Do you know why the heart is used as an iconic symbol for love and romance? Believe it or not, its use goes right back to the Romans when they used a little heart-shaped seed as a natural form of herbal contraception.
Did you know that a lot plants make their own phytohormones, such as phytoestrogen which is very similar to the oestrogen that humans later evolved? Many members of the Apiaceae family contain phytochemicals that affect hormones. Silphium was one of them. It was a type of giant fennel. The ancient Romans used silphium so widely that we believe they caused its extinction. Silphium's seeds were heart-shaped which is where the use of the heart as an icon for love originated! The Romans ate it extensively and also, like the Greeks before them, mixed it with resin to make contraceptive pessaries! Separately to this silphium was popular as a food spice and after it became extinct it was replaced with its close relative, asafoetida - also reputed to have a contraceptive effect.
Another relative, wild carrot seed Daucus carota, was also used as a contraceptive in ancient times and some empirical modern case studies have also been undertaken. One method recorded is to chew 1 teaspoon of dried carrot seed daily, for three days before and 3 days after having unprotected sex. No guarantees offered but a herbalist in the USA did run a pilot study and says it works!
You can read more about the work with wild carrot by Robin Bennett on wild carrot seed herbal contraceptives. You can also find other anecdotal cases on the Sister Zeus website and information about using tinctures rather than chewing the seeds.
Theoretically, there is no reason why herbs shouldn't work. There are a lot of herbs that should not be taken during pregnancy precisely because they are abortifacients. This means that they can cause a miscarriage by triggering contractions or causing shedding or damage to the lining of the uterus. It is also well-known amongst farmers, that grazing sheep in fields rich with red clover can affect their fertility. Red clover is very high in phytoestrogens. Soy is another. Which is why both are often used to balance hormones, for example: for difficult periods or the menopause.
Many people also do not realise that the first contraceptive pill in the 1960s came from phytochemicals in the plant wild yam Dioscorea villosa. Wild yam contains diosgenin which is a precursor of the semisynthesis of progesterone. It is also used to commercially synthesis cortisone and other steroid products.
In the middle East, a relative called zallouh Ferula harmonis is sometimes used as a male sex supplement but it has been shown in animal trials to actually reduce male fertility (Ayuob, Al-Harbi &Abdulhadi, 2014) so plants do not just affect female fertility but also men's too.
If you want to experiment we would strongly advise that the greatest care is taken. Firstly, don't experiment if you really would mind becoming pregnant. If chemical contraception doesn't suit you, other forms of physical contraception such as condoms may.
Secondly, be aware that if pregnancy did occur and you had been taking an untested herb, there could be an unforeseen, unintended effect on the health of the baby.
The only reliable, natural form of contraception (approved by some religions) that we are currently aware of that has been shown to be completely safe for both women and unexpected babies is the Billings Ovulation Method.