Eli5: If men have millions of sperm per load, and women have hundreds of thousands of eggs, why does it take some couples months or years for the woman to become pregnant? Even if either is person has a lower count, all you need is 1 of each to meet

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Eli5: If men have millions of sperm per load, and women have hundreds of thousands of eggs, why does it take some couples months or years for the woman to become pregnant? Even if either is person has a lower count, all you need is 1 of each to meet

In: 1

Only 1 egg is released during ovulation once a month. Rarely a woman will release 2 eggs at a time and both can be fertilized at the same time which gives you fraternal twins

Only 1 egg at a time each month (ish)

The egg is only available and ready to party for a short and specific period of time each month.

The way to the party is a rough road for the boys, and most won’t survive the tirp.

Lots of sperm ‘die’. Lots of sperm are needed for the hydrolytic enzyme they release in order to that “one” packet of genetic material to make it into the egg.

The egg cells are not generally accessible by sperm. The eggs are kept in the woman’s ovaries until she is *ovulating* – meaning, a single egg is released into the connecting fallopian tube. The egg will travel to the uterus and if it isn’t fertilized, it will be expelled along with the lining of the uterus during menstruation. Only one egg is released at a time (usually, most of the time; occasionally multiple eggs are released, usually still only two).

During menstruation, even if the egg somehow gets fertilized, it’s already being expelled so it can’t implant and develop. Immediately after menstruation, no egg has been released from the ovaries so there’s no egg to fertilize. Pregnancy is not* possible at this time, either.

(* Sperm can survive up to five days inside a vagina/uterus so it is entirely possible for sperm to begin their journey when the woman is not ovulating and survive long enough until she is, and fertilize the egg. It’s not very likely, but it’s possible.)

It’s a common misconception that the egg gets fertilized in the uterus. It *can* happen there, but it usually happens in the fallopian tube. The embryo will begin developing as it continues to travel to the uterus were it will implant into the uterine lining and develop the placenta. If the egg is fertilized after it already enters the uterus, it *may* be too late and the embryo doesn’t have enough time to implant in the uterus before menstruation and the lining is shed along with the embryo.

An embryo can also implant inside the fallopian tube before it gets to the uterus. This is an *ectopic pregnancy* and is a life-threatening emergency requiring emergency surgery to remove.

That’s *all* on the woman’s side of things. Sperm have a torturous journey to get to the egg. Sperm can survive for up to five days inside a vagina and it may take that long to get to the egg. Another misconception is that sperm have to swim all the way there. Really, the vaginal muscles do the bulk of the work drawing the sperm in. If she is not ovulating, the cervix – the sphincter that separates the vagina from the uterus – will be closed and sperm cannot enter. Additionally, the chemistry inside the vagina changes to be more hostile to sperm so that they are less likely to survive.

If she *is* ovulating, the cervix is not wide open. It will be open *millimeters* at most. Most sperm will not get through the tiny opening. Once inside, the sperm have to navigate through the uterus without getting trapped by mucus on the uterine lining. Again, most of the sperm that made it that far will get trapped and never move on. Since (most) women have two ovaries and two fallopian tubes, if – as normal – there is only one egg there’s a 50/50 chance that any given sperm will end up in the wrong tube. It’s possible that no sperm will survive intact and actually make it to the egg. And of the very few that make it that far, they have to successfully implant into the egg *and* then successfully begin developing as an embryo *and* successfully implant into the uterine lining *and* continue to successfully develop.

Nearly every woman who is sexually active without protection will have multiple miscarriages. However, the vast majority of these miscarriages will be at such early stages of pregnancy that it can very easily be mistaken for a heavy period – or even a perfectly normal period. Could be caused by a bad sperm or egg with messed up genes. Genetic abnormalities like Down’s Syndrome are very rare: the vast majority of problems with chromosomes and genes will cause the pregnancy to terminate almost immediately. It could be caused by the embryo just failing to implant. Could be stress-induced, could be a problem with the uterine lining that prevents implantation. It could be a *lot* of things.

There is *so* much random chance involved, and a lot of genetic differences that might make it easier for one couple but harder for another. That’s also why males of every species have evolved to ejaculate *millions* of sperm: since it’s largely up to chance, having millions of sperm trying to get there greatly increases the odds that at least one will.

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