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
These hundreds of thousands of eggs are kept in a place in the woman’s body that the sperm cannot get to (the *ovaries*). Only one egg cell per month is released from the ovaries into uterus (via the fallopian tubes). This egg cell degenerates within 24 hours after release if it isn’t fertilized.
You might think this gives only a 24-hour window for fertilization, but fortunately things aren’t quite so bad, because sperm cells can actually survive for quite a bit longer inside the woman’s body: up to 5 days. So all in all this gives a fertile window of about 6 days (5 days before and 1 day after ovulation) where, if you have sex within this window, a successful fertilization might ensue.
Still, this means that a woman is only fertile about 25% of the time. Also, while some sperm cells survive up to 5 days, many others will die off before then, so the numbers are a lot better if you have sex closer to the moment of ovulation. In other words, the average woman is only *very* fertile for maybe 2-3 days every month or so.
Nevertheless, about 80-90% of couples who are actively trying to get pregnant are successful within the first year (the number is higher if they are able to time their attempts well against the woman’s cycle). Of the remaining 10-20%, a large portion probably have some form of fertility issue. Could be on the man’s or the woman’s side, or both. On the man’s side the main issues are sperm count and quality. You need to not only release enough sperm cells, but they need to be healthy swimmers too. They only get released into the vagina & cervix, after all, and they need to make it all the way to the uterus or even fallopian tubes before they encounter the egg – a journey of about 15 to 18 cm. A sperm cell is only about 0.05 mm long, so that’s over 3000 times its own “body length”: the equivalent of a human traveling (walking? swimming? hard to know what to compare to) about 6 km (4 miles). Not an insurmountable task, but impossible if you can’t move or if you only swim in circles. In general, only about 1% of sperm cells make it that far, and that’s assuming good or average sperm health. So if your numbers are down or your swimmers don’t swim good, the odds are against you.
Also, it doesn’t just take one sperm cell. The egg cell is surrounded by a protective coating and sperm cells make an enzyme that breaks down this coating. But one sperm can’t make it through on its own – it needs help from other sperm.
On the woman’s side, there are other possible issues. For example, there could be problems with ovulation – for instance the woman’s ovaries might not reliably release an egg in every ovulation cycle, but only sometimes. Your odds then depend on how rare this is. There could also be a blockage in the fallopian tubes, meaning the egg cell rarely or never makes it into the uterus. Amongst other issues.
All of these issues get worse/more likely with age, and people generally are having children later and later right now, so we’re seeing more couples struggling to get pregnant.
These hundreds of thousands of eggs are kept in a place in the woman’s body that the sperm cannot get to (the *ovaries*). Only one egg cell per month is released from the ovaries into uterus (via the fallopian tubes). This egg cell degenerates within 24 hours after release if it isn’t fertilized.
You might think this gives only a 24-hour window for fertilization, but fortunately things aren’t quite so bad, because sperm cells can actually survive for quite a bit longer inside the woman’s body: up to 5 days. So all in all this gives a fertile window of about 6 days (5 days before and 1 day after ovulation) where, if you have sex within this window, a successful fertilization might ensue.
Still, this means that a woman is only fertile about 25% of the time. Also, while some sperm cells survive up to 5 days, many others will die off before then, so the numbers are a lot better if you have sex closer to the moment of ovulation. In other words, the average woman is only *very* fertile for maybe 2-3 days every month or so.
Nevertheless, about 80-90% of couples who are actively trying to get pregnant are successful within the first year (the number is higher if they are able to time their attempts well against the woman’s cycle). Of the remaining 10-20%, a large portion probably have some form of fertility issue. Could be on the man’s or the woman’s side, or both. On the man’s side the main issues are sperm count and quality. You need to not only release enough sperm cells, but they need to be healthy swimmers too. They only get released into the vagina & cervix, after all, and they need to make it all the way to the uterus or even fallopian tubes before they encounter the egg – a journey of about 15 to 18 cm. A sperm cell is only about 0.05 mm long, so that’s over 3000 times its own “body length”: the equivalent of a human traveling (walking? swimming? hard to know what to compare to) about 6 km (4 miles). Not an insurmountable task, but impossible if you can’t move or if you only swim in circles. In general, only about 1% of sperm cells make it that far, and that’s assuming good or average sperm health. So if your numbers are down or your swimmers don’t swim good, the odds are against you.
