Every one of us has an incredible story to tell, and it has nothing to do with the milk you snorted out of your nose in grade school– it’s much more meaningful. It’s the story of our birth, from how we transformed from microscopic cells on standby in our father’s pants, to the complex people we are today.

It’s pretty incredible. In the nine months that we all spent in our mother’s womb our developing nervous system works synergistically with our mother’s to manufacture a multibillion celled walking, talking, breathing, and sentient human being. What’s equally amazing is the fact that what starts with just two sets of twenty-three chromosomes, continually multiplies so precisely that it rarely goes wrong– just one chromosomal defect can cause deadly disease or prime the infant for a debilitating disorder.

Take a peek behind the curtain that modern science has lifted to learn what happens from conception to birth, even at an intracellular level.

Let's start from the beginning, with semen– because, why not?

When a man ejaculates, it’s not just sperm he’s releasing, it’s a fluid with many constituent parts! Sperm, or spermatozoa, leave the testes and mix with fluids from the seminal vesicles, the prostate, and the bulbourethral glands to form semen. In fact, semen is only two to five percent sperm, with the rest being composed of substances such as amino acids, enzymes, fructose, and zinc, which preserves the genetic material in sperm.

Of course, sperm is only half the story, so here’s a crash course on a woman’s anatomy.

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A woman’s reproductive organs consist of her vagina, cervix, uterus, Fallopian tubes, and ovaries. A typical premenopausal woman will ovulate once a month, meaning one of her ovaries releases an egg that descends the fallopian tube towards the uterus to be fertilized. Meanwhile, estrogen is surging which encourages the lining of the uterus to thicken. This will provide nutrients to a fertilized egg but will shed (i.e. a menstrual period) if the egg goes unfertilized.

The odds are stacked against life.

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Once the semen enters the vagina, the sperm only has 12 to 48 hours to fertilize the egg before they die– and it’s estimated that nearly eighty-five percent of sperm are defective and cannot swim properly. The remaining fifteen percent of sperm are quickly whittled down even further since despite the fact that the woman’s egg release molecules to lure the sperm towards the correct Fallopian tube, some sperm still swim up the wrong one.

From the moment the sperm reach the egg in the Fallopian tube, they begin competing for access.

Each sperm releases enzymes that degrade the surface of the egg’s membrane. Once the first sperm has successfully penetrated, the egg membrane repels the other sperm by emitting electrical signals that cause small sacs to release a swell of fluids. This process effectively kills the remaining sperm and is referred to as "cortical reaction."

From this point on the woman’s body takes over (which depending on how you look at pregnancy, is either a very morose or very encouraging fact).

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A fertilized egg is called a zygote, and within about four days it balloons to one hundred cells, transforming into a blastocyst. The blastocyst continues to descend the Fallopian tubes until it has implanted itself in the uterine lining, near day six. Pregnancy officially commences!

Not all fertilized eggs end up implanting in the uterine lining.

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Some do not fully descend and implant in the Fallopian tubes instead.  This is referred to as an ectopic pregnancy and can be life-threatening to the mother. Early symptoms include pelvic pain as well as possible vaginal bleeding and if left unaddressed, the growing embryo can rupture the Fallopian tube, causing a hemorrhage.

Despite the lack of a visible “baby bump" the first trimester is action packed and is a critical period for development.

A portion of the cells from implantation have generated the placenta and the remaining cells become the embryo. During this time, the embryo is especially vulnerable to toxic insults like alcohol and infection, as well as miscarriage, most often due to major genetic abnormalities that make the developing embryo inviable.

The embryo is divided into a three-layered disk of cells, with each layer destined to become a different portion of the body.

The outer layer, or the ectoderm, generates the skin, hair, nails, and nervous system. The middle layer, or mesoderm, develops into the heart, bones, and muscles, and the inner layer, or endoderm, becomes the intestines and lungs.

The Neural Tube formation is arguably the more fascinating part of the first trimester.

