it works both ways. i notice a lot of guys i know who quit on the gym after a baby. that first year is tough bro, you are having to take care of something 24/7 so it is brutal on your sleep and health
my advice is don't be pushy with her
also don't you dare make the mistake of making any smart comment about her weight or shape she is in. you aren't talking to a gym buddy here playing around, anything you tell her she will remember the insult and hold it against you forever lol.
bottom line is you brung in a new being into your lives, so you are going to have to share that being and accept that she isn't going to be your gym buddy for a while until that kid is old enough to stay with the gym care.