Obesity can make it harder to conceive but also the pregnancy more complex.
I'm sure you have googled the potential complications of obesity in pregnancy and are aware of the increased risk to both yourself and your baby, although many have uncomplicated pregnancies, some will end in serious complications.
You will rarely hear people on here say they think x or y complication was caused by their weight - over or under.
You have a few options...
Google 'counterweight plus' it would allow you to lose the required weight within a year.
Likewise looking into a gastric band or gastric sleeve may be an option.
Experience within this area tells me that a more radical change may be the most beneficial.
You have perhaps dieted several times in the past. If 'eat less and exercise more' worked everyone would be thin. This is really not that simple.
Please also don't think the methods suggested above are set to make you 9st they certainly won't. We are talking 3-5st off. Realistically, with motivation 4st in maintenance and minimal regain during pregnancy.
If you want to ask me more please pm me.
Best wishes
xx