The average weight gain in a pregnancy is about 12-14kg. It's generally made up of the following:

baby = 3 to 4 kg
placenta = 0.5 kg
amniotic fluid = 1 kg
uterus = 1 kg
blood volume = 1.5 kg
breasts = 0.5 kg
Fat stores for breastfeeding = 3.5 kg
Fluid retention = 1.5 kg

I wouldn't worry too much about your weight though. My ob didn't even ever weigh me once during my pregnancy. My concern was that during the first trimester I lost weight, but I was assured that it was fairly normal and that weight change during pregnancy can be hugely different from one woman to the next