If you pump before birth then your body is going to make real milk. But you don't want to give your baby that milk from the start. They need the colostrum first. It gives them the antibodies and protein they need plus it's gentle enough for their digestive system. They need that for a few days and when your real milk comes in, your baby is ready for it. You have to remember that your milk is made exactly for the baby, pumping before hand messes all that up and isn't good for either of you.
U can't pump before hand because ur milk doesn't come in until a few days after birth. When u first have ur baby u will produce colostrum which is packed with nutrients. But I've never heard of anyone pumping before the baby and not to mention if u pump it can cause u to go into labor because of nipple stimulation