Wikipedia

Prenatal development

(Redirected fromFetal development)
"Antenatal" redirects here. Not to be confused withAntinatalism.

Prenatal development(fromLatinnatalis'relating to birth') involves thedevelopment of the embryoand of thefetusduring aviviparous animal'sgestation.Prenatal development starts withfertilization,in the germinal stage of embryonic development, and continues in fetal development untilbirth.

Inhumanpregnancy,prenatal development is also calledantenatal development.Thedevelopment of the human embryofollowsfertilization,and continues asfetal development.By the end of the tenth week ofgestational age,theembryohas acquired its basic form and is referred to as afetus.The next period is that of fetal development where many organs become fully developed. This fetal period is described both topically (by organ) and chronologically (by time) with major occurrences being listed by gestational age.

The very early stages ofembryonic developmentare the same in allmammals,but later stages of development, and the length of gestation varies.

Terminology

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In the human:

Stages duringpregnancy.Embryonic development is marked in green. Weeks and months are numberedby gestation.

Different terms are used to describeprenatal development,meaningdevelopment before birth.A term with the same meaning is the "antepartum" (from Latinante"before" andparere"to give birth" ) Sometimes "antepartum" is however used to denote the period between the 24th/26th week ofgestationalage until birth, for example inantepartum hemorrhage.[1][2]

Theperinatal period(from Greekperi,"about, around" and Latinnasci"to be born" ) is "around the time ofbirth".Indeveloped countriesand at facilities where expert neonatal care is available, it is considered from 22 completed weeks (usually about 154 days) ofgestation(the time whenbirth weightis normally 500 g) to 7 completed days after birth.[3]In many of thedeveloping countriesthe starting point of this period is considered 28 completed weeks of gestation (or weight more than 1000 g).[4]

Fertilization

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Main article:Human fertilization
Asperm cellfertilizinganegg cell

Fertilization marks the firstgerminal stageofembryonic development.Whensemenis released into thevagina,thespermatozoatravel through thecervix,along the body of theuterus,and into one of thefallopian tubeswhere fertilization usually takes place in theampulla.A great many sperm cells are released with the possibility of just one managing to adhere to and enter the thickprotective layer surrounding the egg cell(ovum). The first sperm cell to successfully penetrate the egg cell donates itsgenetic material(DNA) to combine with the DNA of the egg cell resulting in a new one-celledzygote.The term "conception" refers variably to either fertilization or to formation of theconceptusafter itsimplantationin the uterus, andthis terminology is controversial.

The zygote will develop into a male if the egg is fertilized by a sperm that carries aY chromosome,or a female if the sperm carries anX chromosome.[5]The Y chromosome contains a gene,SRY,which will switch onandrogenproduction at a later stage leading to the development of amalebody type. In contrast, themitochondrial DNAof the zygote comes entirely from the egg cell.

Development of the embryo

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Main article:Human embryonic development
The initial stages ofhuman embryogenesis

Following fertilization, the embryonic stage of development continues until the end of the 10th week (gestational age) (8th week fertilization age). The first two weeks from fertilization is also referred to as the germinal stage or preembryonic stage.[6]

Thezygotespends the next few days traveling down thefallopian tubedividing several times to form a ball of cells called amorula.Furthercellular divisionis accompanied by the formation of a small cavity between the cells. This stage is called ablastocyst.Up to this point there is no growth in the overall size of the embryo, as it is confined within a glycoprotein shell, known as thezona pellucida.Instead, each division produces successively smaller cells.

Theblastocystreaches theuterusat roughly the fifth day after fertilization. The blastocysthatchesfrom the zona pellucida allowing the blastocyst's outer cell layer oftrophoblaststo come into contact with, and adhere to, theendometrialcells of the uterus. The trophoblasts will eventually give rise to extra-embryonic structures, such as theplacentaand the membranes. The embryo becomes embedded in the endometrium in a process calledimplantation.In most successful pregnancies, the embryo implants 8 to 10 days after ovulation.[7]The embryo, the extra-embryonic membranes, and the placenta are collectively referred to as a conceptus, or the "products of conception".

