What is a safe alternative to douching in pregnancy?

Not douching. Douching is never recommended. The vagina is a self-cleaning organ and douching can push harmful bacteria into the vagina or change the pH balance which allows infections to occur. Gentle cleaning of the outside of the vagina with soap and water is all that is necessary and safe.
Don't. You do not need to douche. Normal bathing is adequate and besides, the vagina cleans itself. If you are having a problem with discharge, see your doctor.