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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 edie 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 we found a specific kind of stem
cell in the ovaries called a egg oniel
stem cells which can create new oocytes
even in adulthood since then other
studies and 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 a hundred and twenty thousand
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
oscillation 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 to 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 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 efficient does occur precisely
two weeks after menstruation starts then
yes the two to seven 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 six and twenty-one 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 to 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 quote 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 in cilia waving hair
like cells in the fallopian tube has
these moves 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
tubule isthmus their interactions with
the epithelium the inner layer of the
reproductive tract can either hyper
activate 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 it 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
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
inheritance 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 parents
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
hypothesize 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 C a
cell that will become an ovum undergoes
a process called meiosis 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 pair's 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 four dollars 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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