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Most adults think they have a pretty good understanding of how people make more people, but there are a lot of things related to human reproduction that we get wrong.  Like you've probably heard that you can't get pregnant if you're on your period, but that's not necessarily true, or you might think that it's the fastest swimming sperm that gets to fertilize the egg, except that's not true either.

Sometimes we get these things wrong because scientists are still making new discoveries and they've learned a few things since our generation last took Sex Ed, and sometimes it's just misinformation.  Either way, we here at SciShow are here to set the record straight so let's talk about sex.

Now, one of the most durable beliefs when it comes to human reproduction is that  people are born with all of the egg cells they'll ever have, and to be fair, this was kind of the dominant idea for a long time, but in the last 15 years or so, new discoveries seem to have turned this on its head.  While we tend to call them eggs, when we're talking about human reproduction, what we really mean are ova, female reproductive cells released during ovulation.  They're produced in ovaries from immature cells called oocytes, and since the 1950s, it's been thought that oocytes are all made during fetal development, so the ovaries were essentially thought of as long-term storage facilities, and we knew this no-new-oocytes after birth idea wasn't true for other animals, but we thought mammals, including humans, were special, but since the mid-2000s, the evidence has been mounting that we were just wrong.

The first big break was a mouse study in 2004.  They found a specific kind of stem cell in the ovaries called oogonial stem cells, which can create new oocytes even in adulthood.  Since then, other studies in mice, rats, cows, non-human primates, and pigs have confirmed that these stem cells can contribute to the pool of eggs used in natural reproductive cycles, and in 2012, we found similar stem cells in people, too, as well as evidence that the human versions can produce oocytes.  

So although we have yet to definitively demonstrate that these stem cells make new eggs in adult humans that they do in other animals and their existence in people suggests that in the end, humans may be able to make new eggs during their lifetime.

This is a good example of science correcting itself, but it's also cool because this discovery could lead to new treatments for infertility.  It's possible we could coax these stem cells to make more eggs when we want them to, including outside of the human body, which we've already done in mice.

Now, for our next myth, the human menstrual cycle, the cyclical bodily process of preparing for a possible pregnancy including ovulation and getting a period is said to be 28 days long with ovulation occurring two weeks after the last period starts, but that's just an average and it's not necessarily an accurate one.  First off, only about 13% of people actually have 28 day cycles according to a 2019 study of over 120,000 users of a menstrual cycle tracking phone app.  65% of the participants had cycles between 25 and 30 days, but overall, the range was huge, and some people had highly variable cycles where the length was somewhat unpredictable, but more to the point, the researchers also found that the idea that ovulation occurs precisely on day 14 of a cycle wasn't accurate for most of their participants.  

In fact, the average length of the follicular phase, the first part of the menstrual cycle which starts with menstruation and continues until ovulation, was nearly 17 days.  Even for participants with a 25-30 day cycle, it was a little over 15 days on average, and this seems to get shorter with age.  Participants in their mid-20s had follicular phases that were, on average, 3.4 days longer than people in their mid-40s, so the actual timing of ovulation was quite variable, both between individuals and within a person's lifetime, even if their cycles were normal, which kind of leads on to the next myth.

You might have heard that you can't get pregnant on your period.  So here's the thing.  Sperm can survive for days after sex.  That's why the fertile window, the time in a person's cycle when they can get pregnant if they have unprotected sex, is generally defined as the day of ovulation plus the five days preceding it.  Now, if ovulation does occur precisely two weeks after menstruation starts, then, yes, the 2-7 days you're on your period falls outside the fertile window, but we just learned that that isn't always the case.

Back in 2000, scientists published a paper drawing on data from over 200 women in North Carolina who were trying to plan their pregnancies.  The scientists were able to estimate each woman's day of ovulation based on urinary hormone levels.  One of the participants ovulated a mere eight days after her period started and got pregnant.  Between 6 and 21 days after their last period started, the women had, at a minimum, a 10% chance of being in their fertile window.  Only about 30% of participants had their fertile window entirely within the days of the menstrual cycle identified by clinical guidelines, which, for this group, was days 10-17. 

This means that, yes, if your period is less than a week long and you ovulate two or more weeks after it starts, it's not possible to get pregnant by sex during your period, but not only are the lengths of menstrual cycles highly variable, when a person actually ovulates during their cycle varies, too, and that means that for some, it's entirely possible to get pregnant from "period sex".

