If you have a pregnant friend, then make her day by offering for her to borrow your gently used maternity clothes. My personal rule of thumb when sharing clothes is to only loan out what you'd be OK with not getting back. Spills, rips, and dry-cleaning mishaps can happen to even the most careful, and it's simply not worth putting tension on a friendship over a piece of clothing.
Another alternative, if you're looking to make some extra cash on your clothes, is to resell them at a local consignment shop or on a site like Tradesy. This is a good option if you're certain that you're done having kids, or if you have pricier pieces that are still in good condition but you're tired of wearing.