Inspired by the where to find colourful babygros thread, I realised I am completely dumfounded by the range of babygros combined with cooler/warmer months!
I was in Marks the other day, contemplating how we have a lot of newborn stuff but how it would be good to get 3-6, 6-9, 9-12 etc. We intend to keep our little cub (Jungle Book inspired in-joke name for our baby until we meet them!) in babygros for a long time so not massively interested in trousers or dresses.
I'm due in mid-October so all our newborn stuff is long legged, long sleeved.
3-6 months will still be the end of winter into Spring
6-9 will be early/mid summer summer
9-12 will be late summer/Autumn
I was looking at the range of long sleeved, short sleeved, and sleeveless babygros and got myself into a right pickle trying to figure out roughly at what age it would need short sleeve or sleeveless/legless etc. Factor in the unpredictability of English weather and I was completely malfunctioning in Marks and left 
Thinking about it now, it doesn't seem that complicated but I think I am half baby-brained, and half not confident of general rules of thumb on these things! Apologies about the total novice, over-thinking nature of this thread!