As the neural tube gives rise to our central nervous system, or our brain and spinal cord. Within the embryo’s mesoderm lies the notochord, and together they release a chemical that compels the undifferentiated ectoderm cells to thicken along the dorsal midline of the body. They generate the neural plate and eventually, the neural tube. These neuroepithelial cells divide and differentiate, creating neurons and neuroglia– which extend from the neural tube. This process is influenced by both chemical and physical signals, as molecular signals and glial fiber scaffolding guide the growth of immature neurons. However, at a certain threshold, these neurons become less sensitive, likely meaning that the sensory and intellectual capacity of a child is at least partly determined before this stabilization period. By week four, the neural tube is completely closed.

By week ten in gestation, the embryo is developed enough to be considered a fetus.

 At about this interval, the fetus is only one inch long and weighs one-thirty-ith of an ounce. The major tissue and cell types have been defined, so from here it’s a process of fine-tuning.

Between weeks eleven and fourteen, the mother may choose to undergo preliminary tests to assess the likelihood that her child has a genetic anomaly.

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Early tests can screen for chromosomal abnormalities, like trisomy twenty-one (Down syndrome) and trisomy eighteen (Edwards syndrome). The screening involves an ultrasound as well as a blood draw. The ultrasound examines the nuchal translucency, or the fluid behind the fetus’ neck, and the blood draw evaluates pregnancy-associated plasma protein-A (PAPP-A) and human chorionic gonadotropin (HCG). However, if a mother screens positive during this test, this does not guarantee her child has such genetic defects, only further testing is able to diagnose the fetus.

A parent may decide to follow their screening test with a formal diagnostic test, typically using a chorionic villus sampling (CVS) or amniocentesis.

In addition to trisomy defects, other major diseases of concern include cystic fibrosis, spina bifida, Tay-Sachs disease, sickle cell anemia, and thalassemia. CVS can be completed before amniocentesis, as early as ten weeks. When a mother undergoes CVS, she has a sample of her chorionic villi removed from the placenta. Chorionic villi project from the placenta and mimic the fetus’ genetics, meaning the DNA from the fetus can be accurately analyzed. Amniocentesis involves inserting a syringe past the abdominal wall, the uterine wall, and then finally, the amniotic sac wall. Once a portion of the amniotic fluid is extracted, the tissue that it contains can be analyzed similarly to samples from a CVS.

At thirteen weeks the fetus actually creates urine and releases it, though it isn’t absorbed by the mother’s body.

Instead, it is ejected into the amniotic sac, becoming part of the amniotic fluid. Don’t confuse the placenta with the amniotic sac! The placenta acts as a barrier to segregate the mother and fetus’ blood supply while permitting nutrients to transfer to the fetus via the umbilical cord. The amniotic sac is more or less the fluid-filled bag you’re confined to for nine months.

After looking something like an underwater alien, fourteen weeks marks the approximate period that the fetus’ sex can be determined.

It’s about three and a half inches long and just over an ounce heavy, but its neck and lower limbs are also beginning to lengthen.

Sixteen weeks marks the moment that the fetus can erect its head and its eyes can wander about.

Although they are only four and a half inches long, their heart is fully formed and their liver is producing red blood cells.

At about week eighteen, the fetus starts to look real human.

It can hear things, its digestive system begins working, and it may even start to do some flips in the womb! A fine hair called lanugo also springs up on the fetus’ head and its bones are gradually hardening.

At this point, the fetus begins to look like an alien again, as it develops a gooey coating on its body, called the vernix caseosa.

For as gross as this may look, it’s actually critical to protecting the fetus’ skin from chapping and abrading in the amniotic fluid. As the pregnancy progresses to about week twenty-two, meconium is made in the intestinal tract. Meconium is the first poo a baby will have and is completely sterile!

If a fetus is not brought to term before twenty weeks it’s considered a miscarriage, whereas any time between twenty and thirty-six weeks is a preterm birth.