Rapid growth occurs and the embryo's main features begin to take form. This process is calleddifferentiation,which produces the varied cell types (such as blood cells, kidney cells, and nerve cells). A spontaneous abortion, ormiscarriage,in thefirst trimesterof pregnancy is usually[8]due to major genetic mistakes or abnormalities in the developing embryo. During this critical period the developing embryo is also susceptible to toxic exposures, such as:

Nutrition

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The embryo passes through 3 phases of acquisition of nutrition from the mother:[9]

  1. Absorption phase:Zygote is nourished by cellular cytoplasm and secretions in fallopian tubes and uterine cavity.[10]
  2. Histoplasmic transfer:Afternidationand before establishment ofuteroplacental circulation,embryonic nutrition is derived fromdecidual cellsand maternal blood pools that open up as a result of eroding activity oftrophoblasts.
  3. Hematotrophic phase:After third week of gestation, substances are transported passively viaintervillous space.

Development of the fetus

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Further information:Development in humans

The first ten weeks ofgestational ageis the period of embryogenesis and together with the first three weeks of prenatal development make up thefirst trimesterof pregnancy.

From the 10th week of gestation (8th week of development), the developing embryo is called a fetus. All major structures are formed by this time, but they continue to grow and develop. Because the precursors of the organs are now formed, the fetus is not as sensitive to damage from environmental exposure as the embryo was. Instead, toxic exposure often causes physiological abnormalities or minor congenital malformation.

Development of organ systems

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Development continues throughout the life of the fetus and through into life after birth. Significant changes occur to many systems in the period after birth as theyadapt to life outside the uterus.

Fetal blood

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Hematopoiesisfirst takes place in theyolk sac.The function is transferred to theliverby the 10th week of gestation and to thespleenandbone marrowbeyond that. The total blood volume is about 125 ml/kg of fetal body weight near term.

Red blood cells

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Megaloblastic red blood cells are produced early in development, which become normoblastic near term. Life span of prenatal RBCs is 80 days. Rh antigen appears at about 40 days of gestation.

White blood cells

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The fetus starts producingleukocytesat 2 months gestational age, mainly from thethymusand thespleen.Lymphocytesderived from the thymus are calledT lymphocytes(T cells), whereas those derived frombone marroware calledB lymphocytes(B cells). Both of these populations of lymphocytes have short-lived and long-lived groups. Short-lived T cells usually reside in thymus, bone marrow and spleen; whereas long-lived T cells reside in the blood stream.Plasma cellsare derived from B cells and their life in fetal blood is 0.5 to 2 days.

Glands

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Main article:Development of the endocrine system

Thethyroidis the firstglandto develop in the embryo at the 4th week of gestation.Insulinsecretion in the fetus starts around the 12th week of gestation.

Cognitive development

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Electricalbrain activityis first detected at the end of week 5 of gestation.Synapsesdo not begin to form until week 17.[11]Neural connections between thesensory cortexandthalamusdevelop as early as 24 weeks' gestational age, but the first evidence of their function does not occur until around 30 weeks, when minimalconsciousness,dreaming,and the ability to feel pain emerges.[12]

Initial knowledge of the effects of prenatal experience on later neuropsychological development originates from the Dutch Famine Study, which researched the cognitive development of individuals born after theDutch famine of 1944–45.[13]The first studies focused on the consequences of the famine to cognitive development, including the prevalence of intellectual disability.[14]Such studies predateDavid Barker's hypothesisabout the association between the prenatal environment and the development of chronic conditions later in life.[15]The initial studies found no association between malnourishment and cognitive development,[14]but later studies found associations between malnourishment and increased risk forschizophrenia,[16]antisocial disorders,[17]and affective disorders.[18]

There is evidence that the acquisition of language begins in the prenatal stage. After 26 weeks of gestation, theperipheral auditory systemis already fully formed.[19]Also, most low-frequency sounds (less than 300 Hz) can reach the fetal inner ear in the womb of mammals.[20]Those low-frequency sounds include pitch, rhythm, and phonetic information related to language.[21]Studies have indicated that fetuses react to and recognize differences between sounds.[22]Such ideas are further reinforced by the fact that newborns present a preference for their mother's voice,[23]present behavioral recognition of stories only heard during gestation,[24]and (in monolingual mothers) present preference for their native language.[25]A more recent study withEEGdemonstrated different brain activation in newborns hearing their native language compared to when they were presented with a different language, further supporting the idea that language learning starts while in gestation.[26]

