Quinn didn't sit completely unsupported until 7 & half months. He didn't roll over in the 'correct' developmental order either & only learnt to roll after he'd been crawling a while

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All of my children (I have 4) have met the milestones at different ages, but it is hard to not compare esp if you have one that does it 'by the book'.
I think that as long as they are progressing, even if not in the typical pattern, then try not to worry. It's hard not to be bothered by it tho, especially when helpful people ask "isn't he/she sitting/walking/sleeping through/potty trained etc. yet?" Grrr.
On a positive note though...my 11 year old was really late to do anything (walked at 17+ months, didn't talk until 3) has started at grammar school this year having passed his 11+, so it just goes to show that it doesn't matter a jot now, but I did tie myself up in knots worrying at the time!