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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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.
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