Original subtitles

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[narrator] At the beginning

of our nine months

in the womb,

we all start off the same,

a generic fetus.

By eight weeks,

our bodies are built,

but we're not

yet individuals.

But, from now on, a series

of dramatic events

will take place, making

every one of us unique.

The latest cutting-edge

science means

we can hone in

on the weeks, days,

even hours

when our faces form,

our fingertips develop...

and our skin

takes on its color.

[crowd cheering]

And it's not

just our bodies.

Our brains will receive

powerful hormones

that influence

our personalities.

[man]

If I turned around and

there was a giant wave,

I would be almost

in a zombie-like state.

And I would be drawn to it.

[narrator] And our feelings

about sex and gender

may even be established

long before birth.

Your mom and dad

think you're a boy,

then you're actually a girl.

[narrator] In this episode,

we will follow

the magical metamorphosis

that happens to us all

as we transform from

a generic eight-week fetus

into the unique

individuals we are.

[rooster crowing]

[indistinct chatter]

[narrator] In this remote

corner of the Caribbean,

the sense of tradition

is strong.

Men and women tend to play

very different roles.

But this village harbors

a strange secret.

There are some people

living here

for whom life has been

far from simple.

It's eight weeks

after conception

and a pivotal change

is about to take place.

From a generic fetus,

the very first

signs of individuality

are about to emerge.

And this tiny body

is about to reveal

if it is male or female.

It's a momentous change

that will determine

the entire course of life,

but if it doesn't

happen as it should,

then life can be

pretty confusing.

[speaking Spanish]

[translator] I remember

that I used to wear

a little red dress.

It did not feel good.

I didn't like to wear it.

It made me

feel uncomfortable.

[narrator] When 24-year-old

Jonny was born,

his parents thought

he was female.

They named him Felicitas

and raised him as a girl.

[translator] I never liked

to dress as a girl.

And when they bought me

toys for girls,

I never bothered

playing with them.

When I saw a group of boys,

I would stop to play ball

with them.

All I wanted to do

is play with the boys.

[narrator] At the moment of

conception, we all inherit

a set of

genetic instructions

that makes us physically

male or female.

But for the first few weeks,

we all look the same.

Until around eight weeks,

when genetically

male embryos

release a critical hormone.

Testosterone.

This is what triggers

the momentous

physical transformation.

That makes males

grow a penis.

But very occasionally,

the body doesn't respond

to this critical surge

of testosterone...

which is what

happened to Jonny.

In the narrow window of time

when his body should have

become fully male,

it didn't process

the hormone correctly.

So the change

was incomplete.

And this is why

Jonny's penis

didn't grow while

he was in the womb.

[speaking Spanish]

[translator] I was born

at home instead

of a hospital.

They didn't know

what sex I was.

[narrator]

Jonny is not alone.

One in 90 of the males

in this village are born

without a penis,

and they're brought up

as girls.

Like two-year-old Gaudalin.

Shortly after birth,

doctors noticed

something unusual

about her body.

[woman speaking Spanish]

[translator] They sent her

for an ultrasound...

And they noticed a problem.

[interviewer speaking

Spanish]

[translator] She had

two little testicles,

one on each side.

[interviewer through

translator] And now,

with that knowledge,

do you still treat her

like a girl?

[translator] Yes,

I think she's a girl.

[narrator] Gaudalin,

like Jonny,

is biologically male.

She inherited

the Y chromosome

that should have

turned her into a boy.

For the first eight

weeks in the womb,

whether male or female,

we all have

a small protrusion

called a tubercle

where our genitals

will eventually grow

and a set of internal glands

called gonads.

If you're male,

a single gene

on the Y chromosome

instructs the gonads

to become your testicles.

Only now, do they produce

a powerful type

of testosterone

called dihydrotestosterone.

This transforms the tubercle

into a penis.

But if you're female, your

gonads become your ovaries.

You don't make

dihydrotestosterone

and your tubercle

becomes a clitoris.

Jonny and Gaudalin

lack the enzyme

that produces

dihydrotestosterone.

So, they couldn't grow

a penis in the womb...

