There is no reason to eat for 2 while pregnant - but it is so easy to do!! And people encourage you to by telling you not to "diet".
You probably won't lose weight while you are pregnant but if the key is not to gain too much then and you can't do that on your own then you need the help of a structured plan.
Slimming World is fab for that. It shows how healthy the eating plan is actually that it is the only one you CAN follow during pregnancy.
I am 36 weeks pregnant and have put on 1 stone 10lbs since my booking in appointment at 10 weeks. I'm expecting to gain about 2 stone in total. Maybe more than you want to but I was at Target weight and within the healthy weight range for my height when fell PG so would expect to gain more than someone who is already obese.
I lost 2stone 8lbs with SW and have carried on going all through my pregnancy and its' been great. I gained 2.5 stone with my second pregnancy and was already overweight to start with so I've felt much more in control this time.
Also - having the support of SW meant that I didn't worry that I was cutting out essential foods or doing something unhealthy.
I would suggest joining a group - you do need to get the approval of your midwife.
The key is to go to group and be aware and keep an eye on it!