Your Baby’s Progress
Once it has implanted, the embryo secretes chemicals that have two functions. First, they signal to your body that the embryo has arrived, and this triggers a number of changes in your body: Your ovulation cycle stops, the mucus in your cervix thickens, your uterine wall softens, and your breasts begin to grow. Second, your immune system is suppressed so that the embryo is not treated as foreign and rejected, but is allowed to grow.
In addition, an outer layer of the blastocyst develops into a protective cocoon around the embryo. This cocoon will create the rudiments of the placenta and the support system in which the embryo will grow- the amniotic sac (the watery balloon in which it will float), the chorion (a safety cushion around the amniotic sac), and the yolk sac (which will manufacture blood cells until the liver takes over). The chorion then grows finer like projections, the chorionic villi, which the cocoon burrows firmly into your uterine lining.
The Cells specialize
Throughout these early weeks, the embryo’s cells become more specialized. There are now three layers of them, each destined to create different organs of the body. The innermost layer forms a primitive tube that will later develop into the lungs liver, thyroid gland, pancreas, urinary tract, and bladder. The middle layer will become the skeleton, muscles (including the heart muscle), testes or ovaries, kidneys, spleen, blood vessels, blood cells, and the deepest layer of skin, the dermis. The outer layer will provide the skin, sweat glands, nipples (and breasts, if it is a girl), hair, nails, tooth enamel, and the lenses of the eyes. These three cell layers differentiate to create an entire human body.
The Embryo’s Support System
The villi of the growing placenta intermingle with the maternal blood vessels of the uterine wall in such a way that they eventually become surrounded by lakes of blood. Maternal blood flows in and around these spaces, and because it is divided by only a cell or two from fetal blood, exchange of nutrients and waste between fetus and mother can occur in this blood space. The placenta is a hormone factory pumping out hormones, such as human chorionic gonadotropin (hCG), that are designed to support a healthy pregnancy.
Until the sixth week, the embryo’s blood cells are supplied by the yolk sac; after the end of the third week, blood circulation is pumped by the baby’s own heart.