Day 0

Week 0

Hello, first cell

Right now you are one cell—and that cell already holds the full recipe for a whole human being, written in DNA.

Size: ~0.1 mm

Day 22

Week 3

A heart that starts young

A simple tube of muscle begins pulsing rhythmically—the first organ to take on a steady job of moving fluid through the growing body.

Size: ~2 mm · <0.02 g

Day 35

Week 5

A brain takes shape

The tube of nerve tissue that sealed shut about a week ago now swells into the beginnings of a brain—billions of future connections packed into something still smaller than a pea.

Size: ~7 mm · <0.1 g

Day 56

Week 8

Blueprint for every organ

Miniature versions of the stomach, liver, kidneys, lungs, and more are all sketched in—each system claiming its place in the plan.

Size: ~3 cm · ~3 g

Day 84

Week 12

Stretch, yawn, practice

You rehearse motions you will need later—big yawns, long stretches, and a growing catalog of twists and kicks.

Size: ~9 cm · ~45 g

Day 168

Week 24

Sound and flavor

Voices filter through fluid-filled ears, while swallowed water carries faint tastes from what a birth parent eats.

Size: ~35 cm · ~760 g

Day 266

Week 38

Ready to meet the world

Lungs, heart, and vessels complete the switch from fluid life to air-breathing life—a full-term baby is prepared for the first cry.

Size: ~51 cm · ~3.4 kg