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| Biological terms |
| Sexual reproduction |
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Humansexual reproduction, to produceoffspring, begins withfertilization. Successful reproduction typically involvessexual intercourse between a healthy,sexually mature andfertilemale andfemale.[1] During sexual intercourse, sperm cells areejaculated into thevagina through thepenis, resulting in fertilization of anovum to form azygote.[1]
While normal cells contain 46chromosomes (23 pairs),gamete cells contain only half that number, and it is when these two cells merge into one combined zygote cell thatgenetic recombination occurs.[citation needed] The zygote then undergoes a defined development process that is known as humanembryogenesis, and this starts the typical 38-weekgestation period[a] for theembryo (and eventuallyfoetus) that is followed bychildbirth.
Assisted reproductive technology also exists, likeIVF, some of which involve alternative methods of fertilization, which do not involve sexual intercourse; the fertilization of the ovum may be achieved byartificial insemination methods.[2]
In order for human reproduction to be achieved, an individual must have undergonepuberty first, requiringovulation in females and thespermarche in males to have occurred prior to engaging insexual intercourse or achievingpregnancy through non-penetrative means. Before puberty, humans areinfertile, as their genitals lack reproductive function (only being able to dischargeurine).
Legal factors also play a vital role in the achievement of human reproduction: aminor under theage of consent cannot give legal consent to sexual intercourse or artificial alternatives to reproduction, the former case of which is liable to have the older party charged withstatutory rape, depending on jurisdictions.[3][4] Even for minors above the age of consent,comprehensive sex education advises both consenting parties to usecontraception to avoid bothsexually transmitted infections andearly, unplanned/unwanted pregnancies. Pregnancy in girls under the age of 15 is especially discouraged due to their reproductive systems having yet to reach full maturity.[5]
The male reproductive system contains two main divisions: thetesticles wheresperm are produced, and thepenis wheresemen isejaculated through theurethra. In humans, both of these organs are outside theabdominal cavity. Having the testicles outside the abdomen facilitates temperature regulation of the sperm, which require specific temperatures to survive about 2-3 °C less than the normal body temperature i.e. 37 °C. In particular, the extraperitoneal location of the testicles may result in a 2-fold reduction in the heat-induced contribution to the spontaneous mutation rate in male germinal tissues compared to tissues at 37 °C.[6] If the testicles remain too close to the body, it is likely that the increase in temperature will harm the spermatozoa formation, making conception more difficult. This is why the testes are carried in an externalscrotum rather than within the abdomen; they normally remain slightly cooler than body temperature, facilitating sperm production.
Malegerm cells produced in the testes are able to perform specialDNA repair processes during meiosis that act to repair DNA damages and to maintain the integrity of thegenomes that are to be passed on to progeny.[7] Two of these DNA repair processes arehomologous recombinational repair andnon-homologous end joining.[7]
The female reproductive system likewise contains two main divisions: the external genitalia (thevulva) and the internal genitalia.
Theovum meets with thesperm cell: a sperm may penetrate and merge with the egg, fertilizing it with the help of certain hydrolytic enzymes present in the acrosome. The fertilization usually occurs in thefallopian tubes, but can happen in the uterus itself. Thezygote then becomesimplanted in the lining of the uterus, where it begins the processes ofembryogenesis andmorphogenesis. When thefetus is developed enough to survive outside of theuterus, the cervix dilates and contractions of the uterus propel it through the birth canal, which is thevagina, and thereby gives external life to the newborninfant. This process is calledchildbirth.
The ova, which are the female sex cells, are much larger than the spermatozoon and are normally formed within theovaries of the female fetus before birth. They are mostly fixed in location within the ovary until their transit to the uterus, and contain nutrients for the laterzygote andembryo. Over a regular interval known as themenstrual cycle, in response to hormonal signals, a process ofoogenesis matures one ovum which is released and sent down the fallopian tube. If not fertilized, this egg is flushed out of the system throughmenstruation.
