Hi there,
I did the dukan diet for 4 months and I didn't have a period for the entire time. I wouldn't recommend it if you seriously want to get pregnant as it totally messes up your hormones and your body doesn't get the nutrition it needs to conceive a healthy baby. It's true that you lose weight really quickly to start with but then it's really slow, so you may as well do weight watchers or similar which is a really healthy way to get ready for conception. This was my experience and it's not worth the extra stress! I feel so much better now I've stopped and started eating a more balanced diet.