Causing abortions was really really common.
Tansy tea is one that was mentioned on Game of thrones and from the Tansy wiki page "During the Middle Ages and later, high doses were used to induce abortions."
Rue another plant "It is UNSAFE for both mother and unborn child to take rue during pregnancy or breast-feeding. Rue can cause uterine contractions, which can cause a miscarriage. That’s why rue is used to cause an abortion. But it also has serious effects for the mother. Some women who have tried using rue to cause an abortion have died." -from webMD
From the Abortifacient wiki page
"In English law, abortion did not become illegal until 1803. English folk practice before and after that time held that fetal life was not present until quickening. "Women who took drugs before that time would describe their actions as 'restoring the menses' or 'bringing on a period'."[21] Abortifacients used by women in England in the 19th century (not necessarily safe or effective) included diachylon, savin, ergot of rye, pennyroyal, nutmeg, rue, squills, and hiera picra,[22] the latter being a mixture of powdered aloe and canella.[23]"
Add that to poor healthcare, high infant mortality, malnutrition, natural family planning etc as you can see why not every family was 14 children strong.