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Generally, those born at twenty weeks have a twenty to thirty-five percent chance of survival, with the odds of surviving preterm birth increasing two to three percent per day until about twenty-six weeks gestation. After this point, most preterm infants survive, though growth and developmental delays may be present throughout the lifespan.

Week twenty-six marks the point that the fetus glows up and starts packing on the cute fat stores we all love so much!

I do hope the fetus takes in this moment, as it will be the last time it ever wishes to store fat. The fetus is spending the majority of its time in rapid eye movement (REM), which is the stage of sleep that’s typically associated with dreaming!

Week twenty-seven to thirty signifies the start of the third trimester and is also a time of rapid brain development for the fetus.

The gyri, or crests of brain folds, and the sulci, or crevices of brain folds, mature allowing for more grey matter to cover the cerebral cortex. Cortical grey matter increases about fourfold! Grey matter represents the network of neurons that sets humans apart as the most intelligent animal known.

Super Fat

Did I say fat? I meant phat! At week thirty-one, the fetus escalates its fat storage as most of its major development is complete. Lanugo, the odd body hair that the fetus has been developing, begins to fall off and is actually eaten by the fetus. We certainly had some interesting tastes prenatally.

By week thirty-two, the fetus’ lungs are nearly fully developed and its reflexes are intact.

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This period also signifies when the fetus descends head down in the amniotic sac to prepare for birth. This head-first position is referred to as a vertex presentation.

Although the fetus certainly isn’t familiar with what a “vertex presentation" is, only about three percent of fetuses don’t make this transition.

How is this possible? The answer is simple. The fetus is simply adjusting to the most comfortable position, as things get tight by the eighth or nineth month.

If they head in the other direction, it is called a breech birth.

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Breech births are more common after the first pregnancy, likely because the womb has been stretched from the first go around! There are many techniques to turn a baby and they become more difficult to pull off as the due date approaches, so it’s important to identify if this is an issue for the mother as early as possible.

At this point, the mother’s body has transformed in multiple ways to accommodate the little fetal parasite it’s hosting.

At the point that a mother is ready to give birth, some notable changes to her metabolism and physiology have occurred. An increase in hormones such as estrogen begin to remodel and relax the joints and soft tissues, prolactin hormones rise to prepare the mother for breastfeeding, plasma and blood volume increase by about forty to fifty percent as a result of the mother’s heart working thirty to fifty percent harder. The list goes on, but just know that pregnancy generally becomes really, really uncomfortable by the last trimester.

When the fetus decides to make the grand exit and become an infant, there are several major signs that occur.

The mother will release something called a mucus plug. Yes, a mucus plug,– it’s the barrier of mucus that protects the opening to the uterus. This can be expelled a few days or just a few minutes prior to the onset of labor. The mother’s uterus will begin to contract at least every 6 minutes and her cervix will begin to dilate.

The active phase of labor phase begins when the cervix is dilated to about 5 centimeters.

The cervix becomes incorporated with the lower portion of the uterus and as contractions continue, the upper portion of the uterus slowly pushes the baby towards the birth canal. When the diameter of the cervix has reached 10 cm it is wide enough to fit the infant’s cranium. Reaching this point in labor usually takes about four hours and normally no longer than twelve.

“Fetal expulsion" is the technical term, but I’ll let you come up with whatever fun moniker you’d like.

At this point, the infant’s head has passed through the pelvic inlet. “Crowning" is a term to describe when the infant’s head is visible and marks the very last moments of labor.

Oh, you thought that delivering the baby was the first and last concern of labor? Nope.

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After the infant is introduced (cue famed Lion King scene) the mother must deliver the placenta. Expelling the placenta is actually critical to the mother’s health, as even if a fragment is retained inside the uterus, the flesh becomes necrotic and can catalyze sepsis.

The ball of zygotic cells is now a fully formed baby, ready for cute mittens, gross burps, and first words!

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At the risk of looking sentimental, creating and giving birth to a baby truly is a miracle.