i read this in my pregnancy book and thought it might help you.....
Although we are all different and different factors will make up each of our own final figue...these are just average amounts and what the final figure is made up of...

Blood Volume = 1.3kg
Breasts = 0.4kg
womb = 1kg
baby = 3.4kg average baby weight 7 pounds 8 ounces.....
Placenta = 0.7kg
amniotis fluid = 0.8kg
fat = 3.5kg
retained water = 1.5kg this can get up to as much as 4.5kg

total 12.6kg or 27 pound 13 ounces

In my first pg i put on 11 kg and had a 6 pound 4 ounces baby.... second one i put on 9kg and had an 8 pound 2 ounces baby...this time so far ive only put on 4kg?????