But extraordinarily, they

can do so later in life.

It's an extremely

rare condition

in the rest of the world,

but in this village,

it's so common

that it's

completely accepted.

Nine-year-old Carla has been

showing signs that

she's not actually a girl

for some time.

[woman through translator]

When she turned five,

I began to notice changes.

Whenever she saw

one of her friends,

she wanted

to fight with him.

Her muscles and chest

began growing.

You could see she was

going to be a boy.

[narrator] Carla is

about to enter puberty,

a time when all males

get another surge

of testosterone.

Soon, her testicles

will descend

and her penis will

start to grow.

Because this time

Carla doesn't need

the dihydrotestosterone

to make her genitals grow.

She, like Jonny, is

known as a guevedoce,

which means "penis at 12."

Her whole life is

about to change.

[woman through translator]

She began asking

at eight years old,

"Cut my hair, I don't

want it in braids."

[interviewer through

translator] You said

you want to cut your hair.

How will you feel

with short hair?

[translator]

I'll feel good, happy.

[narrator]

Carla's transition to

boyhood is made easier

because there's another

guevedoce in the family.

Her cousin Catherine.

He officially became

a boy at 11,

but he hasn't

changed his name.

Together, they're off to the

barbers for the first time.

[speaking Spanish]

When I see him

with short hair,

I think it'll help me

understand he's a boy.

I love her however she is.

Girl or boy, it makes

no difference.

[barber] A new you!

[mother]

What do you look like?

-[speaking Spanish]

-[translator] A man!

You used to have a sister.

Now what do you have?

[child] A brother.

[teacher]

From now on, this girl

is going to be a boy.

He is a boy,

you must respect him

and call him Carlos.

Be friendly and don't

make fun of him.

Now you all know this,

so, if you don't do

what I say,

you'll be punished.

[narrator]

It's 14 years since

Jonny made

his transformation

into a male.

[translator]

When I changed, I was

very happy with my life.

[narrator]

While most of them

will not have

children of their own,

their dreams for the future

are wide open.

[interviewer speaks Spanish]

[translator] Would you like

to get married

and have a family one day?

[Jonny through translator]

I'd like that very much.

I'd love to have

a partner who

stood by me

through good and bad.

[narrator] The embryo

is almost an inch long,

and the magnificent process

of becoming an individual

starts to take shape.

A structure fundamental

to our sense of identity.

Where more than

40 muscles allow us

to share our emotions

with the whole world,

the face.

One in over seven billion.

Even if you're

an identical twin,

no one has a face

exactly like yours.

It all begins

around four weeks,

when the embryo is no bigger

than an apple seed.

Over four miraculous

weeks, five sheets

of cells converge...

Eventually knitting

together to form

the unique human face.

But if anything disrupts

this precisely

timed process,

the jigsaw

can't be completed.

Oliver's face wasn't

finished in the womb.

He was born four weeks ago

with a cleft lip.

Before his arrival, Oliver's

parents, Dave and Kerry,

knew he was going

to be different.

Yeah, we found out

about the cleft lip

-at the 20-week scan.

-Yeah.

I was just worried

for the baby, really.

[Dave] You just want

everything to be perfect,

don't you?

When something is

clearly, you know,

not going to be 100%,

that's the sort of

worrying part,

is when there's no doubt

that it is, you know,

something there that

will need taking care of.

[Kerry] It's something

that can be fixed, so,

in the scheme of things

it's not so bad.

[narrator] Oliver's plastic

surgeon, Felicity Mehendale,

is preparing to operate

on his lip.

Cleft lips and cleft palates

are actually the most common

congenital condition

that we see.

So, they give us a huge

amount of information

about how our

human face is formed.

I think it's so easy

to look at a human face

and take for granted

that that's the way

it always was.

But to think that

my eyes started out here,

my nose started up there,

my top lip was

in three separate bits,

and... I had a one

in 1,000 chance that

a bit of my top lip

wouldn't have zipped up.

[narrator] Oliver is

now three months old

and strong enough

for surgery.

[interviewer] How are you

feeling about today?

