For what purpose are tansy leaves used in the novel, Weyward?

Both Violet and Grace depend on Jennet’s tincture of tansy leaves, a potion that causes them to miscarry their babies. Grace insists that Altha fix the tincture for her because John beats her each time she gives birth to a stillborn baby. She fears that he will kill her if she gives birth to another dead baby. Violet wants to abort her baby because she does not want to be tied in marriage to the man who raped her.