I won't hold it against you if you've never wondered where babies come from in movies. But, it's good to know the details to understand how it works and ease out the planning should you be commissioned to shoot a lifestyle brand's advertising campaign. The general aim is to keep the babies safe on the day, to let them get the rewards for the work they put in once they turn 18, and to give the production team options and solutions, like booking twins or triplets so you either have more time to shoot the baby scenes as you can literally swap out the infants. This video shows how it works in more detail.
It's not something you think about often. You're focusing on your work, spend time with agents and people in your industry and then your friend gives birth to a child and starts a clothing label for a baby and needs some images for the marketing materials and website. This video will be a perfect starting point for you to get the correct information and execute the shoot professionally.
What I took out of this video is that you need to know the amount of time you have with the child per day. It's short and you'll have to do light tests with a doll so when the baby is called upon the setup is done and ready to go. Due to the video, every time I see an advert or movie with a baby in the scene I think about how it got there and try to imagine what the shooting day entailed and that's part of the creative process for me now, with other, non-baby photos and videos too.