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The chance of conceiving quickly does depend on a woman's age. Women are most fertile between the ages of 20 and 24. The average person is born with around 300,000 eggs and steadily loses them as she ages. Males go through their own form of menopause called andropause, which involves a drop in testosterone production in men who are age 50 or older. Conditions involve lowered testosterone levels and similar symptoms, but doesn't lead to a shut down of reproductive organs.

In this world, the situation is different. Males lose their fertility completely around 40s and 50s. Women grow eggs throughout their lifespan and don't hit menopause. However, it is well known that pregnancies after a certain age are considered high risk and can lead to complications. How best to safely extend this time span without thus problem?

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    $\begingroup$ If you can declare that menopause isn't a factor, why can't you just declare that pregenacies from ages 35-60 aren't a problem either? $\endgroup$ – kingledion Nov 11 '18 at 4:28
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    $\begingroup$ @kingledion Those are two separate problems. Menopause could be considered a biological clock: when it hits, no more natural conception. Pregnancy becoming more dangerous as a woman grows older is rather different. I know I'm grossly simplifying here, but even if menopause likely developed (from an evolutionary perspective) due to age-related pregnancy risks, they are not the same problem. Removing menopause would do nothing to alter those risks in itself. $\endgroup$ – Palarran Nov 11 '18 at 8:26
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Okay, your science is WAY off.

1) While it's true that women lose a fair number of eggs from their ovaries every month, well beyond the number of mature eggs that are released, your actual numbers are not consistent with known science. Look it up again and please edit with the correct numbers.

2) I am not sure which years are the peak of fertility for women (and I don't trust your numbers) but just because you say a 5 year span is the peak does not mean fertility is especially difficult during other years. Most women will conceive within 6 months of trying up to the late 30's. Then it starts to slow down, but conception is not particularly uncommon into the early 40's. Even into the mid-40's, it happens more than you think. Some women can conceive later but that's unusual.

3) You state: "It is well known that pregnancies over the age of 35 are considered high risk and can lead to complications." How do I say this nicely? I can't really...this statement is utter and complete baloney. In fact, it's extremely insulting.

What you are thinking of is the magical number 35 in regards to risk for Down Syndrome. That's where the risk goes up to about 1 in 250 (it doesn't even hit 1% until age 40).

There is no extra risk at age 35 in regards to the pregnancy itself or for complications. The only additional risk is a tiny increase in chromosomal abnormalities when the gamete is formed. That is it. There is no difference in pregnancy or birth. Very young age of course is a high risk factor but being in your late 30's or your 40's is not a high risk factor. I had two kids in my 40's; I know exactly what risks are real and which ones are made up.

4) We already have plenty of women with insufficient egg fertility carrying fetuses to term. A fair number of women conceive with egg donors. Occasionally someone is avoiding a genetic problem being passed on. Some women carry babies made with other women's eggs because they are acting as surrogates. But most women who use egg donors do it because they are experiencing infertility. This can be early menopause or other issues when they are younger. Or it can be infertility due to age. While conception this way is more difficult and there are early miscarriages, there are no complications due to a woman being close to or past the onset of menopause during pregnancy (aside from needing to take hormones in many cases).

In fact, there are cases of fully menopausal women in their 50's and 60's getting pregnant (with medical assistance of course) and delivering healthy babies.

The problem with menopause is with conception. If your species does not experience menopause then conception can happen (perhaps it is more difficult with age, perhaps not). Pregnancy will be the same as for younger women. As long as the pregnant woman does not have age-related health issues, problems with energy/endurance, bone stability, etc, there should be no additional risk factors to pregnancy or birth.

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This question is complicated, so I will focus more on the male aspect you have laid out. The issue with it is that, unlike the eggs of females, sperm are not suited to live for long periods of time. They degrade over time, and as such need to be constantly replenished. If there was a static number of long-lasting sperm, then that wouldn't fit your criteria either, as men do not ejaculate on a fixed timer, like women do with their periods, so the infertility age would vary drastically depending on sexual behavior. So, the only way to make sure male fertility only lasts a certain amount of time would be to involve the testes degrading over time, which brings in its own host of issues.

The testes are not only responsible for sperm production, but are the primary source of testosterone in males. Testosterone is a very important hormone, contributing to hair growth, muscle gain, and emotional stability, among other things. If a man were to have his testes degrade at an old age, then it is likely that he would suffer from a massive reduction in hair growth (not enough to incur complete baldness, but likely enough to induce a receding hairline, if not already present, and to inhibit the growth of beards), an increase in muscle atrophy, and a complete change in mood, which will differ from person to person, but will most likely be erratic at best.

The other method to ensure infertility would be exposure to radiation, which would be complicated to implement naturally in an earth-like environment, since that would require a constant exposure of a higher-than-normal level over the lifetime of the individual to achieve the desired age-based outcome, which could be easily avoided through relocation. Also, radiation affects individuals of both genders, so you would likely have to hand wave some reason as to why women are unaffected.

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