The menstrual cycle, fertilization and the embryo

As the embryo grows, two embryonic membranes form. The outer one is the chorion and the inner one is the amnion. The amnion surrounds the embryo with a fluid that protects it from shocks and from drying. The chorion surrounds the amnion and grows finger-like projections into the endometrium. There the capillaries of the mother and of the embryo come close together, and this makes the placenta. The embryo is joined to the placenta by the umbilical cord, about 70 cm in the book. Through the cord digested food, vitamins, water, salts and oxygen go to the embryo, and waste and carbon dioxide return to the placenta. The substances pass by diffusion and the bloods normally do not mix. The placenta also makes progesterone from the fourth month. It lets drugs, alcohol, nicotine and viruses pass from the mother’s blood to the embryo, and these can cause serious harm, so a pregnant woman asks her doctor about any medicine, alcohol or smoking. The book divides development into three stages of three months. In the first, the nervous system and heart begin in month 1, the hands and eyes become distinct, the testes form by week 6 and the ovaries by week 12, and the embryo responds to stimuli. In the second, the heart beat can be heard, the bones harden, the sense organs develop and the size grows. In the third, the brain and the other systems keep maturing while growth slows. In the ninth month the placenta loosens and progesterone falls.