Pregnant ladies get a concession - most of your weight will be water (in your tummy and your thighs so don't worry)
Please don't worry about having to diet during pregnancy. Your doctor will let you know if theres a problem with your weight gain - but DON'T use a normal BMI calculator - it doesn't work in pregnancy.
You can find help sheets that tell you what all the weight gain is in pregnancy - baby, placenta is a huge hunk, water, fat (mostly in your boobs remember).
If your putting it on - then your body probably needs it :)
ALso, don't pay attention to the NHS guidelines for 'normal' weight gain in pregnancy.
I started off at 50kg, and am now towards the end of my pregnancy at 75kg - that's half my origional body weight! The website said 10kg was a normal gain, so i asked my doctor if i was morbidly obese now and he said not to be so silly, and that my weight gain was perfectly healthy, considering my origional body weight.
So... let go... relax and enjoy the one time in your life when you can eat extra cake without having to feel guilty.