Welcome to brain Stuff, a production of iHeart Radio. Hey brain Stuff, Lauren Vogel Bomb Here. The human body is complicated, and a lot of us who are wandering around in one have a perhaps less than complete idea of how it works. So let's talk about it today. Let's talk specifically about how conception works. Don't worry or apologies, I'm
not getting graphic. We're mostly talking about cells. Most people who are assigned female at birth are born with millions of immature eggs, which are contained in fluid filled sacks called follicles within the ovaries. Hundreds of these will mature over the course of the person's lifetime about one per month,
from the onset of puberty through menopause. People who get a period get them in cycles that repeat about every twenty eight days, though it's normal for cycles to last anywhere from one to thirty five days, or even as much as forty five day is for young people. Between periods, ovulation happens in which a mature egg releases from one of the two ovaries. Ovulation usually occurs about a week before or after a woman's period, though it can be
very irregular and can occur even during the period. Itself. Here's how it happens. Well. Once every cycle, the hypothalamus in the brain sends a signal to the pituitary gland to release follical stimulating hormone. This hormone prompts several follicles to begin developing the immature egg that they each contain.
One of these will usually grow dominant over the others, and within two to three days following its maturity, the follical will rupture and release the egg out of the wall of the ovary and into the fallopian tube, which connects the ovary to the uterus. Some people feel an
ache in their ovary during ovulation. You can also sometimes tell if you're ovulating because your cervical secretions will be wetter and more slippery, and ovulation usually causes a small dip in body temperature followed by a spike, so people often measure their temperature when trying to detect ovulation. But back down to the cellular level, the follicle that additionally released the egg sends out a call for increased estrogen
and progesterone production. These hormones are the body's que that an egg is now mature and that it should get ready to nurture it in case it gets fertilized, and normally only one egg passes from either of the two ovaries through its fallopian tube and only once per cycle. Sometimes two eggs or rarely more, are released within a
single twenty four hour span. If multiple eggs are fertilized, it can result in a multiple fraternal pregnancy, but a mature egg only has about twenty four hours to find a partner, a key master to its gatekeeper, a sperm cell that can penetrate the eggs outer layer. It's normal for three out of tens firm cells to be abnormally formed, and for four out of ten to be bad swimmers.
The odds are poor for any single sperm cell. For one thing, it has about a quarter billion competitors that will be joining it in the vagina within minutes of ejaculation. Most of the sperm cells released will die due to the acidic nature of the vagina. They're viewed initially, at least by the person's immune system, as foreign bodies that should be destroyed. From there, they must enter the cervix, swim through the cervical mucus, enter the uterus, and find
the opening to the fallopian tube. The flopian tube is where fertilization usually occurs. As opposed to the vagina. It's a really friendly, nutrient filled environment for both the sperm and the egg. The fallopian tubes also contain tiny hair like structures called cilia that helped pass the egg through the tube from the ovary toward the uterus. The journey is only about four inches or ten centimeters in length,
but takes several days. Of the millions of sperm that initially enter the cervix, there may be anywhere from one to a couple hundred that arrive at the fallopian tube. They can survive there for a few days, and like I said, an egg survives for about twenty four hours, so a sexual encounter either before or after ovulation can result in a pregnancy. A sperm have receptors that allow
them to detect eggs, or rather to detect progesterone. Eggs are surrounded by cells that release progesterone, and sperm cells get really active when an egg is present. A progesterone makes sperm so active that they've sloughugh off layers of proteins that surgeon activity. Along with the loss of those proteins enables sperm to pierce the egg. This process is called capacitation. The head of the sperm, once making contact with the exterior of the egg, will more or less
pop releasing enzymes that allow it to cross through the barrier. On. Once a sperm cell penetrates the exterior of the egg, fertilization occurs. The sperms DNA payload is delivered as it's absorbed by the egg, and as the two combine, a complete genetic blueprint is formed. Once a single sperm enters the egg, the eggs protective protein covering changes and doesn't
allow other sperm to enter. Usually, meanwhile, the uterus, prompted by those hormonal signals released by the follicle that formed the egg, will have created an internal lining a called an endometrium, a rich in blood and nutrients that's prepared to house and nurture the egg should it become fertilized. If no fertilization occurs, the egg disintegrates into the uterine lining and the uterus sloughs it off and passes it
out through the vagina during the person's period. But if fertilization does occur, that's conception and the fertilized egg is known as a zygote. Information encoded in the collective DNA of the egg and sperm cell instructs the zygote to continue its development. Within twenty four hours after forming, a zygoat will begin the process of cellular division. A solid cluster of cells forms up within what's called a blasticyst, hollow structure that the massive cells continues to develop within.
If everything goes right, the cluster of cells inside the structure will become an embryo, while the outer wall will become the placenta and other nutrient providing tissues that the growing embryo will need. But first, the zygoat makes that four day journey down the fallopian tube towards the uterus, aided by those tiny cilia lining the tube. By the fifth day, the zygote should have made its way into the uterus, and by the sixth the massive cells will
hatch from the thin walled sack. And now the blasticyst is ready to make direct contact with the endometrium. If it does, the endometrium and the blasticyst exchange hormones, allowing the blasticyst to connect to the uterine wall. A process known as implantation. People may experience about forty eight hours of light bleeding or spotting during this process, which can
sometimes be mistaken for a period. As this process occurs, the cervix will be closed with a mucous plug, with the zygote now known as an embryo, safely in the blood and nutrient rich uterine lining. The pregnancy has taken a big step toward viability because it's only in this environment that the embryo can grow and that the pregnant person's body can withstand the enormous nutritive demands of that growing embryo. Again, this is assuming that everything goes right.
Human bodies are complicated and there are all kinds of things that can go wrong that aren't anybody's fault. Only one third or fewer zygotes survived to become embryos. About one in every fifty will implant somewhere that isn't the
uterus and won't have to be removed. That's called an ectopic pregnancy, and it can't be reimplanted because, as we've covered, the implantation is part of the growing embryo, and even after a pregnancy is clinically confirmed, About one in ten will end in spontaneous miscarriage before twenty weeks, and sometimes the dead embryo will have to be removed. Sometimes the developmental process goes wrong after that and the fetus will
never be able to live outside of the womb. Pregnancy is a whole other episode or mini series, really, but all of these possibilities are why it's so important for people who can become pregnant to have access to reproductive information and healthcare. We like to say around here that the world is understandable and worth understanding, and that includes the worlds within ourselves. Today's episode is based on the article understanding the conception process on how stuff works dot Com,
written by Tom Shave. Brain Stuff is production of iHeart Radio in partnership with how stuff works dot Com, and it's produced by Tyler Clang. Before more podcasts from my heart Radio, visit the I heart Radio app, Apple Podcasts, or wherever you listen to your favorite shows,