Pregnancy is the process by which a mammalian female carries a live offspring from conception until it develops to the point where the offspring is capable of living outside the womb. It starts from the fertilization of an embryo (conception) to form a zygote, and ends in birth, miscarriage or abortion. In humans, pregnancy takes approximately 40 weeks between the time of the last menstrual cycle and delivery. It is divided into three trimesters of three months duration each. The first trimester carries the highest risk of miscarriage, the unintentional abortion of a fetus. It is often a result of defects in the fetus, its parent, or damage caused after conception. Fertilization The first step of pregnancy usually begins with sexual intercourse where haploid male gametes in the form of sperm cells are deposited into the vagina. The semen produced by the male contains not only sperm cells but also sugars, proteins and other substances to help keep the sperm viable - human sperm generally survive for about 48 hours in the female body. Sperm have a long flagella, which they use to swim and are the only human cell with this property. These cells, having divided by meiosis from germ cells in the male testes, only have one half copy of the chromosomes, making them haploid. Typically, between 100 million and 300 million of these cells are released at one orgasm. Oocytes are the female egg cells, and their role is to fuse with one sperm cell to form a fertilized zygote which will then grow in the uterus to form a developing fetus. These cells are produced by meiosis in the ovaries and stay in a state of suspended animation until they become activated by hormonal changes in the woman's menstrual cycle. Typically, only one egg cells is released during each menstrual cycle. At ovulation, the fimbriae at the end of the fallopian tube move over the ovary to catch the released egg. If fertilization is to take place, the sperm usually meet with the egg in the fallopian tube. Note that the sperm cell would have had to swim from the upper vagina through the cervix and across the length of the uterus before reaching the fallopian tube (quite a task considering how small a sperm cell is). Once there, the successful sperm swim towards the oocyte and each attempt to fertilize it. Each sperm cell contains a small pouch of enzymes that it uses to break through the outer layer of the oocyte in order to fuse with it. This can take up to twenty minutes. Once the oocyte fuses with a single sperm cell, the cell membrane of the oocyte changes and no other sperm are allowed into the oocyte. The fusion of the oocyte and sperm nuclei completes the first stage of pregnancy. In vitro fertilisation (IVF), an artificial means of fertilization where sperm and egg cells are mixed in a test tube, has, in recent years, become an alternative to the traditional way of fertilization for infertile couples. Pre-implantation At this point, there exists a single totipotent cell with the potential to create an entire human being. Mitotic cell division is the next process to occur, where each cell doubles to produce another diploid cell. The zygote divides to produce two smaller cells called blastomeres roughly every 20 hours. These cells get progressively smaller until about 4 divisions have taken place resulting in 16 individual cells. This cluster of 16 cells is known as a morula and will leave the fallopian tube and make its way to the uterus. Ectopic pregnancy If the developing bunch of cells implants anywhere other than the uterus, it is an ectopic pregnancy. The most common location for ectopic pregnancies is in one of the fallopian tubes although ectopic pregnancies can potentially arise in any tissue that the fertilized zygote can get to - this potentially includes anything within the abdomen. When the zygote implants itself in an ectopic position, the tissue it burrows into is not suited to the development of the embryo. This usually results in complications when the developing embryo grows too big for the poor blood supply of the ectopic location to sustain it any more. When this happens, the ectopic pregnancy is at risk of rupture which can lead to life threatening blood loss. An ectopic pregnancy can be a life-threatening condition. Post-implantation A blastocele is a small cavity on the center of the cells, and the developing cells will grow around this. There will be a flat layer of cells on the exterior of this cavity, and the zona pellucida will remain the same size as before. Cells are growing increasingly smaller to fit in. This new structure with a cavity in the center and the developing cells around it is known as a blastocyst. The presence of the blastocyst means that two types of cells are forming, inner cell mass growing on the