The embryonic period runs from the third to the eighth week after fertilization. In these weeks every major organ begins to form, so harm at this stage can cause major birth defects.
Three germ layers
In week 3 the embryo forms three cell layers. Ectoderm becomes skin and the nervous system, mesoderm becomes muscle, bone, heart and blood, and endoderm becomes the lining of the gut and lungs.
Neural tube
A groove along the back of the embryo folds into a tube that becomes the brain and spinal cord, and it closes by about day 28. Folic acid before and early in pregnancy lowers the risk of neural tube defects.
Heart and circulation
The heart starts as a simple tube and begins to beat around day 22. Over the next weeks it loops and divides into four chambers.
Placenta and membranes
The placenta forms an exchange surface between maternal and embryonic blood without the two mixing. The amnion fills with fluid that cushions the embryo.
At the bedside
This critical window often passes before a person knows they are pregnant, so ask about medicines, alcohol and folic acid in anyone who could become pregnant. Many drugs that are safe later in pregnancy are risky in weeks 3 to 8.
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Written by Making Synapses. Topic reference: OpenStax, Anatomy and Physiology 2e (no text adapted).