Speaking of pregnancy, the sperm part of conception is usually framed as a mad race.  All the sperm are trying to beat each other to the egg waiting at the finish line, but while sperm can and do swim, the one that fertilizes the egg isn't necessarily the best swimmer of the group.  The female reproductive tract carries the sperm along, weeds out contenders, boosts up others, and in the end, has a huge role in deciding which sperm wins.

For example, muscular contractions within the uterus may help move the sperm along.  There's also the flow of secretions and cilia, waving hair-like cells, in the fallopian tube.  As these move sperm along, though, some are pulled from the race.  Immune defenses in the vagina, cervix, and uterus can eliminate sperm, and cervical mucus can weed out abnormal ones, and the sperm that do survive still aren't racing to the finish.  When sperm get to the fallopian tube, they may be held or slowed down at a structure called the tubal isthmus.

There, interactions with the epithelium, the inner layer of the reproductive tract, can either hyperactivate the sperm and prepare them for fertilization if the egg is ready to go, or slow the whole process down, preserving them until the time is right.  This pause may also prevent the egg from being fertilized by multiple sperm at once by only allowing a few sperm at a time to get close to it.

It may even narrow down which particular sperm get to the egg.  The fallopian tube seems to be able to separate sperm based on their genetic content, including sex chromosomes or whether they have damaged DNA.  So if the fastest sperm doesn't pass this screening check, it's not getting anywhere near an egg.

Now, when a sperm finally does reach the egg, it doesn't battle its way in as if it was breaking through a wall.  The process of fertilization doesn't rely on brute force, but rather on protein interactions between the sperm and various parts of the egg, so it's more like a complex series of secret handshakes, and if the sperm gets any of these wrong, it might be kept out of the egg and it turns out sperm might also be involved in determining what genes end up in the egg. 

See, back in the middle of the 2010s, one lab looking at the inheritence patterns of genes in mice and humans started to notice something weird.  One of the foundational tenets of genetics is that the half of a parent's genome that's passed on to its offspring is random each time.  That should result in a predictable mix of combinations of genes in the next generation, but the researchers were seeing cases where some combinations were far more common than they should be, so they investigated what might be going on using mouse models, but it wasn't any of the usual suspects like variants that are lethal when combined.  In the end, they came to the conclusion that eggs are somehow able to select certain sperm based on their DNA, or vice versa.

They hypothesized this might be because B vitamins somehow act as signaling molecules on the sperm and egg, basically helping with those secret handshakes.  This is an appealing idea because B-vitamin supplementation during pregnancy has been shown to alter the ratios of genetic combinations or maybe we're just kind of wrong about how eggs mature.  You see, a cell that will become an ovum undergoes a process called myosis, which essentially cuts its genome in half, leaving it ready to combine with the half genome the sperm will provide.

To do this, the cell first doubles its entire genome, creating copies of each of its 23 pairs of chromosomes, and then divides into two daughter cells, the copies sticking with their clones.  One of these daughter cells is destined to be the egg and the other one becomes one or two supportive polar bodies whose genes don't end up in the embryo.

Then, the egg-to-be splits again, separating the chromosomes from their copies to create cells with one copy of each of the 23 chromosomes, so the order goes first, sort the pairs, then split them into single strands, but that last split doesn't happen until after fertilization.  The thing is, if the order of divisions is reversed and the clones split into single strands first, you get an egg-to-be with both variants of the original chromosomes, and that means it's possible signals from sperm could influence the results of that final division, thereby choosing which chromosomes end up in the egg, which could explain the skewed ratios scientists have seen, and this weird reverse meiosis has been seen in humans, though we'll need more research to confirm whether sperm actually have any influence over the last step. 

In the end, when we look at the most up-to-date science, it's clear that we've been getting a lot of things wrong about human reproduction.  The fastest sperm isn't necessarily the one that fertilizes the egg.  The female reproductive system has ways to transport, delay, direct, and select sperm, and sperm may even influence the genetic makeup of the eggs that they fertilize, and when you combine that with what we've learned about the menstrual cycle and its variability, it becomes clear that we have lots more to learn about human reproduction before we can really say we know how it all works.

If this list of reproductive myths has you curious to learn more about sex, well, I've got some good news.  Our monthly Patreon podcast, SciShow After Hours, tackles all kinds of Not Safe for Work science, so if you pledge $4 or more on Patreon, you can learn a whole lot more about reproductive biology in humans and other animals.  Head on over to patreon.com/scishow to learn more.

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