Do not diet to loose weight, you can try and keep your weight steady and put on a minimal amount.
If you have a sensible high protein diet of grilled/boiled/steamed lean protein and plenty of vegetables + complex wholegrains and nuts and pulses + good fats in small amounts, you should not put on any weight during the first trimester, very little in the second and gain steadily in the third when baby needs it most.
Think about it this way: every bite counts to make your baby strong and healthy and keep you in optimal health to prepare your body for labour, motherhood and breastfeeding.
I put on 3 stone for my first. Now just 1 and 1/2 with still one month to go.
Your worst enemy is gorging on carbs because you restrict yourself at mealtimes and then overcompensate when your sugar levels drop.
If you snack, snack on protein/calcium rich foods.They keep your blood sugars steady and fill you up.
Then keep the sensible diet throughout breastfeeding (which is more of challenge because you'll always be hungry!!). You use up 500 calories a day, so should shed the pounds easily if you do not give in at that time.
Take care