I wasn't diagnosed with GDiabetes with my first, but she was born 4kg a week earlier than her EDD by natural vag delivery. The day before I went into labour my ob had estimated that she would only be 3.5kg!
With my second I was diag with Gdiabetes and had to go on a low carb diet to control my Gdiabetes. Bub #2 gained weight steadily and was induced at 40 weeks, all weights and measurements were average and her weight corresponded with what my ob thought she'd be. My ob reckons that I had Gdiabetes with my first but it just wasn't detected. My ob was always concerned about how big bub got due to my small size. So it's not only how big bubba is but it also depends on the mother's pelvis as bub has to fit through that to get out.
As for contractions... dunno if it varies with different babies and whether it's your first, but my first labour took 24 hours from the first felt contraction (not painful) to the birth. If we're talking about active labour then mine took about 7 hours.
I just had my second bub last week and had no signs of labour when the drip was put in. Once they put the drip in and broke my waters, it took 15 minutes for contractions to kick in. The pain started to build up and it got quite painful after 1.5 hours. I had my baby by the third hour. It happened so quick and to me it felt very painful at the time, but when I think about it, I was relieved that the pain came fast and painful rather than long and painful because my body recovered much faster this time round and I didn't have to stare at the clock for too long =)
To teach is to learn twice