Week

No longer just a mass of cells, your baby — officially called an embryo — is beginning to take on a distinct form.

The embryo has divided into three layers. From these layers, all tissues and organs will develop. In the top layer, a groove develops and then closes to form the neural tube, which will eventually develop into your baby's brain, spinal cord, spinal nerves and backbone. It runs along the midline of the body, from the top to the bottom of the embryo. The closure of the neural tube begins in the embryo's midsection. It proceeds upward and downward from there, like a double zipper. The top portion thickens to begin forming the brain.

From the embryo's middle layer of cells, the heart and the circulatory system are taking shape. A bulge at the center of the embryo will develop into your baby's heart. By week's end, the earliest blood elements and blood vessels have formed, both in the embryo and the developing placenta.

Your baby's first heartbeats occur at 21 to 22 days after conception, although you and your health care provider can't hear them yet. It may be possible to see this motion on an ultrasound, however. With these changes, circulation begins, making the circulatory system the first functioning organ system.

Your baby also has an inner layer of cells, from which lungs, intestines and the urinary bladder will develop. This week, not much is happening in the inner layer. It will be awhile yet before those areas take shape.

At the moment of conception, your baby was a single-celled zygote and was microscopic in size. By the fifth week of your pregnancy, three weeks after conception, your baby is about 1/17 of an inch long, about the size of the tip of a pen.

My First Baby

My First Baby

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