Week 6: your baby is the size of a sea firefly

Your embryo is now the size of a sea firefly, a tiny ostracod or seed shrimp about 3 mm across that lives in shallow coastal waters. Tucked inside its two-part shell, it looks like a translucent grain of rice with legs.

Its superpower is light. When disturbed, the sea firefly spits out a cloud of glowing blue chemicals into the water, a burst of bioluminescence bright enough that Japanese soldiers once collected and dried them to read maps by at night.

The chemistry behind that glow, a reaction between a molecule called luciferin and an enzyme, turned out to be so useful that related systems are now used in laboratories worldwide to make cells light up on command.

Meanwhile, inside you

Your embryo this week is growing their own dramatic features: the beginnings of eyes, ears, and a neural tube that will become the brain and spinal cord.