Abortion was much more common in the middle ages than most people know. A number of common plants act as abortifacients; the herb pennyroyal, for example.
Roman women used a half lemon, partially squeezed out, as an ancient equivalent of the “morning after pill”. It was inserted cut-face first into the vagina and maneuvered over the cervix; apparently this was quite effective in preventing pregnancy.
I seem to recall a high school biology teacher saying that sperm cells are PH sensitive, so I guess it makes sense.