Help!

[Mumto2nearly]

Sorry, this is going to be long! I have been a "lurker" on MN for some time now and have enjoyed the discussions and gained valuable advice and information but it is not until now that I have needed personalised advise from you all. I hope that is ok.
A little background....
I have been married approx 4 years and have a gorgeous 2 year old girl and I am expecting my second baby in approx 4 weeks. Between my DD and my soon to be DS I had 2 relatively late miscarriages, at 11 and 12 weeks (the second a day after a perfectly healthy scan picture). I work pretty much full time and my husband works away from Monday to Friday almost always.Until last night, I thought I had an exceptionally happy marriage. You have probably guessed that this is where the need for advice kicks in!
Last week, I was sat with my husband when he was on his works messaging service (allows staff to communicate between offices on a real time basis)and there was a message from a girl, lets call her Sarah, which said something along the lines of "yippee, you are back and talking to me". I commented that this seemed a little odd and my DH said she was being sarcastic. I though nothing more of it. Then last night, he turned over in bed and put his arm round me and said "I love you, Sarah". Well, I leaped out of the bed (not easy at 8 1/2 months pregnant as I am sure you will all appreciate!) and said what did you say and he said it again (he had, had a few drinks). At this point I got out of bed in a blind panic and grabbed his mobile to see what was on there.
Well, there were some awful, depraved pornographic video clips and lots of pictures of womens breasts/bums and some more pornographic type images. DH then woke up and came in and said they were sent by 2 of his male friends and indeed upon watching one of them again I could see that there was indeed a man in one of the videos that wasn't my husband.
Being furious by this point, I demanded he get his work laptop because I wanted to check his email. Upon opening his emails, I found lots of emails between him and Sarah. They started with "getting to know you" questionnaires, family pet, favourite colour that sort of nonsense. But they progressed to one from her regarding sexual conquests, favourite positions etc etc. There were also work emails where they were both clearly trying to get on the same jobs - where they would both clearly have to stay in the same hotels.
My husband has acknowledged that the emails are not entirely appropriate and he can understand why I am upset and tbh he is being lovely. The emails were also not at a pornographic level like the pictures on his phone so I believe that they are not from Sarah. However, I have this awful sinking feeling like I am in a bad episode of Trisha where I am being the completely stupid and naive wife believing all he says.
I don't know what to do and I feel like my entire world is crumbling around me. I would completely acknowledge that being pregnant after 2 miscarriages hasn't made me the most attentive wife but I thought he understood that. I feel like I don't know the man I am married to, my loving husband has turned into a depraved and dishonest man that I don't recognise in the space of a few hours.
I have considered contacting Sarah to get her side of the story (I have her work email and her mobile phone number). I have considered walking out and never coming back but I love him and can't begin to imagine my life, or my children's lives without him. Can anyone offer any advice as to how to deal with this?

[Mumto2nearly]

Thanks SoupDragon, I have reposted the message there.

[kittypants]

blimey! hes being lovely-i bet he bloody is!personally i wouldnt be there.

[SoupDragon]

I have no advice but I would recommend you copy this and repost it in the "relationships" topic. You may get more responses there.

Good luck.