
Just be there and talk to her. A lot of it is mental and unfortunately there’s not a whole lot you can do about that. Sometimes it’s hard to believe you look good if you don’t think you do even if others are saying it. I personally have never been through a pregnancy but it’s a rough time and having someone be there is really nice
Listen to what she needs, and be supportive. Don’t try to “solve” the problem by, for example, suggesting ways to lose weight or that she buy new clothes. Make sure she knows you think she’s beautiful but also understand that that’s not enough on its own, she’s still going to feel her own feelings/insecurities. Don’t praise other, smaller, women’s appearances in front of her or praise how she looked before the birth. Focus on praising her as a person also, not just on how she looks
As a woman (who has not been blessed by childbirth yet) Do not sign her up for the gym unless she specifically asks and is the one to bring it up first. She will also need plenty of time to recover before even thinking of working out. Seeing you go may either be motivation for her to go or fuel her insecurities as guys typically loose weight faster than women (it’s an unfair reality)