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DNA is mapped out from the beginning -

Your sex and inherited genetic characteristics (such as eye color, hair color, skin and body type, etc.) are determined at the moment of conception.

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Your rear end... -

Believe it or not, in the first few weeks after fertilization, you're just a small group of cells, called a blastula, which reportedly bursts open, making an opening which you now use to excrete waste!

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Yes, that's right -

This opening, called a blastopore, is the first of your proto-organs to start forming, and the rest of your body develops from there. So yes, it’s true, we all start as butts!

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Nerve cells -

Before a body grows from that cluster of cells, it will grow several layers around it of varying density and formation.

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Brain and spine -

Something of a little dot and a line extending from it will develop around the fifth or sixth week. These formations are the beginnings of your brain and spine! They'll help tell your body how to react to its own growth.

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Placenta -

The placenta also develops around this time, which helps pass nutrients, oxygen, and water from the mother's blood into the embryo via umbilical cord.

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Heart -

Within five weeks, some odd-looking tissue forms a little s-shaped tube near the center of your dinosaur-looking shape, and that will soon learn to move together, making a heartbeat.

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Limb stumps -

Between the fifth and eighth weeks, little miniature stumps form where your arms and legs will eventually be. Your embryonic tail will also start to recede around this point.

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Face -

Between the fifth and eighth weeks, your head will form around your brain and spine, and tissue will start to separate and grow into very basic features.

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Mouth and nostrils -

An opening will begin to form for a mouth, and the beginnings of little nostrils and nasal passages will start to develop.

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Eyes -

Around the seventh week, your eyes start to appear as two small, dark, sightless circles on either side of your face, not yet exhibiting color.

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Eyelids -

After that, eyelids grow over your eyes. The eyes will also grow connections to the brain so that you can understand and control your vision as you exit the womb.

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Genitals -

Up until around the eighth week of development, all human embryos seem female, but grow without actual sexual features. Once those X and Y chromosomes kick in, things begin to get a little different down below, though you won't be able to know the sex for a while longer.

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Teeth -

Around the eighth week, some little spots will appear in your mouth, which are the first buds of primary (or "baby") teeth. At birth, babies have a full set of 20 primary teeth hidden within the gums.

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Hands and feet -

Around week 10, you leave the embryonic stage and become a fetus (though still just 1 inch long), which includes sprouting your stumps into limbs with hands and feet, as well as webbed fingers and toes.

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Bones -

At around 10-12 weeks, bones take shape, coming together to form a skeleton, though they will remain soft to make for an easier birth.

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Functioning kidneys -

At around 10 weeks, kidneys are formed and actually start producing urine.

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Fingernails and toenails -

In the 10-12 week range, different tissue begins to develop on your fingers and toes, which are rudimentary nails. By weeks 13-16, the nails will start to harden.

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Muscles -

The fetus can feel touch with its nerve endings, but around week 12 is when muscles begin to develop. Back then you could twitch, then later wiggle, and eventually graduate to little kicks.

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Internal organs -

Your internal organs start developing from around week five, growing outside of your body, and your stomach tissue later forms on top. At this stage, your intestines and liver will grow, and by week 14 organs settle around your chest and stomach cavity. Males' prostates will form, and females' sex organs settle in the pelvis.

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Ears -

After your organs have the groundwork laid out and are starting to work together, the outer ear forms starting in week 13. The inner ear has been developing for a while, but the outer ear will finish taking shape around the 24th week.

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Eggs -

Females in the womb start to develop hundreds of thousands of eggs in their ovaries around week 16. In a way, moms already have their future grandkids in them!

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Eyebrows and eyelashes -

By the end of week 16, your skin becomes less transparent, and your eyebrow ridges, eyebrows, and even eyelashes will start appearing.

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Fat and sweat glands -

At week 17, the fetus' arms, legs, and trunk catch up to the size of its head, and it starts plumping up. Body fat is deposited under the skin, and sweat glands develop.

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Fingerprints -

Around week 21, your fingers and toes change texture, and these new ridges are your lifelong fingerprints.

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Lungs -

Your lungs are the last of your vital organs to develop fully, as before air you'll be "breathing" amniotic fluid. You do, however, start coughing, hiccuping, and more around week 23.

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Lungs -

Your lung cells then start producing surfactant, a substance that allows the air sacs in your lungs to inflate and expand, preparing you for your grand entrance into the world.

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Billions of brain cells -

Also starting around week 23, billions of brain cells start to develop over the next couple of weeks, which will control all your movements, senses, and involuntary body functions like breathing.

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Growing and practicing -

Although you've got most everything developed by about week 30, you continue to grow, to practice moving, and to strengthen right up until birth.

See also: Mind-blowing things you had no idea happened in the womb.

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