When are babies able to sit up by themselves?

6-11m: Babies can usually prop sit briefly by 6 mo but get to sitting over a wider range 6-11 mo.The expected average is around 7-8 mo. Prematurity and other factors can delay the emergence of this talent. Beyond 9 mo i begin to have concerns.

Gross motor skills: develop head-to-toe. With baby in prone position, put a toy in front of her to encourage her to get her head & chest up & reach for the toy at 4 mos. In the 5th month, she should balance herself with her forearms, then shift her weight to one arm, reaching for a toy with the other. A tripod sit, with arms forward on the floor is at 5-6 mos., then maintain a sit.when placed, then get to a sit 8 mos