MCQ Anatomy Answer 012
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While the third-week embryo is a primitive trilaminar plate, in the fourth week, the heart begins to form and begins beating almost immediately. Hematopoiesis occurs in the yolk sac, and a primitive circulatory system connects the capillary plexuses of the yolk sac and chorion to the embryo. Partitioning of the atrium also begins in the fourth week. During the fifth week, cardiac septa form and the atrioventricular (AV) cushions fuse. By the sixth week, the heart is close to fully formed. This early sequence for the heart explains why it is so difficult to try to prevent congenital malformations of the cardiovascular system from occurring, since the mother of a 6-week-old fetus is only about 8 weeks from her last menstrual period, and may have assumed that she just "missed a period" (a very common phenomenon) for reasons other than pregnancy 
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