Editing Development of the human body (section)

==Before birth==
{{Main|Prenatal development}}
Development before birth, or '''prenatal development''' ({{ety|la|natalis|relating to birth}}) is the process in which a [[zygote]], and later an [[embryo]], and then a [[fetus]] develops during [[gestation]]. [[Prenatal development]] starts with [[human fertilization|fertilization]] and the formation of the [[zygote]], the first stage in [[Human embryonic development|embryonic development]] which continues in fetal development until [[childbirth|birth]].

===Fertilization===
{{Main|Human fertilization}}
[[Image:Sperm-egg.jpg|right|thumb|Sperm fertilizing an egg]] 
Fertilization occurs when the sperm successfully enters the ovum's membrane. The [[chromosome]]s of the sperm are passed into the egg to form a unique genome. The egg becomes a [[zygote]] and the germinal stage of embryonic development begins. The germinal stage refers to the time from fertilization, through the development of the early embryo, up until implantation. The germinal stage is over at about 10 days of gestation.<ref name="germinal stage">{{cite web|title=Germinal stage|url=http://medical-dictionary.thefreedictionary.com/germinal+stage|work=Mosby's Medical Dictionary, 8th edition.|publisher=Elsevier.|access-date=6 October 2013|archive-date=28 April 2021|archive-url=https://web.archive.org/web/20210428005411/http://medical-dictionary.thefreedictionary.com/germinal+stage|url-status=live}}</ref>

The [[zygote]] contains a full complement of [[genetic material]] with all the biological characteristics of a single human being, and develops into the embryo. Embryonic development has four stages: the [[morula]] stage, the [[blastula]] stage, the [[gastrula]] stage, and the [[neurula]] stage. Prior to implantation, the [[embryo]] remains in a protein shell, the [[zona pellucida]], and undergoes a series of rapid mitotic cell divisions called [[cleavage (embryo)|cleavage]].<ref name="TortoraG">{{cite book |last1=Tortora |first1=G |last2=Derrickson |first2=B |title=Principles of anatomy & physiology |year=2012 |url=https://archive.org/details/principlesanatom00tort_737 |url-access=limited |publisher=Wiley |isbn=978-0470-91777-0 |page=[https://archive.org/details/principlesanatom00tort_737/page/n1218 1183]|edition=13th.}}</ref> A week after fertilization the embryo still has not grown in size, but hatches from the zona pellucida and adheres to the lining of the mother's [[uterus]]. This induces a [[decidual reaction]], wherein the uterine cells proliferate and surround the embryo thus causing it to become embedded within the uterine tissue. The embryo, meanwhile, proliferates and develops both into embryonic and extra-embryonic tissue, the latter forming the [[fetal membranes]] and the [[placenta]].  In humans, the embryo is referred to as a [[fetus]] in the later stages of prenatal development.  The transition from embryo to fetus is arbitrarily defined as occurring 8 weeks after fertilization. In comparison to the embryo, the fetus has more recognizable external features and a set of progressively developing internal organs. A nearly identical process occurs in other species.

===Embryonic development===
{{Main|Human embryonic development}}
[[Human embryonic development]] refers to the development and formation of the human [[embryo]]. It is characterised by the process of [[cell division]] and [[cellular differentiation]] of the [[embryo]] that occurs during the early stages of [[Human development (biology)|development]]. In biological terms, human development entails growth from a one-celled [[zygote]] to an adult [[human being]]. [[Fertilisation|Fertilization]] occurs when the [[sperm]] cell successfully enters and fuses with an egg cell ([[ovum]]).<ref name="germinal stage"/> The genetic material of the sperm and egg then combine to form a single cell called a zygote and the germinal stage of [[prenatal development]] commences. The embryonic stage covers the first eight weeks of development; at the beginning of the ninth week the embryo is termed a [[fetus]].

The germinal stage refers to the time from fertilization through the development of the early embryo until [[implantation (human embryo)|implantation]] is completed in the [[uterus]]. The germinal stage takes around 10 days.<ref name="germinal stage"/> During this stage, the zygote begins to divide, in a process called [[cleavage (embryo)|cleavage]]. A [[blastocyst]] is then formed and implanted in the [[uterus]]. Embryonic development continues with the next stage of [[gastrulation]], when the three [[germ layers]] of the embryo form in a process called [[histogenesis]], and the processes of [[neurulation]] and [[organogenesis]] follow.

In comparison to the embryo, the fetus has more recognizable external features and a more complete set of developing organs. The entire process of embryonic development involves coordinated spatial and temporal changes in [[gene expression]], [[cell growth]] and [[cellular differentiation]]. A nearly identical process occurs in other species, especially among [[chordate]]s.

===Fetal development===
{{Main|Fetus}}
A '''fetus''' is a stage in the human development considered to begin nine weeks after fertilization.<ref name="nursing">Klossner, N. Jayne, [https://books.google.com/books?id=B47OVg25g-QC&dq=fetal+stage+begins&pg=PA103 Introductory Maternity Nursing] (2005): "The fetal stage is from the beginning of the 9th week after fertilization and continues until birth"</ref><ref>{{cite web|url=http://www.americanpregnancy.org/duringpregnancy/fetaldevelopment1.htm|title=First Trimester |date=1 May 2012|website=American Pregnancy Association|url-status=live|archive-url=https://web.archive.org/web/20090423022520/http://www.americanpregnancy.org/duringpregnancy/fetaldevelopment1.htm|archive-date=23 April 2009}}</ref> In biological terms, however, prenatal development is a continuum, with many defining features distinguishing an embryo from a fetus. A fetus is also characterized by the presence of all the major body organs, though they will not yet be fully developed and functional and some not yet situated in their final location.
[[Image:Prenatal development table.svg|thumb|center|upright=3.5|Stages in [[prenatal development]]]]

===Maternal influences===
{{See also|Placenta}}
The fetus and embryo develop within the [[uterus]], an organ that sits within the pelvis of the [[mother]]. The process the mother experiences whilst carrying the fetus or embryo is referred to as [[pregnancy]]. The [[placenta]] connects the developing [[fetus]] to the [[Uterus|uterine]] wall to allow nutrient uptake, thermo-regulation, waste elimination, and gas exchange via the mother's blood supply; to fight against internal infection; and to produce hormones which support pregnancy. The placenta provides oxygen and nutrients to growing fetuses and removes waste products from the fetus' blood. The placenta attaches to the wall of the uterus, and the fetus' [[umbilical cord]] develops from the placenta. These organs connect the mother and the fetus.  Placentas are a defining characteristic of [[placental mammal]]s, but are also found in [[marsupials]] and some non-mammals with varying levels of development.<ref name="Pou92">Pough ''et al.'' 1992. Herpetology: Third Edition. Pearson Prentice Hall:Pearson Education, Inc., 2002.</ref> The [[Homology (biology)|homology]] of such structures in various [[Viviparity|viviparous organisms]] is debatable, and in invertebrates such as [[Arthropod]]a, is [[Analogy (biology)|analogous]] at best.