When my younger son was about 9 months old, he suddenly stopped bearing weight. He stopped sitting up, scooting or pulling up.
I panicked. My older son had just been diagnosed with autism, and to see his little brother, who’d been progressing so well, suddenly stop was frightening, however misguided it was.
It turned out that my younger son was just a little hypotonic, meaning he had low muscle tone. His “abrupt” decline probably wasn’t as pronounced as I thought it was (I was probably hyper-vigilant because of the older son’s diagnosis), but it was still something to be addressed.
We had him evaluated by what, in Texas, is called Early Childhood Intervention (most states have a similar program for children from birth to 3 years old). He was assigned a wonderful occupational therapist who appeared to come twice a week to play games and cuddle with him, but the therapist was really performing targeted exercises to help make him stronger.
Within a few weeks, he was back to proper sitting, scooting and pulling up to standing.
There is a huge range of normal development in babies, but if you ever have any question, consult a medical professional. It’s always better to 1.) have peace of mind and 2.) be proactive. If there is something to address, the earlier you get on it, the better for your baby. If there isn’t, at least you can SLEEP like a baby!