Some women have low progesterone levels at the end of their cycle, this is known as a luteal phase defect; the luteal phase is the second half of the cycle, after ovulation has occurred. Under normal circumstances, the ruptured egg follicle turns into a progesterone-generating corpus luteum which prevents a bleed from occurring for two weeks; if there is a successful pregnancy, the placenta is able to generate enough progesterone to sustain the pregnancy after 12-14 days. If there is a luteal phase defect, the corpus luteum dies away too early and the fall in progesterone triggers a bleed which would usually end any attempted pregnancy.
If you are worried about low progesterone levels, you may have come across Vitex. Vitex is a native Mediterranean herb, also known as 'chasteberry' or agnus-castus. Vitex is only for use by women, it is thought to be harmful for sperm production in men; indeed the term 'chasteberry' comes from a time when it was used to curb sexual urges of soldiers in medieval times!
Vitex can be very effective for some women but can be counter-productive for other women, causing their cycle to shorten or the bleed to become extremely light so it is not suitable for everyone. While it can increase progesterone levels in the second half of the cycle, if you do not have a luteal phase issue it can interfere with the maturing of the egg in the first half of the cycle. To try to reduce this risk, some people suggest to take it only during the second half of the cycle, and it seems the best way to take it is as a tincture.
Another thing to be wary of is that Vitex also has an effect on dopamine levels so if you have a history of depression or you have a tendency to a low mood at the end of the month then it is probably not a good idea to take it, or if you do take it, be careful that it does not worsen these symptoms.
Diet and stress levels have a significant impact on progesterone levels, so if you suspect you have a luteal phase defect, it maybe better to look to make improvements to your diet and reduce stress before considering Vitex as a solution.