Here's a link to a cool article I found talking about women marathon and half marathoners continuing to race. There's also an email at the bottom- the woman wants to create a database of women who have raced during pregnancy- how they did, felt, etc.

http://ubermotherrunner.com/...-history-and-safety/

I know everyone is different during pregnancy and the author is NOT encouraging women runners to do marathons while pregnant, she is just saying that it shouldn't be discouraged because there is little evidence that it is actually bad for you.