interior of the blastocele and cells growing on the exterior of it. In 24 to 48 hours, the blastocyst's barrier, the zona pellucida breaches. The cells on the exterior of the blastocyst begin excreting an enzyme which erodes epithelial uterine lining and creates a site for implantation. The blastocyst also secretes a hormone called human chorionic gonadotropin which in turn, stimulates the corpus luteum in the mother's ovary to continue producing progesterone, which acts to maintain the lining of the uterus so that the embryo will continue to be nourished. The glands in the lining of the uterus will swell in response to the blastocyst, and capillaries will be stimulated to grow in that region. This allows the blastocyst to receive vital nutrients from the mother. The hormone human chorionic gonadotropin is the substance that pregnancy tests test for the presence of. Placental Circulation System The cells surounding the blastocyst will now destroy cells in the uterine lining forming small pools of blood, which in turn will stimulate the production of capillaries. This is the first stage in the growth of the placenta. The inner cell mass of the blastocyst divides rapidly, forming two layers. The top layer will become the embryo and cells from there will be used in the amniotic cavity. At the same time, the bottom layer will form a small sac. If the cells begin developing in an abnormal position, an ectopic pregnancy may also occur at this point. Several days later, chorionic villi in the forming placenta anchor the implantation site to the uterus. A system of blood and blood vessels will now develop at the point of new-forming placenta, growing near the implantation site. The small sac inside the blastocyst will begin producing its blood cells. These are small cells with no nuclei, known as hematopoietic. For the next 24 hours, connective tissue will develop between the developing placenta and the growing fetus. This will later develop into the umbilical cord. Cellular Differentiation Following this, a narrow line of cells will appear on the surface on the embryo. Its growth shows the fetus will undergo gastrulation, which is a process where the three layers of the baby will develop. The ectoderm, mesoderm and endoderm are the layers which develop. The narrow line of cells begin to form the audoderm and mesoderm. The ectoderm begins to grow rapidly as a result of chemicals being produced by the mesoderm. At this point, the three layers that surround the embryo, the ectoderm, mesoderm and endoderm will give rise to all the various types of tissue in the body. The endoderm will later form the lining of the tongue, digestive track, lungs, bladder and several glands. The mesoderm will form muscle, bone and lymph tissue, as well as the interior of the lungs, heart, reproductive and excretory system. It will also give rise to the spleen, and will be used in the production of blood cells. The ectoderm will form the skin, nails, hair, cornea, lining of the internal and external ear, nose, sinuses, mouth, anus, tooth enamel, pituitary gland, mammary glands, and all parts of the nervous system. Approximately 18 days after fertilisation, the embryo will have divided to form much of the tissue it will need. It will be shaped like a pear, where the head region is larger than the tail. The embryos nervous system is one of the first organs to grow. This system begins growing in a concave area known as the neural groove. Also, the blood system is continuing to grow networks which allow the blood to flow around the embryo. Blood cells are already produced and are flowing through these developing networks. Secondary blood vessels also begin to develop around the placenta, to supply it with more nutrients. Blood cells will begin to form on the sac in the center of the embryo, as well as cells which will begin to differentiate into blood vessels. Endocardial cells begin to form the musculature which will become the heart of the embryro. At about 24 days past fertilisation, there is a primative S-shaped tubule heart which begins beating. The flow of fluids travelling throughout the embryo will begin at this stage. Pregnancy in science fiction There are numerous science fiction, utopian and dystopian novels revolving around sexual reproduction, pregnancy and infertility. Some examples: * Margaret Atwood: The Handmaid's Tale (1985) * Anthony Burgess: The Wanting Seed (1962) * Charlotte Perkins Gilman: Herland (1915) (parthenogenesis in an all-female society) * P.D. James: The Children of Men (1992) (universal infertility) * Marge Piercy: Woman on the Edge of Time (1976) (fetuses raised externally in breeders rather than in the female womb) * John Wyndham: The Midwich Cuckoos (1957)
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