Good luck with the pregnancy.There will be other marathons.

I would be extremely reluctant to advise anyone to do it - not because I think it's definitely harmful - but because many early pregs end in a miscarriage and if (and I hope not) yours did, I would feel culpable.However were I to make the choice for myself I would weigh up the evidence that I was a fit healthy regular runner who had prepared adequately and had checked this websites advice on running in pregnancy and could adopt all advice such as hydration, temperature control, HR control and then do it.I would also change my mind and pull out if it did not go well. I would accept a slower time. I wouldn't beat myself up if anything went wrong with either the mara or my preg if I had taken all these precautions.Next marathon, I would take other precautions!