There are many alternatives for water-resistant materials.
Water-resistance means that the fabric will prevent some liquid or moisture from travelling through, but will often work only to a certain level. So may not be suitable for a heavy flow.
Natural water-resistant fabrics include wool, which can hold many times its weight in water before feeling wet and can be felted for more protection.
Polyester fleece and nylon can also be used to form a water-resistant barrier, and are sufficient for some women to prevent leaks through pads. These materials are often sufficiently waterproof to be used to make wetbags.