Try identifying things that speak love to her and show them to her. Small, insignificant things that speak directly to her heart and help her feel safe and loved. Just express them without any expectation of return and then let it speak for itself to her.
Her primary partner right now has a tremendous advantage. She's there. She's seeing what's going on. She has some understanding of what she is going through, and she's going to be the one who gets to do the bulk of the support. That doesn't mean you can't be there and show your love as well. It does mean you will have to tread very carefully through this and focus primarily on her and her needs and let your own be on a back burner for her.
Most relationship experts will actually encourage people to not make any major relationship decisions during pregnancy or within a year of giving birth. The hormones and life changes are just SOO overwhelming that it's hard to know if you're reacting from those or making a decision you're going to be glad you made in five years.