I had my 6 and 3.5 yo at my last birth. We prepared them by watching a LOT of videos from YouTube that I had prescreened. I know that I labor fairly quietly with some "mooing" towards the end, so I tried to find videos like that. I also am not one to be hung up on how exactly the birth will go down (water or land, standing or squatting, I'm flexible) so I showed them a variety. We looked at some books too and did a lot of discussion. They each volunteered for different aspects of the birth itself - like fetching water or blankets, cutting the cord, etc. My kids (still, 2.5 years later) think "God puts the baby there" as far as anatomy so I did explain that there's a "pee tunnel, a poop tunnel and a baby tunnel" because they were wondering if baby would come out while I was using the bathroom. lol! We explained that my belly would squeeze the baby out of the tunnel, that it would take awhile (about 4 hours, but that's eternity when you're little!), that baby would be "slimy". I did tell them that it hurt some, but not forever and that I would need them to be quiet and probably not touch me while a squeeze was happening. They thought vernix was "baby sunscreen" because I explained it was like lotion to keep the baby's skin protected.(we live in a very sunny climate and use sunscreen frequently, lol!) Toward the end, I get a lot of "practice" bh cx and I showed them my belly sort of deforming so they could get an idea of what it would be like.
My mw was awesome at involving them in my prenatals, letting them use the doppler, pointing out body parts she'd palpate, etc. That really helped them know I was cared for and get them super excited for their sibling.
As labor began, they were very excited, but very respectful of mommy. When they were bored they went off to watch a movie, but my 6yo was taking "orders" from me on his little pad of paper, bringing me water and snacks. My 3.5 yo was laying on the floor looking thru the side of the birth pool asking if the baby was here yet. They hung out, kissed me between cx and stayed out of the way when dh joined me in the tub. They were making observations in relation to our discussions too - like "Oh mommy! You're doing that hoola hoop thing we saw on the movie". I will say that they were far less of a bother to me than I thought they would be and I remember dh trying to shush them and get them to move back when I was in labor land... I remember thinking they weren't bothering me like he thought they were, but I was unable to communicate that then.
We're going to do it all again in early January with our youngest (2.5yo) present, but we'll have someone there for him expressly. Last time we did not have anyone around for the kids since I knew my youngest at that time would be fine to follow directions etc and had some friends on call just in case. (but we didn't make the call) I don't know that my youngest will follow instructions as needed and don't want the older 2 to miss out on their part of the birth because they are needed to care for their brother.
We're all little birth junkies around here! lol!
Maybe more info than you wanted... but I hth!
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