Embryonic Period

The third through eighth weeks of prenatal development — the stage when the basic body takes shape. The developing human is called an embryo.

What happens

• The body plan is laid down — three primary cell layers form, each giving rise to particular tissues and organs.
• Organs begin to form — the heart starts beating, and the brain, spine, limbs, and major organs all begin to appear.
• By the end of week 8 the embryo has a recognizably human form, though it is only about 2–3 cm long.

Key idea

This is the most vulnerable stage of development: because every major structure is forming for the first time, disruptions — from infections, alcohol, or certain drugs — can cause lasting birth defects.