A Brief Explanation of the Menstrual Cycle and How it Relates to Niddah

(Starting with my general disclaimer that will apply to almost every single post on this blog – I am not an expert in any of this. I am, however, an academic at heart, and I try to gather information from reliable sources.)

To talk about niddah, about fertility, about natural family planning, we need to start by talking about menstrual cycles. The menstrual cycle is literally at the center of all of these different topics. While there are several resources online that explain menstrual cycles, I’m going to give a brief summary here so we can start to place everything else in context. My sources are listed at the end of the post.

In a future post, I’ll describe more about what it means to be in “niddah” status, how one counts unclean versus clean days, and how one exits “niddah” status. But for purposes of this particular post, all you need to know is that niddah status begins on the first day of menstrual bleeding, and while someone is in niddah status they are halachically forbidden from having sex. Also worth noting that halacha is defined as “Jewish law”, and for purposes of this post I will be using halacha to mean the Orthodox understanding of the “rules” and “laws” of Judaism.

The “menstrual cycle” is the cyclical shedding and rebuilding of the uterine lining. It is defined as starting on the first day of menstrual bleeding (commonly called a period) and ending the day before the next period starts. It’s governed by several hormones produced in various parts of the body that interact with each other, and is broken up into four phases which I’ll describe below.

1. Menses – this is the actual shedding of uterine lining if pregnancy did not occur in the previous cycle. It’s characterized by a large drop in the hormones progesterone and estrogen, as well as menstrual blood coming out of the vagina. The onset of menses marks the first day of the menstrual cycle. The menses phase varies in length from person to person, but it usually lasts somewhere between 2 and 7 days, with the median being 5 days. The onset of menses would also be the point that someone would enter “niddah” status. Per Sephardic minhag, one must count a minimum of four “unclean” days regardless of the actual number of days of bleeding. Per Ashkenazic minhag, the minimum number of days is 5.
2. Follicular phase – This phase begins on the first day of menses and ends at ovulation. It’s marked by the production of the follicle-stimulating hormone (FSH) which signals to the ovaries to produce somewhere between 5 to 20 follicles, each containing one immature egg. The healthiest of the eggs will mature fully and be released at ovulation. During the follicular phase, the level of estrogen is also rising. For much of the follicular phase, someone who is following niddah will remain in “niddah” status, as there is a set number of seven “clean” days before they can immerse in the mikveh and leave “niddah” status.
3. Ovulation – This is the release of the matured egg from the ovary. It’s preceded by a sudden increase in the luteinizing hormone (LH), which causes the egg to be released. Once released, the mature egg will be able to be fertilized for approximately 24 hours. If the egg has not been fertilized during this period then it gets reabsorbed into the body. The day before ovulation is the peak of someone’s “fertile window” – meaning someone who is trying to become pregnant should attempt to have sex the day before they ovulate to maximize their chances of achieving pregnancy.
4. Luteal phase – These are the remaining days of the menstrual cycle, where the egg makes its way down the fallopian tubes on its way to the uterus. This phase can last anywhere from 10 to 17 days. During this time, the level of progesterone in the body increases. If a fertilized egg gets to the uterus and implants, then pregnancy occurs. If not, the levels of estrogen and progesterone rapidly drop, and the cycle starts again.

People who menstruate are often described as having “regular” or “irregular” cycles. A “regular” cycle generally means a level of predictability – each cycle has the same number of days and bleeding lasts the same number of days in each cycle. Someone who has “irregular” cycles may not be able to predict the start date of their next period.

We’ve learned an egg can only be fertilized in the 24 hours after ovulation. It’s also important to note that sperm can survive in the body for up to five days. Therefore, we can refer to someone’s “fertile window” as being the five days before ovulation and the one day after ovulation, meaning that someone can theoretically only get pregnant during 6 days of their cycle.

Remember that someone would remain in niddah status for a minimum of 11 or 12 days, depending on the minhag they follow. Someone who ovulates before day 11 of their cycle who follows Ashkenazic minhag niddah minhag would be very unlikely to be able to get pregnant naturally, and this phenomenon is called “halachic infertility”. This person would not be infertile by medical standards, but their observance of the halacha around niddah would make them effectively infertile. A study published by Wilcox et al. in 2000 showed that ovulation can occur as early as the eighth day of someone’s cycle (see my sources below).

This issue of halachic infertility deeply bothers me, as I wonder about how many people in the past thought that they were incapable of getting pregnant when they were in fact fertile. My heart breaks for generations of people who were fundamentally capable of pregnancy, but were effectively barred from it by following the rules of their culture and their religion. I know that there are now experts in halachic infertility who are able to suggest medical interventions such as taking medication to delay ovulation. I am glad to know that for those who want to follow the current standards of halacha, there are some options for them. But to me, that doesn’t undo the damage of history.

Sources:

Reed BG, Carr BR. The Normal Menstrual Cycle and the Control of Ovulation. [Updated 2018 Aug 5]. In: Feingold KR, Anawalt B, Blackman MR, et al., editors. Endotext [Internet]. South Dartmouth (MA): MDText.com, Inc.; 2000-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK279054/

Wilcox AJ, Dunson D, Baird DD. The timing of the “fertile window” in the menstrual cycle: day specific estimates from a prospective study. BMJ. 2000 Nov 18;321(7271):1259-62. doi: 10.1136/bmj.321.7271.1259. PMID: 11082086; PMCID: PMC27529.