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Speaker 1: Welcome to brain Stuff, a production of iHeart Radio. Hey

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Speaker 1: brain Stuff, Lauren Vogel Bomb Here. The human body is complicated,

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Speaker 1: and a lot of us who are wandering around in

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Speaker 1: one have a perhaps less than complete idea of how

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Speaker 1: it works. So let's talk about it today. Let's talk

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Speaker 1: specifically about how conception works. Don't worry or apologies, I'm

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Speaker 1: not getting graphic. We're mostly talking about cells. Most people

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Speaker 1: who are assigned female at birth are born with millions

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Speaker 1: of immature eggs, which are contained in fluid filled sacks

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Speaker 1: called follicles within the ovaries. Hundreds of these will mature

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Speaker 1: over the course of the person's lifetime about one per month,

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Speaker 1: from the onset of puberty through menopause. People who get

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Speaker 1: a period get them in cycles that repeat about every

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Speaker 1: twenty eight days, though it's normal for cycles to last

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Speaker 1: anywhere from one to thirty five days, or even as

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Speaker 1: much as forty five day is for young people. Between periods,

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Speaker 1: ovulation happens in which a mature egg releases from one

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Speaker 1: of the two ovaries. Ovulation usually occurs about a week

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Speaker 1: before or after a woman's period, though it can be

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Speaker 1: very irregular and can occur even during the period. Itself.

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Speaker 1: Here's how it happens. Well. Once every cycle, the hypothalamus

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Speaker 1: in the brain sends a signal to the pituitary gland

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Speaker 1: to release follical stimulating hormone. This hormone prompts several follicles

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Speaker 1: to begin developing the immature egg that they each contain.

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Speaker 1: One of these will usually grow dominant over the others,

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Speaker 1: and within two to three days following its maturity, the

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Speaker 1: follical will rupture and release the egg out of the

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Speaker 1: wall of the ovary and into the fallopian tube, which

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Speaker 1: connects the ovary to the uterus. Some people feel an

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Speaker 1: ache in their ovary during ovulation. You can also sometimes

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Speaker 1: tell if you're ovulating because your cervical secretions will be

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Speaker 1: wetter and more slippery, and ovulation usually causes a small

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Speaker 1: dip in body temperature followed by a spike, so people

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Speaker 1: often measure their temperature when trying to detect ovulation. But

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Speaker 1: back down to the cellular level, the follicle that additionally

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Speaker 1: released the egg sends out a call for increased estrogen

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Speaker 1: and progesterone production. These hormones are the body's que that

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Speaker 1: an egg is now mature and that it should get

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Speaker 1: ready to nurture it in case it gets fertilized, and

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Speaker 1: normally only one egg passes from either of the two

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Speaker 1: ovaries through its fallopian tube and only once per cycle.

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Speaker 1: Sometimes two eggs or rarely more, are released within a

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Speaker 1: single twenty four hour span. If multiple eggs are fertilized,

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Speaker 1: it can result in a multiple fraternal pregnancy, but a

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Speaker 1: mature egg only has about twenty four hours to find

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Speaker 1: a partner, a key master to its gatekeeper, a sperm

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Speaker 1: cell that can penetrate the eggs outer layer. It's normal

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Speaker 1: for three out of tens firm cells to be abnormally formed,

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Speaker 1: and for four out of ten to be bad swimmers.

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Speaker 1: The odds are poor for any single sperm cell. For

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Speaker 1: one thing, it has about a quarter billion competitors that

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Speaker 1: will be joining it in the vagina within minutes of ejaculation.

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Speaker 1: Most of the sperm cells released will die due to

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Speaker 1: the acidic nature of the vagina. They're viewed initially, at

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Speaker 1: least by the person's immune system, as foreign bodies that

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Speaker 1: should be destroyed. From there, they must enter the cervix,

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Speaker 1: swim through the cervical mucus, enter the uterus, and find

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Speaker 1: the opening to the fallopian tube. The flopian tube is

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Speaker 1: where fertilization usually occurs. As opposed to the vagina. It's

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Speaker 1: a really friendly, nutrient filled environment for both the sperm

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Speaker 1: and the egg. The fallopian tubes also contain tiny hair

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Speaker 1: like structures called cilia that helped pass the egg through

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Speaker 1: the tube from the ovary toward the uterus. The journey

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Speaker 1: is only about four inches or ten centimeters in length,

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Speaker 1: but takes several days. Of the millions of sperm that

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Speaker 1: initially enter the cervix, there may be anywhere from one

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Speaker 1: to a couple hundred that arrive at the fallopian tube.

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Speaker 1: They can survive there for a few days, and like

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Speaker 1: I said, an egg survives for about twenty four hours,

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Speaker 1: so a sexual encounter either before or after ovulation can

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Speaker 1: result in a pregnancy. A sperm have receptors that allow

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Speaker 1: them to detect eggs, or rather to detect progesterone. Eggs

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Speaker 1: are surrounded by cells that release progesterone, and sperm cells

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Speaker 1: get really active when an egg is present. A progesterone

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Speaker 1: makes sperm so active that they've sloughugh off layers of

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Speaker 1: proteins that surgeon activity. Along with the loss of those

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Speaker 1: proteins enables sperm to pierce the egg. This process is

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Speaker 1: called capacitation. The head of the sperm, once making contact

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Speaker 1: with the exterior of the egg, will more or less

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Speaker 1: pop releasing enzymes that allow it to cross through the barrier. On.

