4 month old!

Brittany

So, my 4 month old baby girl has learend to arch her back. The thing about it is now when I am giving her a bath she does it in the bath tub. smh. I have one of the baby bath tubs, but she still tries to arch her back in the bath tub. It has the little seat were I can sit her butt in, but when she tries to arch her back she goes sideways and I'm scared her ears or her face is going to go under the water. I never walk away when she is in the tub, but even though I am right there it still scared the shit out of me. Does anyone have any tips on making it easier for her to sit still in the tub? And do yalls babies do this?