6-9 months old baby

Don't worry, different babies develop differently. My first started crawling v early at 4 mths and by 6 mths, he was able to walk ard by holding onto furniture and the wall. He walked independently unassisted by 8+ mths while his infantcare peers were still crawling ard. But my 2nd one, now 6 months, only started to scoot ard but still can't fully crawl on all fours. I'm also quite worried since her development is far slower than her brother's but PD said it's nth concerning as she's able to hit other milestones. I've read a research that some babies who develop much earlier in terms of motor skills might develop their speech skills later than average and that's exactly what happened to my boy who spoke later than his peers. For my girl, she definitely babbles so much more than her bro at the same age with more varied syllables.