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.
What is the greatest risk from a harmful exposure during very early pregnancy? Harmful exposures during very early pregnancy have the greatest risk of causing miscarriage. A fertilized egg divides and attaches to the inside of the uterus during the first two weeks of embryo development. Very harmful exposures during this period first four weeks after the first day of your last period may interfere with the attachment of the embryo to the uterus. Harmful exposures during this time can also damage most or all of the cells of the growing embryo.
Problems with uterine attachment and severe cell damage can both result in a miscarriage. Sometimes this miscarriage is before a person even realizes that they are pregnant. The cells of the embryo have a greater ability to recover at this early stage than they do later on in pregnancy.
If an individual does not have a miscarriage, it is expected that the exposures during this time are not likely to cause a birth defect. This damage can cause early miscarriage. However, there are some important exceptions to this rule. Please contact MotherToBaby to discuss your specific exposure with our experts.
What are the greatest risks from harmful exposures during the first trimester of pregnancy? The first trimester of pregnancy is defined as up to the 14th week of pregnancy 13 weeks and 6 days counting since the first day of your last menstrual period.
Harmful exposures during the first trimester have the greatest chance of causing major birth defects. This is because many important developmental changes take place during this time. The major structures of the body form in the first trimester. These include the spine, head, arms and legs. Some examples of these organs are the heart, stomach and lungs. While the heart and stomach completely form during the first trimester, the lungs continue to develop past the first trimester. What are the greatest risks from harmful exposures during the second and third trimesters of pregnancy?
Computed this way, the pregnancy lasts 40 weeks and the embryonic period - accordingly - 10 weeks. Caution is advisable, though, when wishing to calculate the moment of ovulation - and thus fertilization, closely connected with it - because the moment of ovulation can vary and depends on many factors conditioned by the environment and psychological aspects. In embryology the temporal indices i. After the 8th week, the fetus takes on typical human features, even though at the end of the first trimenon, the head is still relatively large in appearance.
The eyes shift to the front and the ears and nasal saddle are formed. The eyelids are also clearly recognizable now. On the body, fine lanugo hairs are formed, which at the time of birth are replaced by terminal hairs.
The physiologic umbilical hernia that arises in the embryonic period has mostly disappeared. In the second trimenon the mother feels the first movements of the child. In the last trimenon the subcutaneous fatty tissue is formed and stretches the still wrinkled skin of the fetus.
The skin becomes covered more and more with vernix caseosa. This is a whitish, greasy substance und consists of flaked off epithelial cells and sebaceous gland secretions.
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