Okay. Nervous.

He's happy.

He's happy.

Totally oblivious to what's

going to happen. [chuckles]

Hi.

Still smiling.

[Mehendale] If I didn't

repair Oliver's lip,

the gap in his lip would

let his gum dry out,

he'd look different,

and importantly

when he started

to say things like

"papa" and "baba,"

he'd have a little bit of

air leaking through his lip

and it would make it

a little bit more difficult

for him

to communicate the way

maybe we take for granted.

Mummy's here, darling.

Mummy's here.

There you go.

There you go.

Right, shall I scrub up?

Local going in, Caroline.

Pop that in

the muscle, Stephanie,

if you wouldn't mind,

just very...

Ah, that's lovely. Thanks,

that's great, thanks.

To bring the base of little

Oliver's nose on the right,

I lift the base of his nose

along with the tissues

of his cheek

off the bone of his

upper jaw, see this?

You can see this bits

going into the columella.

That's nice, thanks.

I'm trying to re-align

tissues and structures

that would have joined,

trying to put them

in a position

that allows them to work

the way they should work,

but I think it's

quite a challenge

because the anatomy

is so beautiful

the way it forms

in the womb,

and it's actually quite

humbling when you try

to recreate that,

as nature intended.

I can just see

Oliver's lip beginning to

come together now.

That's it.

That looks happy.

So, he was asleep after

the surgery for a fair bit

until about teatime,

and when he woke up

and was able to

sort of recognize us.

He did his usual thing,

smiling straightaway.

-Which was amazing.

-It was.

That's kind of when you knew

everything was really

okay, I suppose.

[Kerry] There's hardly

anything there already.

[Dave] It's the craft of,

yeah, what Felicity can do.

They told us that,

before, you know,

it'll be quite a change.

Thought, "Oh, no..."

It is really quite a change.

[narrator] It's now

10 weeks into the journey

and on the tip of

every single finger,

one of our most

unique characteristics

is about to be revealed.

Our fingerprints.

But on some incredibly

rare occasions,

this very hallmark

of our individuality

doesn't surface.

To the authorities,

this woman is invisible.

She can defeat

the most sophisticated

security devices.

Because Cheryl is one

of a handful of people

in the world

who was born with

no fingerprints.

Just a few creases on

the end of her finger.

Cheryl's condition

is known as immigration

delay disease.

At 10 weeks, skin cells

begin constructing

the peaks and valleys

that become fingerprints.

But, due to a faulty gene,

Cheryl's cells couldn't

maintain the structures

and the creases collapsed.

[Cheryl] I worked as

a flight attendant

and after flight

attendant school,

you have to get your

security clearance,

and for most of the students

in my class it took

two weeks.

It took almost 10 weeks

for me to get mine.

[narrator] Most of us

don't think about

our fingerprints,

but they do make

each of us unique.

Even identical twins have

different fingerprints.

But that's not

all about our hands

that makes

each of us distinct.

As the clock races

towards 11 weeks,

another astonishing

milestone takes place

as our hands make their

very first movements.

The myriad of connections

that form the body's

nervous system

is beginning to give us

control over our body.

And, remarkably, even at

this very early stage,

most of us favor

one hand over the other.

In most animals, the split

between left and right

is almost the same.

But 90% of us humans

are right-handed.

And so,

being in the minority,

those who are left-handed

can be at a disadvantage.

Boxer Tom Stalker

is among those

who has to live in a world

skewed to the right.

[Tom speaking]

[laughs]

[narrator] Being left-handed

runs in families,

which suggests

it's a trait passed down

through the genes.

Statistically, you're

more likely to be shorter

or have certain conditions

like dyslexia and migraines.

But it's not all bad.

Tom believes his left hand,

or "south paw," saved him.

[Tom speaking]

[crowd cheering]

[narrator] Now he's

a medal-winning boxer.

[Tom speaking]

[announcer] Tom Stalker!

[crowd cheering]

[narrator] In 2012, he was

captain of Britain's

most successful ever

Olympic boxing team.

This might offer a clue

to why natural selection

hasn't bred out

left-handedness.

