May 1, 2012
Pregnancy is part of the process by which all sexual reproducing creatures continue the existence of their species. Pregnancy, in humans, is typically defined as beginning with the implantation of a fertilized egg in the uterus of a woman. This is after sperm has met and penetrated an egg in the woman’s fallopian tube (“Stages of development,” 2010). From there pregnancy is defined as the gestation period leading up to birth. Since all humans are born as a result of pregnancy, one could say that is a health topic that affects, or is related to, each and every person on the planet. However, it can more narrowly be said to affect women who are currently pregnant, those planning to become pregnant, and any partners they may have who are the fathers of their children or are otherwise invested in the pregnant woman’s life. This topic is a great interest to me because of the complexity of human development. At any point during a pregnancy, it seems as if there are so may things that could go wrong, it is a wonder how any have a positive outcome. However, it seems that the vast majority of pregnancies in the United States do have positive outcomes and the result is the ever expanding human family.
Pregnancy in humans is typically expected to last about 40 weeks and is typically measured in three trimesters. The first trimester occurs from the time of fertilization until about twelve weeks. The second trimester lasts from week 13 until week 28. Finally, the third, and last, trimester lasts from week 29 until birth (“Pregnancy,” 2010). Each of these trimesters is marked by specific stages in fetal development and requires that attention be paid to different aspects of prenatal care.
During the first few weeks of the first trimester of pregnancy – also called the germinal stage, in reference to fetal development – the fertilized egg implants itself in the uterus. Cell division typically begins within 48 hours of fertilization (Cherry, 2012). This fertilized egg is also known as a zygote. Once implanted in the uterus it begins the aforementioned cell division – growing from one cell, into two, into four, and so on – and eventually becomes a blastocyst. Once the eight-cell level has been reached, cell differentiation begins to emerge. This is where the differentiation between the placenta and the embryo begins (Cherry, 2012). Cell division continues at a rapid pace during the first few week of pregnancy. By the fifth week, the blastocyst has three distinct layers that have separated from the placenta: the ectoderm, the mesoderm, and the endoderm (“Pregnancy week by week,” 2011). The ectoderm is the outermost layer of the blastocyst. This layer will become the baby’s outermost layer of skin, central and peripheral nervous systems, eyes, inner ear, and most of the body’s connective tissues (“Pregnancy week by week,” 2011). The mesoderm, as the name would imply, is the middle layer of cells and will serve as the foundation for what will become the baby’s bones, muscles, kidneys, and most of its reproductive system. The endoderm becomes a tube lined with mucous membranes and will serve as the site of the development of the child’s intestines, lungs, and bladder (“Pregnancy week by week,” 2011). This is the innermost of the three layers of the blastocyst. At this point in the pregnancy, a woman will likely not even be aware that she is pregnant. The immune system could recognize the fertilized egg or blastocyst as a foreign object and reject it from the body, or the zygote could fail to implant in the uterine wall in the first place. Researchers estimate that approximately fifty-eight percent of all natural fertilizations – this means that in vitro fertilizations that are placed into the uterus are not included in this estimate – will be rejected by the woman’s body before the can even implant in the uterus (Cherry, 2012). Once implantation is successful and the aforementioned...
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