Well, I can't offer any advice on the horsetail in pregnancy, but you might refer to Susun Weed www.susunweed.com . She is an herbalist specializing in Women's health, including pregnancy. I just ordered 2 of her books and they are very specific about formulas, herbs etc. Here is a link where she describes using herbal vinegars as a calcium supplement. http://www.herbshealing.com/page1066.html
You can also mix 1 part apple cider vinegar (even better use an herbal vinegar on her site) and 1 part molasses as a calcium supplement.