[narrator] We can be

confident that

the proportion

of left-handers hasn't

changed for 10,000 years

because 10% of tools

archeologists uncover

are for lefties.

It seems that long ago,

in hand-to-hand combat,

left-handers like Tom

would have had an advantage.

So, they were more likely

to survive a fight

and pass their trait on

to another generation.

[boxer speaking]

[Tom speaking]

[narrator] And this is

supported by the fact

that although just one in 10

of us are left-handed,

there are three times as

many in one-on-one sports.

[Tom speaking]

[narrator] It's intriguing

evidence that

being different

may offer an advantage,

allowing you to

come at the world

from another angle.

It's 13 weeks

and the tiny embryo,

now the size of a pea-pod,

is growing more and more

unique every day.

And it's around now

that skin begins to

take on its color.

Up to this point, the skin

has been transparent.

But, beneath the surface, a

transformation is underway.

A swarm of cells called

melanocytes spread.

Skin comes in a wide

spectrum of tones,

and depending on ethnicity,

the number of cells

affects how dark

the skin becomes.

Here in Kenya,

most people have an

abundance of melanin.

The pigment prevents the

blistering equatorial sun

from harming healthy skin

cells and causing cancer.

It stops your skin from

absorbing too much sunlight.

But there are some people

who have no melanin at all.

Like 28-year-old Nasoro,

who plays soccer

for Albino United.

[Nasoro through translator]

Playing football helps you

forget all your problems.

My heart loves

playing football.

[narrator]

Nasoro is an albino.

And like most Africans,

he should have dark skin,

but he lacks a key gene,

which means his melanocytes

are unable to store

the melanin his skin needs.

The only place he

produces the pigment

is in the occasional mole,

so he has very little

protection from the sun.

[speaking local language]

Playing out in the sun

used to hurt a lot.

I would get blisters.

[narrator] Nasoro

is from Tanzania

and works part time

washing cars.

But his passion is soccer.

Today, Nasoro and his team

will be playing their first

international game against

a team from Kenya.

[speaking local language]

I've never

met an albino before.

I've only seen them

on television.

[narrator] Nasoro's skin

is unusual

but by no means

one of a kind.

There are thought

to be around

150,000 albinos

in Tanzania alone.

In fact, all of

Nasoro's teammates

have the same condition.

[Nasoro through translator]

In my neighborhood,

there was a lot of talk,

"Oh, look, there goes

the albino.

I hope I don't get

a child like that."

[Nasoro speaking

local language]

[translator] They would spit

on their chests

as protection

against having

a child like me.

It's as if you're

not a human being

but a curse on the family.

It hurts.

I'm looking forward

to the game.

I'll make some new friends.

It'll be good.

[narrator] Because their

skin has no protection,

Albino United only play

once the sun starts to set.

[Nasoro through translator]

After playing football,

I feel good,

because it keeps

my body fit.

I am happy because the game

is happy and makes me happy.

[Nasoro speaking

local language]

[translator] I look like

a person who exercises

and is happy.

-[crowd cheering]

-[whistle blowing]

-[whistle blows]

-[crowd cheering]

[translator]

As Albino United,

we're brothers.

We're like one happy family.

[man through translator]

Although we still

have problems,

the fact that we're together

helps us find solutions.

[narrator] Albino United

shows that problems

and prejudices

can be overcome when you

work together as a team.

Over the last six weeks,

a series of rapid

transformations

have taken place that

make each of us distinct.

In addition to our sex,

we each have

a unique set

of fingerprints,

one of many skin tones,

and an instantly

recognizable face.

But our individuality is

much more than skin deep.

Some of the developments

that are taking place

inside our bodies

during those early months

in the womb

not only influence

who we become,

but our survival

throughout our lifetime.

[knock at the door]

[nurse 1] Hello.

[nurse 2] Hi, Joanne.

[Joanne] Hi.

This is it.

It's my birthday.

It's your birthday.

It's a bit emotional.

It gets like...

Well, everyone does.

Do they? Oh, good.

I'm not...

[narrator] Last year,

Jo was diagnosed with

an aggressive form

of blood cancer.

