6 weeks pregnant, or 4 weeks after conception: closure of the neural tube

At this embryological stage, the neural tube, the nerve groove at the back of the embryo, closes. The neural tube forms the early brain and spine. It’s important to take a 400 micrograms folic acid tablet every day before you're pregnant and until you're 12 weeks pregnant. Folic acid can help prevent birth defects known as neural tube defects, including spina bifida.

By 6 weeks, your baby's heart is beating and the structures necessary for forming the eyes and ears are developing. Small buds will appear on top of the embryo, this later becomes your baby's arms.