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1-13. Gestation

Gestation (pregnancy) is the period of time between conception and the birth of the child. The normal duration of human pregnancy is about 40 weeks or 10 lunar months (28 days each) or 280 days. The time period is calculated by counting from the date of the beginning of the mother's last menstrual period. Even though the child was not conceived until two weeks after this date, the date of the beginning of the last menstrual period is used to calculate the expected date of delivery. Usually, the exact date of fertilization cannot be determined.

a. Embryonic Growth. From the time it has embedded itself in the uterine wall until the end of the eighth week after fertilization, the new, developing organism has a new name--embryo. During this period, the process of organogenesis is taking place. Organogenesis is the differentiation of cells into specific organs and parts.

b. Fetal Growth. At the beginning of the ninth week, the growing organism is referred to by another name--fetus. This term is used for the period of growth and development until delivery. At about the twentieth week, the fetal heart sounds can be heard by placing a stethoscope on the mother's abdomen. The mother can also begin to feel the fetus move.

(1) Placenta. The placenta is a disc-like organ which is formed by tissue from the mother and also from the fetus. The placenta brings nourishment to the fetus and carries away fetal excretions. Hormones such as estrogen and progesterone are secreted by the placenta.

(2) Umbilical cord. The fetus and the placenta are connected by the umbilical cord. The cord has two arteries that carry blood to the placenta and one vein which carries blood to the fetus. The exchange of oxygen and other substances between maternal blood and fetal blood takes place in the placenta. The exchange of substances occurs without any actual mixing of maternal blood and fetal blood since each flows in its own capillaries.

(3) Membranes. Two thin, opaque membranes cover the embryo through its development as a fetus. The amnion, which forms on the eighth day after fertilization, is a fluid-filled sac which surrounds the fetus and then embryo protectively. This sac is more commonly known as the bag of waters. The amniotic fluid serves as a shock absorber for the developing fetus. The chorion, the outermost membrane, is first an outer covering for the embryo and then the fetus. Eventually, the amion membrane fuses to the inner layer of the chorion membrane.

David L. Heiserman, Editor

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Revised: June 06, 2015