[Joanne] It was about

the middle of September

and I started to feel

quite unwell.

I thought I had

a run-of-the-mill virus.

Um, hot sweats, achy.

It was our

daughter's birthday

but I couldn't even stand up

to watch her open any of

her party presents,

and that's really unusual.

[man] Later that evening,

Jo was diagnosed

with leukemia.

[narrator] Jo's best

chance of recovery

is to have

a bone marrow transplant.

But this is far

from straightforward

and it all goes back

to our development.

As soon as we enter

the world, our bodies

are under threat.

Viruses and bacteria are

waiting to find a new host.

Week 14 and the race against

the clock is happening.

Specialized cells that will

hunt and kill invaders

must be formed.

The immune system.

Something most of us will

never have to think about.

Unless in life a part of

the body becomes so damaged

that it has to be replaced.

[Joanne] I just hope that

there's a donor for me.

I guess we're just relying

on a phone call. [laughs]

[narrator] Jo's search

is hard because of

the very personalized

way all our immune

systems develop.

At 14 weeks, a key part

of the immune system

is becoming fully active.

Human leukocyte antigens,

otherwise known as

the HLA system,

detect the telltale traces

of infections within

the cells.

If they find any,

they'll bring them to

the surface of the cell,

so that they can be detected

by other immune cells

and destroyed.

But our HLA systems

are incredibly varied.

There are so many genes

involved in creating HLA's

that it's rare to

find someone else

who shares a similarity.

This huge variety is

problematic for anyone

searching for a bone marrow

donor to match.

But there is an upside.

Having such diverse

immune systems

makes us stronger

as a species...

and unlikely that

we'll all be wiped out

by the same infection.

Fortunately, a worldwide

HLA database of donors

is bringing hope

to people like Jo.

I'm getting ready

for a bone marrow

transplant tomorrow.

I feel incredibly humble.

I can't believe

there is such a person

that exists out there.

He's a 22-year-old male.

He is a, almost 10 out of 10

fully matched.

He's been referred to me

almost like a perfect twin.

I'm guessing we look

nothing like each other.

[nurse] Now this is it.

[Joanne] Hooray.

T-day. [laughs]

[narrator] It's seven months

since Jo was diagnosed

with cancer.

Blood stem cells

harvested from

the bone marrow of Jo's

donor have now arrived.

The stem cells

have come into my body,

it will apparently go round

my blood stream

a couple of times,

and, um, and then it's got

like a homing sensor

and it will actually

gravitate towards

my bone marrow,

which is what

I need it to do.

[nurse] Well done.

Congratulations, Joanne.

-Hurray. Well done.

-[laughs]

-Toast.

-[laughter]

Perfect.

[narrator] Finding a close

match has increased

Jo's chances of recovery,

although the success

of any transplant

is by no means guaranteed.

At the same time that

the immune system

is changing

to keep us fit and strong,

our most vital organ

is rapidly growing

ever more sophisticated.

Week 15 is also

a milestone for the brain.

By now, the cerebral

hemispheres,

the thinking parts

of the brain,

have all the brain cells

they'll ever need.

But now is the magical

moment when

the connections between

these billions of neurons

start to grow.

And it's these

extraordinary connections

that shape our thoughts,

feelings, and emotions

and influence

the very essence

of our personality...

And they dramatically

affect how we behave.

[Laird] I wish I had

the answer to why am I drawn

to these great waves

and to these experiences,

but it's out of my hands.

I'd like to say

I have control of it

but it's...

it's got control of me.

[laughs]

[narrator] Laird Hamilton

is considered

the world's greatest

big wave surfer.

Surfing the most

dangerous waves...

swells up to 70 feet and

traveling 15 miles per hour.

He's completely

revolutionized

big wave surfing

by pioneering

a method to surf

the biggest, heaviest

waves in the world...

being towed into them

at high speed.

I knew when

I was very young,

like,

when I was a little kid,

that... that I wanted to

be best at riding big waves

and I wanted to be

a big wave rider.

[narrator] But such

daredevil behavior may lie

in what happened during his

development before birth.