Also, it doesn’t just take one sperm cell. The egg cell is surrounded by a protective coating and sperm cells make an enzyme that breaks down this coating. But one sperm can’t make it through on its own – it needs help from other sperm.
On the woman’s side, there are other possible issues. For example, there could be problems with ovulation – for instance the woman’s ovaries might not reliably release an egg in every ovulation cycle, but only sometimes. Your odds then depend on how rare this is. There could also be a blockage in the fallopian tubes, meaning the egg cell rarely or never makes it into the uterus. Amongst other issues.
All of these issues get worse/more likely with age, and people generally are having children later and later right now, so we’re seeing more couples struggling to get pregnant.
These hundreds of thousands of eggs are kept in a place in the woman’s body that the sperm cannot get to (the *ovaries*). Only one egg cell per month is released from the ovaries into uterus (via the fallopian tubes). This egg cell degenerates within 24 hours after release if it isn’t fertilized.
You might think this gives only a 24-hour window for fertilization, but fortunately things aren’t quite so bad, because sperm cells can actually survive for quite a bit longer inside the woman’s body: up to 5 days. So all in all this gives a fertile window of about 6 days (5 days before and 1 day after ovulation) where, if you have sex within this window, a successful fertilization might ensue.
Still, this means that a woman is only fertile about 25% of the time. Also, while some sperm cells survive up to 5 days, many others will die off before then, so the numbers are a lot better if you have sex closer to the moment of ovulation. In other words, the average woman is only *very* fertile for maybe 2-3 days every month or so.
Nevertheless, about 80-90% of couples who are actively trying to get pregnant are successful within the first year (the number is higher if they are able to time their attempts well against the woman’s cycle). Of the remaining 10-20%, a large portion probably have some form of fertility issue. Could be on the man’s or the woman’s side, or both. On the man’s side the main issues are sperm count and quality. You need to not only release enough sperm cells, but they need to be healthy swimmers too. They only get released into the vagina & cervix, after all, and they need to make it all the way to the uterus or even fallopian tubes before they encounter the egg – a journey of about 15 to 18 cm. A sperm cell is only about 0.05 mm long, so that’s over 3000 times its own “body length”: the equivalent of a human traveling (walking? swimming? hard to know what to compare to) about 6 km (4 miles). Not an insurmountable task, but impossible if you can’t move or if you only swim in circles. In general, only about 1% of sperm cells make it that far, and that’s assuming good or average sperm health. So if your numbers are down or your swimmers don’t swim good, the odds are against you.
Also, it doesn’t just take one sperm cell. The egg cell is surrounded by a protective coating and sperm cells make an enzyme that breaks down this coating. But one sperm can’t make it through on its own – it needs help from other sperm.
On the woman’s side, there are other possible issues. For example, there could be problems with ovulation – for instance the woman’s ovaries might not reliably release an egg in every ovulation cycle, but only sometimes. Your odds then depend on how rare this is. There could also be a blockage in the fallopian tubes, meaning the egg cell rarely or never makes it into the uterus. Amongst other issues.
All of these issues get worse/more likely with age, and people generally are having children later and later right now, so we’re seeing more couples struggling to get pregnant.
Watch the opening sequence of Look Who’s Talking. Ruined The Beach Boys song but have a visual of fertilization.
Now imagine sperm doesn’t move fast enough, the egg is misplaced or inconsistent, or that the joined cells don’t attach to the uterus properly. (It’s gotta stick.) Those are the major issues. That’s why fertility treatments often result in multiple births— it encourages egg release, depending on the issue.
Watch the opening sequence of Look Who’s Talking. Ruined The Beach Boys song but have a visual of fertilization.
Now imagine sperm doesn’t move fast enough, the egg is misplaced or inconsistent, or that the joined cells don’t attach to the uterus properly. (It’s gotta stick.) Those are the major issues. That’s why fertility treatments often result in multiple births— it encourages egg release, depending on the issue.
Watch the opening sequence of Look Who’s Talking. Ruined The Beach Boys song but have a visual of fertilization.
Now imagine sperm doesn’t move fast enough, the egg is misplaced or inconsistent, or that the joined cells don’t attach to the uterus properly. (It’s gotta stick.) Those are the major issues. That’s why fertility treatments often result in multiple births— it encourages egg release, depending on the issue.
[removed]
[removed]
[removed]
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.
Latest Answers