Moerman’s descriptions are terse, but begin to hint at the flexibility a woman had in controlling her pregnancy and regulating the time of birth, something common in indigenous cultures across the globe. Many plants, like horseradish and dogbane, were recorded as being used for “obstructed menses,” which is just a euphemism for the early stages of pregnancies. The entry for spikenard ( Aralia racemosa) states: “Plant used to promote menstruation when stopped by a cold.” Because pregnancy suppresses a woman’s immune system, common colds can be, well, common early in the pregnancy. Other plants, like New Jersey tea, Ceanothus americanus, were used to abort a pregnancy when a fetus was injured late in the first trimester.
Unfortunately, while we might have a record of what plants were used to control pregnancy, we don’t have as clear a record of why abortifacients were so widespread (source). As mentioned above, timing births would have been important to sync up with seasons of plenty. Most mammals have estrous cycles (a single breeding period during the year). Humans have menstrual cycles and can get pregnant (and therefore give birth) throughout the year, even if this might be disadvantageous. Historic records back to the 1800s bear this out, revealing a seasonality to when people are born. The father north you get from the equator, the earlier the average birthday – Finland’s average is April, Jamaica’s November – coinciding with the start of the growing season. Just like a bird! Other reasons identified for abortion may have related to delaying a first pregnancies, spacing out childbirths, or for the health of the mother.