fetal baby weight .... gestational diabetes ?

my baby weighed 1 lb at my 20 weeks scan after having a look in Google he shouldn't of weighed that much and measured at nearly 22 weeks , this is not normal growth for me I've had 3 tiny babies my son was 6 lb 12 at 40 plus 10 days , my daughters weighed in at 3 lb 10 oz at 34 week's and 4lb15oz at 37 weeks , I'm starting to worry about gestational diabetes ? they didn't say anything other than he's a big boy and measures top end of the scale , he had a big belly and his legs looked pretty chunky , anyone else got a big baby ? I'm not complaining as such but it's made me wonder if gestational diabetes could be a cause as to why he's so big compared to my other 3 baby's ? you can see on my scan pic how big his belly is ?