The good news is that for the main part, the range of weights mean that your pre-pregnancy state is taken into account before doctors and midwives get their 'average ideal' measurement notes out. Plus, they will check your weight at every check-up (these check-ups start sporadically and get more frequent as the birth approaches), so you do not need to become a weighing scale addict in your bathroom.

What is too much weight gain?
The average weight gain is around 11 to 12kg. Beyond this, there is a danger of putting unneccessary strain on your body (back, etc) and that the baby will be overly large.
However, women who are underweight before concpetion may be advised to gain a little more than average, just as those who are overweight will be advised to control their gain.