HI, I am in Davie and have actually had two babies come through my classes who had that surgery in the past years. They ended up being normal developing and their heads and brains grew fine after their surgeries. Get her on her tummy as much as possible to increase her strength before the surgery because she will be on her back for awhile and she will be at risk of developmental delay. Let her be on the floor, on belly with her toys and sister. I am wondering if she is crawling or close to crawling yet. If not make sure she stays down most of the time she is awake. She can practice her skills, work out primitive reflexes, rock on hands and knees, roll, turn over and go to a sit from laying. Good luck with the surgery. The worst part, for the time being, is the scar but eventually the hair covers it. I don't know where you have been but Stelnicki at Memoral East is the go to guy.
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I actually know two babies years ago who had that surgery. They both grew up fine and developed normally. Make sure your baby is crawling or starting to crawl to activate the brain and get it ready for school. If the left and right hemispheres are hooked up and your baby has strength to recover from being on her back for awhile while being in the hospital, she will recover and go back to the business of being a developing baby. Give her lots of tummy time now or put her in situations that require her to use upper body like pulling up her legs and making her balance on her hands using shoulders. The surgery should be fine and the scar fades over time and hair covers it. Good luck!
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I just read your update about the helmet. Put her in occupational therpay at the same time so she gets good vestibular activation and stimulation for her sense of balance. Those helmets screw up the whole sense because they have limited movement at a time when they should be having intense movement. OT helps so much by providing the input that she will need to develop a sense of time and space, attention, staying still and focus. It's all inside the inner ear. That fluid needs to move and move. Good luck!