From what I know....
Echinacea is safe for pregnancy and nursing, but goldenseal is not safe for either one. But one thing to watch for is that many echinacea preparations also have goldenseal in them.
Sage is another one which can make your milk dry up. We prescribe it for mothers who have lost their babies or have given them up or aren't going to breastfeed. The main time of the year to watch out for this is around Thanksgiving, if your family tends to use a lot of sage in the turkey stuffing. :-)
Prune juice or black mission figs are good for constipation. They used to also make a juice from black mission figs, called "Miss Figgy", which was also good, but I haven't see it for awhile. Susun Weed's book has some very good ideas.
From what I have heard, the use of yogurt vaginally is a good old stand-by for yeast infections. You can also use a clove of garlic for yeast or for Beta Strep.
I've heard nothing but good things about raspberry leaf tea during pregnancy, but I recently became aware that some preparations of this tea include nettle, which has diuretic properties, and which could cause high blood pressure and pre-eclampsia. So I recommend using only the RLT that has only raspberry leaves in it.
Dandelion also has diuretic properties, so it would have the same problems as nettle.
I've read that licorice can also cause high blood pressure.
In addition to the books already mentioned, I like Prescription for Nutritional Healing, by Phyllis Balch. It has a very good section on herbs.