Dong Quai (also called Dang Gui or Angelica) is a pretty powerful herb. It nourishes and invigorates the blood. It's used to treat irregular menstruation, bad cramping ,hot flashes. It's mildly estrogenic per "The Infertility Cure". Your cautioned on it's use because it can cause excess bleeding if you are already taking blood thinners (which would include aspirin). It can also cause heart palpitations.
It's said that this herb should never be taken on its own, but in conjunction with a full regimen. Honestly, if you don't have blood issues, I'd stay away from it. What are you hoping to accomplish by taking an herb? Do you already know what's "wrong" with you? Do you have bad periods? A luteal phase defect? PCOS?, etc.
Tha'ts a good place to start. I would talk to a licensed herbalist or at least find a good source that explains what the herbs can or can't do (Like The Infertility Cure") before fixing on an herb. They are pretty potent and it could cause more problems if not approached carefully.
I hope some of this helps. Let me know and