Emmenagogues have historically been used to help women manage their menstruation or fertility.
You need to remember we're looking at a time before effective contraception - a time when people did not consider 'fertile' periods and contraception was an afterthought.
The photo on the right here is Mugwort which falls into the medium strength category of emmenagogues.
The herbs were not always effective, but enough women got the desired result that this knowledge was passed down through generations.
Women understood what caused pregnancy and when, but were powerless to stop their men 'having their way' and many religions frowned on contraception - wasting seed! It is still the case in some countries and communities today.
However, women knew that they would need to start taking their herbs just before their period was due.
Also of course, pre-marital sex was happening - maybe not on the grand scale it is today, but it certainly happened and women knew when their period was due and if they'd been 'careless' and knew to take herbs just before.
This is why we can't say how effective herbal emmenagogues are because of the timing of them - you can't have a controlled experiment and give percentage success rates.
Women still attempt herbal abortions today. Success rates of around 45% are reported, but this would be hard to monitor or prove.
According to common knowledge passed down through the generations, a woman would need to start taking the herbs as soon as possible - not leaving it until the pregnancy has become established.
Some herbs work to prevent the embryo attaching and others work to cause uterine contractions.
If you do try a herbal method, then you need to be committed to seeing it through - the stronger herbs may cause defects in the developing embryo and if the herbs fail, then you would need to be wiling to take surgical measures.
Some may dismiss this, but if you are pregnant and want to stay pregnant, then you should avoid herbs in the medium and strong category during your pregnancy - it's better to err on the side of caution than risk a miscarriage.
Some links to help you get started looking for the herbs you need.
Black Cohosh which is in the medium category. See here for the different categories