but secretly worry I'm missing something
Am 2nd time mum so I should know better, so excuse me but...
Close family member, their baby is 6 months younger than mine, is so advanced!
Sitting up unaided at 4 months etc.
I don't even know what age dc2 sat up at 
My dd 15 months has no desire to walk, bum shuffles at lightning speed so no real motivation. Keep getting 'jokes' that the other baby will be walking long before mine. I really don't care about when they walk as I did the whole obsessive worry with dc1 and he did it in his own time.
MIL, SIL are putting it down to 'salts' that were taken during pregnancy to aid bone structure and swimming.
I only took the same pre pregnancy vitamins I took with first dc.
Is this mad or am I missing something?