I started at 16 stone, lost 2 stone, Put half a stone back on and fell pregnant. This was 10% and a bit more.
We'd been trying for over a year including the time I was at 16 stone and above 14.
I think it did help me, as I have PCOS and weight really impacts on the symptoms.
To be honest though I would have like to be thinner pre pregnancy, if only to make the labour and tax on my body a bit easier. i.e my body only had to pump blood about a 12stone pregnant body and not a 16stone one.
I've seen a woman of 28stone get pregnant, not recommended, she suffered terribly with trouble walking, lost all her teeth, was constantly breathless and had to have specialist intervention throughout her pregnancy.
I'm not saying slimmer people don't have problems but she was only young and all the risk factors associated with pregnancy really ramp up the more overweight you are. I myself am at increased risk of gestation diabetes (which post pregnancy does resolve but leaves you at a 35% increased chance of develeoping diabetes in future), pre-eclampsia, SPD or Pelvic Girdle pain, Going overdue, longer labour, the list goes on. Even now I get dietary advice from the midwife in pregnancy, despite the fact I was happy to continue with SW, and I've told them I will be following the SW rules as a guideline.
Had I been able to choose a weight for pregnancy I would have chosen as close to a healthy BMI as possible.
Also don't forget the men have a part to play in this too - studies and research have shown that men who lose weight at 10% if they overweight actually increase thier sperm quality! So it's not all the womans responsibility.