Talk about it now, with your wife, before she gives birth and you are in the midst of your "frustration". She needs to hear your fears and needs, and you need to hear hers. See if you two can come up with a tentative plan on how you will take care of things during this time. Would she be willing to help meet your needs in another way, such as oral sex, manually, or even as someone suggested, breast sex?... but if she's a breastfeeding momma, that may not be an option. Would she give you the freedom to masturbate beside her, while you look upon her breasts or while you are just enjoying her presence? Or are there other creative things you two can come up with?
But, also have the attitude of grace, because as we know, life can throw us curve balls. Post-partum is a time where a woman's body has just gone through some major changes and possibly even trauma, her hormones are all kinds of crazy, and she will be lacking sleep and will be E.X.H.A.U.S.T.E.D. She is having to learn to be a mom to a new little person, who has a completely different personality and temperament she has to learn, and having baby #2 is totally different than baby #1 because now you can't always "sleep when the baby sleeps".