Just stop taking it, if you become pregnant. Long-term use of infertility herbs, and taking them while pregnant can cause problems, the deeper into pregnancy the more problems they can cause. You can stop taking it, when you find out you're pregnant, and don't have to worry about it. Just keep up good nutrition. Some herbs in high doses can cause convulsions to cause a womb to leave the body.
Everything is in link and its sources, it says how red clover decreases fertility, and can lead all the way to sterility. If using red clover, limit it to topical away from the lower body, and limit its use. Red clover causes livestock to have smaller offspring, and I think it mentioned defects for more severe use of clover. It caused problems for Quails too. Humans have many of the same receptors for fertility as other mammals, (and this extents to birds) so the precaution is for humans too. It's called "red clover disease," and you can also do a search for it.
An herb that raises LH or that tries to cause a period to happen, can cause defects to a developing fetus (later in pregnancy), I think because the convulsions interfere with the fetus, when it is a significant size. Pennyroyal is a mint with probably the most toxins of the Mint genus, and a woman died taking a spoonful extract of it. Pennyroyal has been also used for abortions. Other mints have a negligible amount of the toxic chemical that pennyroyal has, but they all still can cause miscarriages if taken in large amounts.
A tea, however, some herbs may not cause harm. Use this link I pointed out
Medline Plus - Herbs and supplements
Also, with the labeled warnings. It's because the effects of anything are more pronounced to a fetus, than the adult. For instance, pregnant women shouldn't drink alcohol nor smoke, because of the defects that can cause.. Better to be on the safe side, because not all supplements are well documented for pregnancy.