Oocytes (female germ cells) located in theprimordial follicle of the ovary are in a non-growingprophase arrested state, but are able to undergo highly efficienthomologous recombinational repair ofDNA damages including double-strand breaks.[8] This capability allows the maintenance ofgenome integrity and protection of the health of offspring.[8]
Human reproduction normally begins with copulation, though it may be achieved throughartificial insemination, and is followed by nine months of pregnancy before childbirth. Pregnancy can be avoided with the use of contraceptives such ascondoms andintrauterine devices.[9]
Human reproduction naturally takes place asinternal fertilization by sexual intercourse. During this process, the man inserts his erectpenis into the woman'svagina and then either partner initiates rhythmicpelvic thrusts until the man achievesorgasm, which leads toejaculation ofsemen containing sperm into the vaginal canal. The sperm and the ovum are known as thegametes (each containing half the genetic information of the parent, created throughmeiosis).[citation needed] The sperm (being one of approximately 250 million sperm in a typical ejaculation) travels through the vagina and cervix into the uterus or fallopian tubes. Only 1 in 14 million of the ejaculated sperm will reach the fallopian tube. The egg simultaneously moves through the fallopian tube away from the ovary. One of the sperm encounters, penetrates and fertilizes the ovum, creating azygote. Uponfertilization andimplantation, gestation of the fetus then occurs within the uterus.[10][11][12][13]
Pregnancy rates for sexual intercourse are highest during themenstrual cycle time from some 5 days before until 1 to 2 days after ovulation.[15] For optimal pregnancy chance, there are recommendations of sexual intercourse every 1 or 2 days,[16] or every 2 or 3 days.[17] Studies have shown no significant difference between differentsex positions and pregnancy rate, as long as it results inejaculation into the vagina.[18]
As an alternative to natural sexual intercourse, there existsartificial insemination, where sperm is introduced into the female reproductive system without the insertion of the penis.[2] There are also many methods ofassisted reproductive technology, such asin vitro fertilization, where one or moreegg cells are retrieved from a woman's ovaries and co-incubated with sperm outside the body. The resulting embryo can then be reinserted into the womb of the woman.[citation needed]
Pregnancy is the period of time during which the fetus develops, dividing viamitosis inside the uterus. During this time, the fetus receives all of itsnutrition and oxygenated blood from the mother, filtered through theplacenta, which is attached to the fetus'abdomen via anumbilical cord. This drain ofnutrients can be quite taxing on the mother, who is required to ingest slightly higher levels ofcalories. In addition, certainvitamins and other nutrients are required in greater quantities than normal, often creating abnormal eating habits. Gestation period is about 266 days in humans. While in the uterus, the baby first endures a very brief zygote stage, then the embryonic stage, which is marked by the development of major organs and lasts for approximately eight weeks, then the fetal stage, which revolves around the development of bone cells while the fetus continues to grow in size.[19] It is estimated that about 3-5% of couples are infertile and the fecunditity of couples is around 30% for each menstrual cycle.[20]
Labor is separated into 4 stages. The first stage involves latent phase and active phase separated by the dilation of the cervix for 6 to 10 cm. The second stage is the pushing stage. The third stage involves the delivery of the placenta. And the last stage is the contraction of the uterus.[21] Once the fetus is sufficiently developed, chemical signals begin the process of birth, which begins with the fetus being pushed out of the birthing canal. The newborn, which is called aninfant in humans, should typically beginrespiration on its own shortly after birth. Not long after, the placenta eventually falls off on its own. The person assisting the birth may also sever the umbilical cord.
While most ancient human societies believed thatsexual intercourse was necessary for reproduction, the reasons some sex did not result in children, and the mechanism by which mating produced children were not understood. The theory ofpreformationism was popular inAncient Greece andChristendom for centuries. Because they are too small to see with the naked eye, it was only after his invention of the microscope thatAntonie van Leeuwenhoek discoveredspermatozoa in 1677.Mitosis andmeiosis were not discovered until the late 1800s.
Voluntary sexual intercourse with a post-pubescent minor who is younger than the legal age of consent is described as statutory rape. [...] In most states, age of consent is delimited between 16 years old and 18 years old. [...] In almost every jurisdiction, prepubescent children may not engage in any sexual contact. [...] Engaging in sexual contact with a prepubescent child is a serious criminal offense and a felony.
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