Organogensis refers to that period of time during development when the organs are being formed. After an egg has been fertilized, and has been implanted in the uterus, the developing form is known as the embryo. Organogenesis takes place during this embryonic phase. In fact, most organogenesis has begun as early as week five in humans (remember that a normal human pregnancy lasts an average of 40 weeks). Therefore, damage to any of the organ systems of the body which may ultimately result in some type of birth defect usually strikes during this time frame.
By week five, the buds of tissue which will become the limbs are in place. The structures which will become the skeleton, nervous system, and circulatory system of the face, neck, and jaws are in place. A five-week-old embryo has the early developmental structures of the esophagus, stomach, intestine, liver, and pancreas. The heart is already functioning, and continues to develop and change over this period of time. The respiratory system begins developing, as do blood vessels, blood cells, nervous and endocrine organs. Clearly, the most crucial organs of the human form are developing during organogenesis. Essentially, the earlier the injury to these developing buds of tissue, the more severe the ultimate defect. This is because these tiny buds of tissue hold all the primitive cells which should differentiate into the myriad number of cells necessary to create all of the varied organs of the human body.
It is an irony that, during this crucial period of development, when toxins from the outside world can have such devastating effects on the ultimate development of the embryo, many women are not even yet aware that they are pregnant, and are therefore not in the mindframe of protecting the developing embryo from exposure to such harmful substances as cigarette smoke, alcohol, certain drugs or medications, or extremes of heat (as could be experienced in a very hot Jacuzzi).
One of the most infamous agents (teratogens) responsible for widespread deformities during the period of organogenesis is a drug called thalidomide. Thalidomide was administered to women (particularly in Europe in the 1950s) because it was thought to combat the nausea present in early pregnancy. Over time, however, it became evident that babies born of thalidomide-using mothers had very high rates of serious limb deformities. In particular, the long bones of the limbs were either absent or seriously deformed. Furthermore, many of these children had associated defects of the heart and intestine. Thalidomide was ultimately determined to be at fault, causing the most severe defects when given between weeks four and six of pregnancy: the period of organo-genesis. Thalidomide was subsequently withdrawn from the market.