I've been wondering for a long time about how women's cycles work in the society of souls, and whether they even have cycles at all. The reason for this is that, (here's a brief biological explanation for those who don't know) the purpose of menstruation is to rid the body of unused eggs (and to renew the endometrium, but that's not the point here). Women have a limited number of eggs, which are formed during the embryonic stage, while they're were still in the womb (unlike men, whose spermatogenic tissue begins producing sex cells during puberty and continues functioning almost until death). When a woman's eggs run out, she enters menopause.
Now, let's return to the question. We know that the number of eggs is not infinite, and that women's eggs are not produced in the same way as men's sperm. This leads me to wonder about the cycle of female souls. Do they even have cycles? Can they become pregnant? Since Kubo didn't provide any details about this (and I don't blame him, it's just a minor detail that piqued my interest), I decided to come up with a few theories.
1. Souls don't have eggs. More specifically, their eggs run out at a specific time. When a soul reaches the age of 50, which is 50+ years after its birth, its eggs run out, it enters menopause, and the soul continues to exist in the society of souls for as long as necessary. However, this theory raises concerns about pregnancy. Without eggs, it's impossible to conceive a child. And we know that souls can be born directly in the society of souls. Therefore, this theory seems unlikely to me.
2. Souls that enter the society of souls after death and souls that are born there have different physiologies (which is also a questionable theory, but let's consider it). And in the first case, we see the first option described above, and in the second case, either they have a huge supply of eggs that is not enough for 50+ years, but for, let's say, 1,000 years, or they produce eggs indefinitely. The option with a huge supply of eggs seems incredibly unlikely to me, because in that case, women's souls would either have incredibly large ovaries or incredibly small eggs. Neither of these options is advantageous for reproduction, and what is not advantageous in nature does not exist. However, I want to explore the option of egg production in more detail and even make it a separate point.
3. So, let's assume that the physiology is the same, but not everyone undergoes the process of oogenesis (the production of eggs). I believe this is due to the amount of spiritual energy. Let me remind you that for souls, especially for shinigami, everything is tied to spiritual energy, its quantity, presence, or absence. For example, when a shinigami is injured, the first step is to restore their reatsu so that their spiritual body can heal itself (with the help of medical professionals). From this, we can conclude that the health of a shinigami and any other soul with at least some amount of spiritual energy is entirely dependent on reatsu. So, why shouldn't the process of producing sex cells also depend on reatsu? Therefore, I believe that for those women's souls who possess spiritual power, once their eggs produced during their lifetime are depleted, new eggs begin to appear until they reach a "mental menopause" (mental, because it's difficult to understand the process of aging in the anime, so let's assume that the physiological age of a soul is influenced by their mental age).
4. Alternatively, it's possible that souls experience menstruation not on a monthly basis but once a year or even more frequently. However, it's challenging for me to imagine how the body would undergo such a drastic transformation after death.
In conclusion, I believe that option 3 is the most plausible, as it partially explains how souls can reproduce without experiencing overpopulation (although it's possible that there is overpopulation in Rukongai, but, I think, it's mostly because of dead souls, not newborn ones). And this fits into the Bleach worldview, as far as I'm concerned.