That depends -- our brand of prenatal vitamins is also designed for breastfeeding moms, does your packaging say something like this?

As a rule of thumb: most prenatal vitamins I know (source: my mother works in a pharmacy) are suitable pre- and post-. There are usually no special things needed for breastfeeding mothers, compared to pregnant women. Just remember that the mother will need to eat more while breastfeeding, and always have something to drink at hand when settling down to feed.