Idk this is just a thought, do you feel like periods are a continuous training and testing session for pregnancy ?? I mean imagine if you had no cramps and when you were going to deliver your baby you would die of the pain (probably) but cramps give us a rough idea of how it is going to be. Nothing can beat the labour pain but it's like a small test before you give the big exam. I do understand the science behind it but I love to think Nature is playing with everything and find proof. Even the mood swings, tender and sensitive breasts and you are basically ejecting something out of your uterus. This is my sort of Eureka moment, please don't ruin it if you have realised it way before.