants to know more. Maybe, as @anongoodnurse said, he does not really want to hear a lot, just a tinny bit of information will satisfied his curiosity.
Little ones often have their curiosity satisfied with small answers. For the vast majority of questions about any topic at all, I find it most rewarding for the child to do two things:
- Ask them what they think first.
- Provide an answer to the specific question they asked without giving too much more detail. For this particular one, "mommy and daddy decided we wanted you to have a little brother or sister, and mommy has the superpower to make one, with a little help from daddy".
Sometimes there is no need for step 2. Judge your child's reaction. For any follow-up questions, start back at step 1.
Your 5yo isn't going to want a long dissertation on any subject (except maybe the different kinds of Pokémon), and giving them one may cause them to lose interest and leave the conversation frustrated.
Children often are not looking for long, drawn out, "THE TALK" kind of answers to these questions. It's important to not lie (the stork put it there!) because that does nothing for their educational development, but it IS important to keep it age appropriate for understanding, and usually as brief as possible. My suggestion would be to find some age appropriate books on the topic at the local library or off of Amazon. There are literally tons. They can help, and in the meantime, make sure your child knows that your question is important to you and that you are going to answer it, that you want to make sure that you give him the right information first.
You should treat this question the same as you would treat any other questions that your child asks you, which is to answer them truthfully and in as much detail as they are interested in.
They may not be able to comprehend everything ('cell' seems to be a difficult concept IME), but it is possible to find analogies that help them understand. For example I've tried to explain to my almost 5-year old daughter that inside your body there is a book that describes everything about you. The egg and sperm each have half a book inside, such that when they come together a new full book is formed. In this way a baby resembles their mommy and their daddy! This way you can lay a basis for genetics. Just don't forget to explain the mechanical details of baby-making too!
Anyway, it is important to remember that no question is off limits.