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 with conception, the process of fertilization to form a zygote, and ends in childbirth, miscarriage, or abortion.
Human pregnancy takes approximately 40 weeks between the time of the last menstrual cycle and birth(38 weeks from fertilization). It is divided into three trimesters. 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.
The first step of pregnancy usually begins with sexual intercourse where male gametes or sperm 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 flagellum, which they use to swim; they are the only human cells with this property. These cells are haploid, having divided by meiosis from germ cells in the testes and possessing only one half of the chromosomes of ordinary body cells. Typically, between 100 million and 300 million sperm are released in one ejaculation.
Ova, or oocytes, are the haploid female egg cells, and their role is to fuse with one sperm cell to form a fertilized zygote . These cells are produced by meiosis in the ovaries and stay in a state of suspended animation until activated by hormonal changes in the woman's menstrual cycle. Typically, only one ovum 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 ovum. If fertilization takes place, the sperm usually meet the ovum in the fallopian tube, requiring the sperm cells to swim from the upper vagina through the cervix and across the length of the uterus before reaching the fallopian tube—a considerable distance compared to the size of the sperm cell.
Once there, the successful sperm swim towards the ovum and each attempts to fertilize it. Each sperm cell contains a small pouch of enzymes that it uses to break through the outer layer of the ovum in order to fuse with it. This can take up to twenty minutes. Once the ovum fuses with a single sperm cell, its cell membrane changes, preventing fusion with other sperm. The fusion of the ovum's and sperm's nuclei to form a diploid cell completes the first stage of pregnancy.
Alternative methods of fertilization, including artificial insemination and in vitro fertilization, are sometimes used in cases of infertility.
At this point, there exists a single totipotent cell, a zygote, a genetically unique new organism. Mitotic cell division is the next process to occur, where each cell divides 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, called a morula,usually leaves the fallopian tube and makes its way to the uterus. see ectopic pregnancy
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.
Placental circulation system
The cells surrounding the blastocyst now destroy cells in the uterine lining, forming small pools of blood which in turn stimulates 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 now develops at the point of the newly forming placenta, growing near the implantation site. The small sac inside the blastocyst begins producing red blood cells. 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 appears on the surface on the embryo. Its growth shows the fetus will undergo gastrulation, in which the three layers of the fetus, the ectoderm, mesoderm and endoderm, will develop. The narrow line of cells begin to form the endoderm and mesoderm. The ectoderm begins to grow rapidly as a result of chemicals being produced by the mesoderm. These three layers will give rise to all the various types of tissue in the body.
The endoderm will later form the lining of the tongue, digestive tract, 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 systems. 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, teeth, pituitary gland, mammary glands, eyes and all parts of the nervous system.
Approximately 18 days after fertilization, the embryo has divided to form much of the tissue it will need. It is shaped like a pear, where the head region is larger than the tail. The embryo's nervous system is one of the first organs to grow. It begins growing in a concave area known as the neural groove.
The blood system continues to grow networks which allow the blood to flow around the embryo. Blood cells are already being 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.
At about 24 days past fertilization, there is a primitive S-shaped tubule heart which begins beating. The flow of fluids throughout the embryo will begin at this stage.