In humans, pregnancy takes approximately 40 weeks between the time of the last menstrual cycle and delivery. It is divided into three trimesters, each is 3 months long. The first trimester carries the highest risk of miscarriage, meaning the unintentional abortion of a fetus. It is often a result of defects in the fetus, its parent, or damage caused after conception.
The first step of pregnancy begins with sexual intercourse (or alternatively, IVF, i.e. in vitro fertilisation), where totipotent haploid male gametes travel into the vagina. These cells can survive for about 48 hours, in their semen which consists of mostly sugars, proteins and other fluids. They have a long flagella, which they use to navigate and are the only human cell with this property. The cells only have one copy of the chromosomes, making them haploid and they divide by meiosis in the male testes. Typically, between 100 million and 300 million of these cells are released at one orgasm.
Oocytes are the female cells responsible for pregnancy, and their role is to become the first cell of the baby and to allow only one sperm to penetrate into it. These cells begin in the ovaries where they are produced by meiosis, and are also totipotent haploid cells. The move through the fallopian tube where they are to come into contact with the sperm. The sperm navigate from the vaginal opening through the uterus and into the fallopian tube. Once there, the sperm are swim towards the oocyte cell and attempt to impregnante it. The first step is to penetrate the zona pellucida which is a tough membrane enveloping the oocyte. It must bind with an enzyme receptor on the surface, to be allowed to pierce this cell. This takes approximately twenty minutes to occur. Oocyte and sperm nuclei fuse as the sperm enters the nucleus of the cell, completing the first stage of pregnancy.
(see also Trimester)
At this point, there exists a single cell with the potential to create an entire human being, the fertilization can be said to be complete. Mitotic cell division is the next process to occur, whereby each cell doubles to produce another diploid cell. Each cell divides to produce two smaller cells called blastomeres, and this occurs 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 uterus.
Three to four days after fertilization, about 4 cell divisions have taken place in 80 hours and the fetus is ready to grow further. However, it cannot continue growing in the fallopian tube because this could be fatal for the mother. The term for a morula which attempts to grow further in the fallopian tube is known as an Ectopic Pregnancy. At this point, doctors will usually try to abort the birth to save the mothers life. Cell divison continues on the journey to the uterus, and the developing cells will now develop a blastocele.
A blostocele 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 a 24 to 48 hours from now, 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 will secrete a hormone called Human Chorionic Gonadotropic which in turn, will stimulate the mother to produce progesteron. The glands in the mucus 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, and this stage is required for the continuation of pregnancy.
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 divide 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 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.
Following this, a narrow line of cells will appear on the surface on the embryo. Its growth shows the baby 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 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 be the heart of the embryro.
The embryo over the past few days has developed many of its internal organs and systems, as well as developing from the pear-shaped growth of cells to a humanoid-like shape. 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.