I've had some spotting this pregnancy, which isn't the same, but the treatment might be. I have two books I'd recommend looking up. I have Herbs for the Childbearing Year, which has some good herbs to take. Also Herbal Healing for Women by Rosemary Gladstar.
Herbs for the Childbearing Year talked about Black Haw tea, and if unable to use that, then crampbark. Also, to take 200-400 mg of Vitamin E (A-tocepherols, which are supposed to be better quality, better for pregnancy complications). The other book talked more about combining tinctures using cramp bark, false unicorn root, and black haw.
Both also emphasized getting lots of rest.