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Speaker 1: Once a sperm cell penetrates the exterior of the egg,

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Speaker 1: fertilization occurs. The sperms DNA payload is delivered as it's

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Speaker 1: absorbed by the egg, and as the two combine, a

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Speaker 1: complete genetic blueprint is formed. Once a single sperm enters

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Speaker 1: the egg, the eggs protective protein covering changes and doesn't

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Speaker 1: allow other sperm to enter. Usually, meanwhile, the uterus, prompted

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Speaker 1: by those hormonal signals released by the follicle that formed

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Speaker 1: the egg, will have created an internal lining a called

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Speaker 1: an endometrium, a rich in blood and nutrients that's prepared

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Speaker 1: to house and nurture the egg should it become fertilized.

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Speaker 1: If no fertilization occurs, the egg disintegrates into the uterine

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Speaker 1: lining and the uterus sloughs it off and passes it

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Speaker 1: out through the vagina during the person's period. But if

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Speaker 1: fertilization does occur, that's conception and the fertilized egg is

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Speaker 1: known as a zygote. Information encoded in the collective DNA

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Speaker 1: of the egg and sperm cell instructs the zygote to

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Speaker 1: continue its development. Within twenty four hours after forming, a

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Speaker 1: zygoat will begin the process of cellular division. A solid

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Speaker 1: cluster of cells forms up within what's called a blasticyst,

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Speaker 1: hollow structure that the massive cells continues to develop within.

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Speaker 1: If everything goes right, the cluster of cells inside the

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Speaker 1: structure will become an embryo, while the outer wall will

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Speaker 1: become the placenta and other nutrient providing tissues that the

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Speaker 1: growing embryo will need. But first, the zygoat makes that

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Speaker 1: four day journey down the fallopian tube towards the uterus,

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Speaker 1: aided by those tiny cilia lining the tube. By the

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Speaker 1: fifth day, the zygote should have made its way into

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Speaker 1: the uterus, and by the sixth the massive cells will

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Speaker 1: hatch from the thin walled sack. And now the blasticyst

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Speaker 1: is ready to make direct contact with the endometrium. If

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Speaker 1: it does, the endometrium and the blasticyst exchange hormones, allowing

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Speaker 1: the blasticyst to connect to the uterine wall. A process

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Speaker 1: known as implantation. People may experience about forty eight hours

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Speaker 1: of light bleeding or spotting during this process, which can

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Speaker 1: sometimes be mistaken for a period. As this process occurs,

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Speaker 1: the cervix will be closed with a mucous plug, with

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Speaker 1: the zygote now known as an embryo, safely in the

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Speaker 1: blood and nutrient rich uterine lining. The pregnancy has taken

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Speaker 1: a big step toward viability because it's only in this

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Speaker 1: environment that the embryo can grow and that the pregnant

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Speaker 1: person's body can withstand the enormous nutritive demands of that

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Speaker 1: growing embryo. Again, this is assuming that everything goes right.

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Speaker 1: Human bodies are complicated and there are all kinds of

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Speaker 1: things that can go wrong that aren't anybody's fault. Only

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Speaker 1: one third or fewer zygotes survived to become embryos. About

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Speaker 1: one in every fifty will implant somewhere that isn't the

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Speaker 1: uterus and won't have to be removed. That's called an

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Speaker 1: ectopic pregnancy, and it can't be reimplanted because, as we've covered,

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Speaker 1: the implantation is part of the growing embryo, and even

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Speaker 1: after a pregnancy is clinically confirmed, About one in ten

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Speaker 1: will end in spontaneous miscarriage before twenty weeks, and sometimes

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Speaker 1: the dead embryo will have to be removed. Sometimes the

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Speaker 1: developmental process goes wrong after that and the fetus will

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Speaker 1: never be able to live outside of the womb. Pregnancy

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Speaker 1: is a whole other episode or mini series, really, but

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Speaker 1: all of these possibilities are why it's so important for

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Speaker 1: people who can become pregnant to have access to reproductive

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Speaker 1: information and healthcare. We like to say around here that

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Speaker 1: the world is understandable and worth understanding, and that includes

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Speaker 1: the worlds within ourselves. Today's episode is based on the

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Speaker 1: article understanding the conception process on how stuff works dot Com,

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Speaker 1: written by Tom Shave. Brain Stuff is production of iHeart

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Speaker 1: Radio in partnership with how stuff works dot Com, and

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Speaker 1: it's produced by Tyler Clang. Before more podcasts from my

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Speaker 1: heart Radio, visit the I heart Radio app, Apple Podcasts,

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Speaker 1: or wherever you listen to your favorite shows,