At an all-important time,

around 15 weeks,

another surge of

testosterone is sent through

the rapidly developing

brain and body.

And at this moment,

the brain is

delicately sensitive

to hormonal changes.

And it appears

that testosterone

alters the structure

of the brain regions

that process

fear and excitement.

Boys get twice the dose

compared to girls,

and some get

more than others.

Which could

explain why some people

are more thrill-seeking.

If I turned around

and there was a giant wave,

I would be almost

in a zombie-like state

and I would be drawn to it.

It's as simple as it's

just inside of me.

It's just inside my soul.

It's inside my bones.

[narrator] Laird's exposure

to extra testosterone

before he was born

has given him

an inherent

appetite for risk,

something that was

reinforced by the way

his mother raised him.

I think my mom compelled me

to try to do things

that had never been done.

She gave me the...

the belief

that the impossible

is possible.

[narrator]

The adrenalin-seeking

instincts

that Laird was born with

have been reinforced

by his upbringing.

Both nature and nurture

working together

to push him to attempt

the impossible.

Daddy!

[woman] I think he has fear.

He just is

able to process it

and use it in

a different way than,

let's say,

someone like myself.

Can I race ya?

Can I race ya?

I think he needs that

to feel like he's

really living,

and when he doesn't

do it enough,

I feel like he sort of feels

like his life is wasting.

[Laird] Are we

gonna play Whoa?

[children] Yeah!

[Laird] Here we go.

Watch out.

Don't get thrown over!

-[girl] Whoa!

-[both laughing]

[narrator] And Laird is

raising his own children

in the same way he was.

Whoa!

-[girl] Whoa, Dad!

-[laughing]

[Laird] The problem is

once you play Whoa,

you wanna play Whoa

every time.

[girl] Whoa!

[woman]

Oh, that's good, Reece.

You're getting him up.

Laird taught Reece to drive

a jet-ski by herself

at three and half,

literally.

[Laird] I want them

to find their...

their passion, their love,

and I want it to be theirs.

And if they like, and if

it has to do with going fast

or doing something

a little bit scary,

so be it.

[narrator] But as well as

thrill-seeking,

there are other telltale

signs that could reveal

how each and every one of us

may have been affected

at this crucial moment

in the womb.

Greater exposure

to testosterone before birth

has been linked to

how competitive we are.

[man] I love to win.

I want to make sure

that I'm the one

that stands on top

of the podium

at the end of the day.

[woman] Working in

a male environment,

everything I do is

just competitive.

I hate losing.

I hate losing.

It's just one of

my pet hates.

[narrator] But this doesn't

only apply to sports.

It also plays out

in other areas of life.

And even affects how many

risks we take at work.

I do sky-diving. I enjoy

that and I like trading.

[man] I definitely trade

how I am, which,

I think, is with

intuition rather than

deep theoretical analysis.

When there is volatility,

when there is momentum,

it works out fantastically.

When the markets

are quiet, I must be

escorted out of the building

and kept away from

all trading terminals.

[narrator] And there's a

surprising clue as

to how much

testosterone we may have

been exposed to.

A clue that can be found

by looking at the length

of the ring finger.

According to one study

on city traders,

the longer the ring finger

compared to

the index finger,

the greater

the financial success.

Whether you're

a stock-market trader,

a go-cart racer,

or a champion surfer,

what happened at week 15

could have affected

how you behave today.

The length of my ring finger

and the length of

my index finger

are substantially different,

which I guess

leads to a lot

of my problems. [laughs]

[narrator] Testosterone

has a dramatic impact

on our personality, but

this hormone is so powerful

and the brain and body

are so exquisitely

sensitive to it,

that its influence

doesn't end here.

It may also have an impact

on the feeling we have

of being male or female.

What we call our gender.

And testosterone

might actually alter

the very structure

of the brain...

Making it male.

But this happens

almost two months after

the sex of our body

becomes male or female.

So, sometimes

the body's physiology

and the feeling of gender

don't match.

Seven-year-old Mati appears

to be a typical girl.

But she's in a boy's body.

