There are also a bunch of other things that are kicking in right now.
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At 8-8.5 months after the due date (not birth, but due), babies learn to ‘make categories’ (conceptually). This is a huge leap in mental development, and it messes with their behavior, including around food. See the book “The Wonder Weeks” (orthis link) for more about those stages.
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The Opinion Gene starts to kick in, too. They start to know what they do and do not like in a more fine-tuned way, and will let you know, but don’t yet know how to let you know effectively. So the back arching, which is a pretty basic frustrated distress response (less than screaming, just ‘Hey!’), starts kicking up more.
When both my kids did this (and they both did), I took it as a cue for mommy to turn into communication central. Ask questions - they may respond (“Are you all done?” “Do you want down?”). This preps them for the whole communication-is-two-way concept. Also, go ahead and explain, too: “You need to stay in your seat while you eat, just like we do. When you are done eating, you can go run around.” Some kids don’t ‘get’ that much talking, so you can also try: “Not safe! Bottom down. Sit to eat, please.”
Look for native signs (their own sign language) in reply, or consider teaching ‘baby signs’ to help them communicate back. Brendan has a good dozen signs and nearly-comical facial expressions (intentionally used) that he uses to communicate back to me: Hands raised for ‘up’, scowl for ‘don’t like/don’t tell me what to do’, hands extended and turning outward with a cute head tilt and shoulder lift for ‘please’, leaning-forward squat (like a football player) for ‘lets’ play’, arms out/hands up shrug for ‘all gone/don’t know’, hand open palm up and finger curls for ‘come here’, hand sideways/finger flicks for ‘take it away’, hand wave palm down (like pushing) for ‘go away’, angry face for ‘no’, extreme face scrunch (smiley) for ‘I like that’, stern finger point for ‘don’t do that’, hand flap (like you burned yourself) for ‘hot’, hands to head and pat for ‘dangerous/head bonk’, and so forth. Most of these he just used once or twice in a situation, and I responded as if they were stable terms, he grasped that they were useful, and repeated them. Now they’re his signs. Very useful, and really reduces their frustration level.
I generally use a sign, voice (word plus tone), and facial cue together to help them ‘get’ a meaningful term like ‘hot’.
So, yeah, the back arch is normal. She’s frustrated, is my bet - maybe she wants the food faster, slower, warmer, colder, wants to sit at the table, wants a different spoon, wants a different food (like what you’re eating), or even wants to listen to something different, wear something different, etc. - but she cannot tell you, and it drives her nuts. But if you try to end the eating, that’s wrong, so she gets mad. Keep trying different things, and see if you can pinpoint what is bugging her - it may even be the seat is sticky, or warm, or uncomfortable. But don’t get upset if you can’t figure it out - it could be so many things that you can’t cover them all. Worth a shot to try to check the easy/obvious ones, though.
Eventually, they progress to using the back arch for entertainment. Brendan (now 19 months) still loves to sit on my lap and throw himself backwards in an arc. I hold onto his shoulders, and let him flip completely over, until he is standing on his feet on the floor in front of me. He thinks it is funny. In the meantime, listen to the wise women who say ‘ignore the nits!’ and don’t let yourself feel unworthy to be the mom. You are the mom. And normal moms are optimal for babies - you don’t even have to be perfect. (Thankfully, since I certainly am not!)