When should your baby with T21 sit up? I know this is a really big issue that causes a lot of frustration, but let me explain so you can understand and spend your energy on much more important aspects of his development (such as creeping and crawling). :D
Golden Rule: Follow your baby's cues
It is important that the proper developmental steps are followed and at the appropriate time.
It is often tempting to have your baby sit up before he is able to put himself in and out of that position. Until your baby is able to do so, he is not developmentally ready to sit! He has not developed the muscle tone necessary for properly supporting his spine. So, no Bumbo seats or anything that prematurely puts your baby into the sitting position. (Bumbo seats are great once they put themselves into the sitting position.) I understand that this is probably the opposite of what your therapist may say. But think about it. It makes perfect sense.
Proper muscle tone has to be present first to keep from overtaxing joints and causing functional and structural problems later on. You want his muscles supporting him and not his bones or joints bearing on the weight. That's not their purpose. The only way to know if he is ready, is to let him do it himself. And don't worry, he will! Once his back and neck muscles are strong enough to hold himself upright and he's figured out where to put his legs so he won't topple over, he will sit.
Before your baby sits up, appreciate that there are four skills he must master first. These skills are:
Good head control (holding head upright and turning head to follow objects)
Adequate development of trunk muscles
Flexibility of leg and hip muscles, and
The ability to take weight on his arms.
Sitting, done prematurely, will limit the appropriate developmental activities your baby should be doing, such as developing a cross pattern crawl and creep (crawling on hands and knees) down on the floor. So, don't worry, creeping and crawling is more important for the brain than sitting. (See post Crawling: More Important than Sitting or Standing!) I know everyone will ask you if your baby can sit, but just relax, it's not important that he does it early or even "on time." I would tell people, "It's actually best for him to be crawling, so I don't encourage him to sit."
Of course, another danger of sitting your baby too early is the possibility that he will fall backwards and hit his head against the floor or other object. At this age your child is definitely not strong enough to control or even avoid hitting his head when falling backward or sideways.
Before sitting, he will need to figure out how to prop himself up on his arms and hold his chest off the ground as a sort of a mini-pushup. When he is able to sit momentarily without assistance, you should stay nearby to provide support and surround him with pillows to cushion a possible fall.
Also, watch the sitting posture and develop good habits early. Avoid the tilted pelvis or rounded lower back. Scratch the backbone a bit if he's slumping, this will help him sit up straighter.
Soon your baby will figure out how to maintain his balance while seated by leaning forward on one or both arms. Then he'll be able to sit unsupported (which will free his hands for exploring), and he'll learn how to pivot to reach a desired object while sitting. At this point he may even be able to get from his tummy into a sitting position by pushing up on his arms. And lastly, he'll be sitting well without support--and without pressure from you!
Also, once your baby can sit, don't leave him sitting unattended, not only may he fall, but he may also try to gain motion by scooting on his bottom, which makes crawling much harder to achieve. Prevent bottom scooting at all costs! Once he bottom scoots, he has less motivation to crawl properly as well.
Kay Ness, Jett's ND, SENC, Southeastern Neurodevelopmental Consultants
Down Syndrome: What You CAN Do edited by Kim and Quadosha Fish
Crawling: More Important than Sitting or Standing!
Teaching Your Baby to Crawl
Toys that Support Crawling
Low Muscle Tone: What to Do
Deep Pressure Tactile Therapy
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How to Spoon Feed
Getting Your Baby to Communicate