Just tell her how you feel Steph. Maybe dont just blurt it out but pay her a few compliments and make a jokey comment about fancying her. Shes your best friend and may possibly feel the same but,of course,theres a chance she may be completely shocked! If it helps,I found my wife even sexier when she was pregnant.Maybe some of us are just turned on by pregnant women.(I often watch pregnant xxx porn).
Are you attracted to her as a person, or attracted to her being pregnant? If it's a fetish thing, then stay away from her.
You can bring up conversation about how sexually frustrated she must be as a prego, and see how it goes from there. I was happily married when I was preg, but if I had a friend approach me I would have been all over her! I was so overwhelmed with horniness and hubby isn't really into much kinky or anything to help me cum so I was fantasizing about being in a dressing room of a dept store rubbing myself and having a woman surprise me and take over. Still one of my favorite fantasies that never fails to get me off. I wouldn't wait though because she is so full of sexy hormones now that it will be to both of your advantages to connect now.
For grins, I'm going to assume the OPie is female. So, no child support unless they married and she was listed on the certificate.