There are several herbs that should not be used during pregnancy, except under the direction of a trained herbal specialist, and many that have not been adequately studied as of yet:
Herbs that stimulate uterine contractions: birthwort, blue cohosh, cinchona, cotton root bark, ergot, goldenseal, gotu kila, Peruvian bark.
Note that blue cohosh and black cohosh are safe to use later in pregnancy when it is desirable to stimulate contractions.
Herbs that stimulate menstrual flow: agave, angelica, bethroot, black cohosh, chicory, feverfew, hyssop, horehound, lovage, milk thistle, motherwort, mugwort, nasturtium seed, osha, fresh parsley leaves*, pennyroyal, poke root, pulsatilla, rue, saffron, sumac berries, tansy, thuja (white cedar), watercress, wormwood, yarrow.
Herbs which can stimulate or irritate the uterus: eucalyptus, barberry, blood root, broom, goldenseal, coffee, mandrake, tea, nutmeg, osha, yerba mansa, and the mint family members basil*, catnip, lemon balm, marjoram*, oregano, peppermint, pennyroyal, rosemary*, true sage*, and thyme*.
Herbs that affect hormonal function: dong quai, , licorice, motherwort, wild yam.
Harsh herbal laxatives: aloe, cascara sagrada, purging buckthorn, rhubarb, senna, and yellow dock (in large amounts).
Strong herbal diuretics: juniper berries, uva ursi (bearberry).
*Culinary herbs, used in normal small amounts for cooking, are perfectly safe.