Mammalian pregnancy is the period during which a mammalian female carries one or various live offspring from implantation until they develop to the point where the offspring are capable of living outside the uterus. It starts after a fertilized zygote implants in the female's uterus and ends in birth, miscarriage or abortion. A pregnant woman Pregnancy is the carrying of one or more embryos or feti by female mammals or by humans inside their bodies. ... Orders Subclass Monotremata Monotremata Subclass Marsupialia Didelphimorphia Paucituberculata Microbiotheria Dasyuromorphia Peramelemorphia Notoryctemorphia Diprotodontia Subclass Placentalia Xenarthra Dermoptera Desmostylia Scandentia Primates Rodentia Lagomorpha Insectivora Chiroptera Pholidota Carnivora Perissodactyla Artiodactyla Cetacea Afrosoricida Macroscelidea Tubulidentata Hyracoidea Proboscidea Sirenia The mammals are the class of vertebrate animals characterized by the presence of mammary glands... Female symbol Female is the sex of an organism, or a part of an organism, which produces egg cells. ... Implantation occurs when a fertilized zygote attaches itself onto the lining of the uterus. ... Female internal reproductive anatomy The uterus or womb is the major female reproductive organ of most mammals, including humans. ... A sperm attempting to fertilize an egg Fertilisation (also known as fertilization and syngamy) is fusion of gametes to form a new organism. ... A zygote (Greek: ζυγωτόν) is a cell that is the result of fertilization. ... Birth is the process in mammals by which a fetus is expelled from the body of its mother. ... Miscarriage is the common term for the natural or accidental termination of a pregnancy at a stage where the embryo or the fetus is incapable of surviving. ...

Fertilization and implantation

A male and female copulate, the male inseminating the female. The semen fertilizes an ovum or various ova in the uterus or fallopian tubes, and this results in one or multiple zygotes. Sometimes, a zygote can be created by humans outside of the animal's body in the artificial process of in-vitro fertilization. After fertilization, the newly formed zygote then begins to divide through mitosis, forming an embryo, which implants in the female's endometrium. At this time, the embryo usually consists of 16 cells. Male symbol Male is the sex of an organism, or a part of an organism, which produces sperm. ... Female symbol Female is the sex of an organism, or a part of an organism, which produces egg cells. ... A pair of lions copulating in the Maasai Mara, Kenya. ... Insemination is the introduction of semen into the genital tract of a female. ... Semen or sperm is a fluid that contains spermatozoa. ... A sperm attempting to fertilize an egg Fertilisation (also known as fertilization and syngamy) is fusion of gametes to form a new organism. ... A human ovum An ovum (also oocyte, ovocyte, or loosely, egg or egg cell) is a female sex cell or gamete. ... Female internal reproductive anatomy The uterus or womb is the major female reproductive organ of most mammals, including humans. ... Female internal reproductive anatomy The Fallopian tubes or oviducts are two very fine tubes leading from the ovaries of female mammals into the uterus. ... A zygote (Greek: ζυγωτόν) is a cell that is the result of fertilization. ... In vitro fertilization (IVF) is a technique in which egg cells are fertilized outside the mothers body in cases where conception is difficult or impossible through normal intercourse. ... Light micrograph of a newt lung cell in early anaphase of mitosis. ... Embryos (and one tadpole) of the wrinkled frog (Rana rugosa you beezie). ... The endometrium is the uterine membrane in mammals which is thickened in preparation for the implantation, of a fertilized egg upon its arrival into the uterus. ...

Development

After implantation

A blastocele is a small cavity on the center of the embryo, and the developing embryonary cells will grow around it. Then, a flat layer cells forms on the exterior of this cavity, and the zona pellucida, the blastocyst's barrier, remains the same size as before. Cells grow 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. A blastocoel(e) or blastocele or cleavage cavity or segmentation cavity is the central region of a blastula (or blastosphere). ... The zona pellucida is a glycoprotein matrix surrounding the plasma membrane of a mammalian oocyte. ... Embryogenesis is the process of cell division and cellular differentiation which leads to the development of an embryo. ...

The presence of the blastocyst means that two types of cells are forming, an inner-cell mass growing on the interior of the blastocele and cells growing on the exterior of it. In 24 to 48 hours, 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. In zootomy, epithelium is a tissue composed of a layer of cells. ...

Placental circulation system

The cells surrounding the blastocyst now destroy cells in the uterine lining, forming small pools of blood, which in turn 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 becomes the embryo, and cells from there occupy the amniotic cavity. At the same time, the bottom layer forms a small sac. (If the cells begin developing in an abnormal position, an ectopic gestation may also occur at this point.) Human placenta shown a few minutes after birth. ...

