- [Narrator] Within a week of conception,
your fertilized egg, known as a blastocyst,
will make its way to your uterus.
The egg is about the size of a pen tip.
In days, the cells in the egg
arrange themselves into groupings.
The inner cell mass will become your baby.
The outer cells will become the amniotic sack and placenta.
The blastocyst then sheds its protective casing
in a process called hatching
and burrows into the lush uterine wall.
Around week five, your developing baby
is the size of a sesame seed.
The cells that once formed the blastocyst's inner cell mass
begin organizing and arranging,
giving shape to the young embryo
and forming primitive organs.
Your baby's brain and spinal cord are visible
through his translucent skin.
Right around this time,
your baby's circulatory system also forms
and his heart begins to beat.
Your baby looks more like a tiny tadpole than a human.
He's drawing nutrients and oxygen
through the newly formed placenta and umbilical cord.
By week nine, the embryonic tail is gone.
Your baby's looking more human every day,
with protruding limbs and fingers,
a defined nose, mouth, and eyes, and tiny earlobes.
Your new resident is about the size of a grape
and weighs a fraction of an ounce.
It's hard to believe how rapidly one cell evolves
in such a short time into the unmistakable body of a baby.
