I don't. The last thing my heavy, hopefully pregnant, Basset bitches needed was to be lugged in and out of a bath. If she's mucky, sponge her down behind. Should be enough?
ps If she's a coated breed, perhaps a hygienic trim might help, especially prior to whelping.
With my Papillons (that normally get bathed once a week to once a fortnight and really do need it) I wait until the season has finished. With the vulva still being so open during the season I don't want to risk getting shampoo etc inside it. My Malinois only get bathed once in a blue moon so not at all with them.