I need some advice about my son's relationship.

My son has been in a relationship with his girlfriend for a year and a half now.  I have always liked her.  She went out of her way to endear herself to me.  She hugs me and tells me she loves whenever I see her and ends every phone conversation by telling me she loves me.  My only problem with her is that she comes from a kind of messed up family with a mom who has a prescription drug problem, which isn't her fault; however, I see some of the same tendencies in her.  She has back pain and fibromyalgia at age 23 for which she gets Norco from her doctor fairly regularly.  She has insomnia and is on Ambien chronically.  She has anxiety and takes Xanax.  I recently became aware that she bought Xanax illegally.  I confronted her and my son about this and she said her mother had stollen hers and that her doctor would not refill the prescription because it was too soon.  I think she has a problem.  My son believes everything she says. 

A few days ago, my son and the GF were fighting.  She left their house at 11 p.m. at night and did not come home all night long.  At 4 a.m. my son calls me in a panic because he woke up and she wasn't home.  He said it wasn't like her.  He wanted me to come and get him and go look for her.  His car is in the shop.  I said to wait until it was light outside and if she wasn't back I would come over.  Well, she wasn't back so I went over and got him.  He said he thought she could be in a wreck or something bad had happened.  I had my suspicions, but said nothing.  We didn't find her at any of her family's houses and so we decided to retrace her steps.  We found her at one of her friends' houses, passed out in bed with another guy.  My son was devastated and I had to drag my son out of the house to keep him from beating up the guy.  She of course says nothing happened.  She drank too much and passed out.  I was angry, but left them to hash it out between the two of them.  A few days later, my son wants me to come over and talk to the GF and reassure her that I don't hate her.  I told my son I didn't hate her.  I feel like I never knew her and that she put on a really good sweet girl about.  I also told my son that I was absolutely not going to sit there and let her lie to my face.  He said he believed her, that nothing happened, that she wouldn't do that to him, she loved him.  He said she just passed out.  So I said, yeah in bed with a guy, when there was a perfectly good EMPTY couch sitting in the living room.  He didn't want to hear any of what I had to say.  He has his doubts, I can see it, but he's willing to put blinders on, what can I do?  So I said fine, I can fake it, if she can.

Yesterday, my son calls me to tell me that she is pregnant.  He wants my approval and support.  I am sick about it.  I couldn't breathe when he told me.  He said "please mom, I love you and need you to be happy for us.  I can't do this without your support."  I'm having dinner with them tonight and I am at a loss as to what I'm going to say.  I keep telling myself that it's done, she's pregnant and my son believes it is his, and me lecturing them and voicing my disappointment won't change that fact.  What do I do?  What do I saw?  I want to cry.  This is all on top of the fact that I have started to go through menopause and I just can't handle any of it!