Thanks ladies! I think I'll still try to run the half marathon in 3 weeks if I feel ok on the day, I have pressure of sponsorhsip that i've commited to too! I think I'll go for a gentle pace and even walk if I don't feel right - I had been training for a PB but think that's out of the window now and just aim to get round and enjoy.

I did continue running when pregnant with my 2nd child (and remember doing a 10K when about 3 months pregnant), but I just ended up so tired that I don't think I lasted much longer than this - 11 years ago now so can't remember exactly!  It probably didn't help having a toddler at home as well, so may be different with a first child.  I do know of other women, though, who have managed to run almost up to the birth, which is very impressive. Helen is right that it just depends on how you feel and definitely worth talking to your GP as well.