You do not develop deep friendships by being shallow and looking the other way. I think you owe it to your friend to be honest with her. Whatever you say needs to be said out of love and not out of judgement. And you need to make that clear! Tell her that you will be her friend no matter what, and you won't badger her about it, but tell her how you feel and what you are afraid of. Speak in facts, not opinions. Do not dominate the conversation.. listen to her!
On that note, I want to share a personal story.
When I married my husband, we had only been together 7 months. Fast forward 2 1/2 years later. We have a 3 month old child and are about to get divorced.
I think it takes a year to REALLY know a person. It gets harder to hide behind a facade. DO NOT GET MARRIED JUST BECAUSE YOU'RE PREGNANT! I don't think you should make any big decisions pregnant, because your emotions are such a roller-coaster, and wedding planning is extremely stressful.. which is bad for baby in utero.
Say to your friend:
I care about you/ love you and
I'm feel worried about you because....
I'm feel scared because...
I want to be honest with you because I care, and
You deserve the best..
I will be your friend no matter what, and will not be judgemental towards you and your decisions,
But I owe it to you as a friend to be honest with you
Because I care/love you...
If you can't say it, write it.
Then listen to what she has to say without interrupting.