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 develops between the developing placenta and the growing fetus. This later develops into the umbilical cord. The arterial system The blood vessels are part of the circulatory system and function to transport blood throughout the body. ... Human red blood cells Red blood cells are the most common type of blood cell and are the vertebrate bodys principal means of delivering oxygen to body tissues via the blood. ... Connective tissue is any type of biological tissue with an extensive extracellular matrix and often serves to support, bind together, and protect organs. ... Fetus at eight weeks A fetus (alternatively foetus or fœtus) is an unborn vertebrate offspring after the embryonary stage. ... Newborn at 45 seconds. ...

Cellular differentiation

Following this, a narrow line of cells appears on the surface on the embryo. Its growth makes the fetus undergo gastrulation, in which the three layers of the fetus, the ectoderm, mesoderm and endoderm, 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 give rise to all the various types of tissue in the body. 1 - blastula, 2 - gastrula; orange - ectoderm, red - entoderm. ... The ectoderm is outermost of the three germ layers of the developing embryo, the other two being the mesoderm and the endoderm. ... Mesoderm is one of the three germ layers in the early developing embryo, the other two layers being the ectoderm and the endoderm. ... The endoderm is one of the three germ layers of the developing embryo, the other two being the ectoderm and the mesoderm. ...

The endoderm later forms the lining of the tongue, digestive tract, lungs, bladder and several glands. The mesoderm forms muscle, bone and lymph tissue, as well as the interior of the lungs, heart, reproductive and excretory systems. It also gives rise to the spleen, and produces blood cells. The ectoderm forms 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. Many animals have longer and more flexible tongues than humans. ... For the Physics term GUT, please refer to Grand unification theory The gastrointestinal or digestive tract, also referred to as the GI tract or the alimentary canal or the gut, is the system of organs within multicellular animals which takes in food, digests it to extract energy and nutrients, and... The heart with relation to the lungs (from an older edition of Grays Anatomy) This x-ray of the human chest shows the lungs as dark regions The lung is an organ belonging to the respiratory system and interfacing to the circulatory system of air-breathing vertebrates. ... A bladder is a pouch or other flexible enclosure with waterproof or gasproof walls. ... A gland is an organ in an animals body that synthesizes a substance for release such as hormones, often into the bloodstream (endocrine gland) or into cavities inside the body or its outer surface (exocrine gland). ... A top-down view of skeletal muscle Muscle is a contractile form of tissue. ... Grays illustration of a human femur, a typically recognized bone. ... In mammals including humans, the lymphatic vessels (or lymphatics) are a network of thin tubes that branch, like blood vessels, into tissues throughout the body. ... The heart and lungs (from an older edition of Grays Anatomy) The heart (Latin cor) is a hollow, muscular organ that pumps blood through the blood vessels by repeated, rhythmic contractions. ... A sex organ, or primary sexual characteristic, narrowly defined, is any of those parts of the body (which are not always bodily organs according to the strict definition) which are involved in sexual reproduction and constitute the reproductive system in an complex organism; namely: Male: penis (notably the glans penis... The excretory system is the system of an organisms body that performs the function of excretion. ... The spleen is a ductless, vertebrate gland that is not necessary for life but is closely associated with the circulatory system, where it functions in the destruction of old red blood cells and removal of other debris from the bloodstream, and also in holding a reservoir of blood. ... Model of the layers of human skin In zootomy and dermatology, skin is an organ of the integumentary system; which is composed of a layer of tissues that protect underlying muscles and organs. ... Nail or nails can mean Look up nail in Wiktionary, the free dictionary Nail (anatomy) Nail (engineering) The Nails, a band The Flash animation collection NAILS. This is a disambiguation page — a navigational aid which lists pages that might otherwise share the same title. ... A female with brown hair. ... The cornea is the curved, transparent layer that covers the front part of the eye and protects its inner structures. ... A left human ear. ... Human nose Anatomically, a nose is a protuberance in vertebrates that houses the nostrils, or nares, which admit and expel air for respiration. ... A sinus is a pouch or cavity in any organ or tissue, or an abnormal cavity or passage caused by the destruction of tissue. ... Sagittal section of nose mouth, pharynx, and larynx. ... Male Anatomy In anatomy, the anus is the external opening of the rectum. ... Types of teeth Molars are used for grinding up foods Carnassials are used for slicing food. ... Located at the base of the skull, the pituitary gland is protected by a bony structure called the sella turcica. ... Mammary glands are milk-secreting adaptations of sweat glands and are the characteristic of mammals which gave the class its name. ... An eye is an organ that detects light. ... The nervous system of an animal coordinates the activity of the muscles, monitors the organs, constructs and processes input from the senses, and initiates actions. ...

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 organic systems 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 begin to form on the sac in the center of the embryo, as well as cells which begin to differentiate into blood vessels. Endocardial cells begin to form the myocardium. Cellular differentiation is a concept from developmental biology describing the process by which cells acquire a type. The morphology of a cell may change dramatically during differentiation, but the genetic material remains the same, with few exceptions. ... In the heart, the endocardium is the innermost layer of cells, embryologically and biologically similar to the endothelium that lines blood vessels. ... Myocardium is the muscular tissue of 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.