She's transgender.

It kind of means that

your mom and dad think

you're a boy,

then you're actually a girl.

So, you have to change

all of that.

She just seemed

like a regular baby.

Nothing out of the ordinary.

We taught her baby sign

early on, which was great

because she was

able to communicate

with us early.

Except that then we were

thinking that we were

getting to know "him,"

Matthias.

Noah, come.

[man] At the age of one

and a half is, I would say,

when we started noticing

a difference.

It was around that time

that Mati, Matthias

at that point,

was able to communicate

more with us

and express preferences

for, um, clothing,

for, uh, games and toys.

[Mati] Come on,

let's get in the tent.

[Noah] Okay.

You can kind

of see a rainbow.

Right?

Yup. It's a light.

[Noah] Look into my eyes.

Look into my eyes.

[Mati] I don't think I'm

different from him

except that

he's a boy and I'm a girl.

This is my bathing suit

I made up.

No, no. Oh! Wait.

Boys are

a little more active

and girls are

a little more calm.

What? Spider-Man!

Help! Spider-Man, help!

I am Spider-Man.

[Mati] Help! Spider-Man!

[woman] People have

all sorts of ideas about

how it comes to be

and I think the bottom line

is it's hard to say how.

It just... it just is.

I just know what I see

and I know that,

you know, without a doubt

if you popped all our

brains out right now,

her brain would

match my brain.

[narrator] We're still

uncertain why some people,

like Mati,

feel the way they do.

But there are theories.

One explanation is that

there's an alteration

to a gene that

makes the brain less

sensitive to testosterone.

So, Mati's brain followed

a path more typical

of a female.

That's when I was zero

and that's me

putting my feet

in the camera.

I think I was about three,

three when I started

growing long hair.

[woman] She turned five

shortly after that,

she started kindergarten,

and I knew in my heart

that she needed to start

as a girl because

she was going to be meeting

friends who she would have

through eighth grade

and beyond.

And she transitioned

first at school.

A lot of people

in the family

and even some of our friends

were still beginning

to understand

what... what we were

looking at

and what is happening

with Matthias

and what is being

transgender, you know, so,

that was all still

relatively new to us,

but I knew if nothing

else, that at school

it was important for her

to be seen for who she was.

[narrator] So Mati

successfully managed

to convince her parents

that he was a she.

She changed her name and by

the time she started school,

she was officially a girl.

This is when I was six.

All this is when I was six.

[man] The thing

that I've found hard

it's like it's my

little boy and...

and you wanted to play

and, and you have

this vision and things

that you're gonna be

doing with your little boy

and... and it's pretty hard.

It's something that

you think about,

"Well, how is it gonna be

with the family,

with the friends,

with neighbors,

and everyone in the

community and at work?"

And then also,

"What's that gonna say

about...

about me as a father?"

[woman] I was told that

the doctors will watch for

the stages of puberty.

They block the testosterone

that would be coming

through her body

and my hope is that she

could take cross hormones

and be on the same

time frame as her peers,

female peers

in terms of puberty.

[narrator] Estimates

vary, but perhaps

one in 10,000 of us

is transgender.

[woman] This is

just another variation

of human condition.

How we come out.

It might not be crazy,

rampant, happening

all around you,

but it's happening,

and as people are finding

the confidence to

stand up and say,

"Yes, that's happening

in my family,"

you'll probably realize,

"Hey, that happened

with my neighbor,

that happened

with my cousin,

that happened with

my co-worker."

[narrator] We are all

shaped by what happened

in the months before

we were born.

Everything from our sex...

To the color of our skin...

The way we smile...

And even our

very personality.

The middle weeks in the womb

have been a time of

rapid development

and personalization.

Moment by moment...

different, one-of-a-kind

aspects of us were formed.

Each change is the result

of an infinite number

of possibilities.

And it's these differences,

forged long before

we were born,

that make each

and every one of us

the extraordinary, varied,

and unique creatures we are.

Next time,

the final 16 weeks.

[laughs]

[narrator]

Our senses switch on.

Bones harden.

And our lungs get ready

for us to take

our first breath.