Growth rate

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Further information:Fetal origins hypothesis

The growth rate of a fetus is linear up to 37 weeks of gestation, after which it plateaus.[9]The growth rate of an embryo and infant can be reflected as the weight pergestational age,and is often given as the weight put in relation to what would be expected by the gestational age. A baby born within the normal range of weight for that gestational age is known asappropriate for gestational age(AGA). An abnormally slow growth rate results in the infant beingsmall for gestational age,while an abnormally large growth rate results in the infant beinglarge for gestational age.A slow growth rate andpreterm birthare the two factors that can cause alow birth weight.Low birth weight (below 2000 grams) can slightly increase the likelihood of schizophrenia.[27]

The growth rate can be roughly correlated with thefundal heightof the uterus which can be estimated by abdominal palpation. More exact measurements can be performed withobstetric ultrasonography.

Factors influencing development

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Further information:Alcohol and pregnancyandSmoking and pregnancy

Intrauterine growth restrictionis one of the causes oflow birth weightassociated with over half ofneonatal deaths.[28]

Poverty

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Poverty has been linked to poor prenatal care and has been an influence on prenatal development. Women in poverty are more likely to have children at a younger age, which results in low birth weight. Many of these expecting mothers have little education and are therefore less aware of the risks ofsmoking,drinkingalcohol,anddrug use– other factors that influence the growth rate of a fetus.

Mother's age

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Women between the ages of 16 and 35 have a healthier environment for a fetus than women under 16 or over 35.[29]Women between this age gap are more likely to have fewer complications. Women over 35 are more inclined to have a longer labor period, which could potentially result in death of the mother or fetus. Women under 16 and over 35 have a higher risk of preterm labor (premature baby), and this risk increases for women in poverty, women who take drugs, and women who smoke. Young mothers are more likely to engage in high risk behaviors, such as using alcohol, drugs, or smoking, resulting in negative consequences for the fetus.[30]Premature babies from young mothers are more likely to have neurological defects that will influence their coping capabilities – irritability, trouble sleeping, constant crying for example. There is an increased risk ofDown syndromefor infants born to those aged over 40 years. Young teenaged mothers (younger than 16) and mothers over 35 are more exposed to the risks of miscarriages, premature births, and birth defects.

Drug use

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Main article:Drugs in pregnancy

An estimated 5 percent of fetuses in the United States are exposed to illicit drug use during pregnancy.[31]Maternal drug use occurs when drugs ingested by the pregnant woman are metabolized in the placenta and then transmitted to the fetus. Recent research displays that there is a correlation between fine motor skills and prenatal risk factors such as the use of psychoactive substances and signs of abortion during pregnancy. As well as perinatal risk factors such as gestation time, duration of delivery, birth weight and postnatal risk factors such as constant falls.[32]

Cannabis

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Main article:Cannabis in pregnancy

When usingcannabis,there is a greater risk of birth defects, low birth weight, and a higher rate of death in infants or stillbirths.[33]Drug use will influence extreme irritability, crying, and risk forSIDSonce the fetus is born.[34] Marijuana will slow the fetal growth rate and can result in premature delivery. It can also lead to low birth weight, a shortened gestational period and complications in delivery.[33]Cannabis use during pregnancy was unrelated to risk of perinatal death or need for special care, but, the babies of women who used cannabis at least once per week before and throughout pregnancy were 216g lighter than those of non‐users, had significantly shorter birth lengths and smaller head circumferences.[35]

Opioids

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Opioidsincludingheroinwill cause interrupted fetal development, stillbirths, and can lead to numerous birth defects. Heroin can also result in premature delivery, creates a higher risk of miscarriages, result in facial abnormalities and head size, and create gastrointestinal abnormalities in the fetus. There is an increased risk for SIDS, dysfunction in the central nervous system, and neurological dysfunctions including tremors, sleep problems, and seizures. The fetus is also put at a great risk for low birth weight and respiratory problems.[36]

Cocaine

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Main article:Prenatal cocaine exposure

Cocaine use results in a smaller brain, which results in learning disabilities for the fetus. Cocaine puts the fetus at a higher risk of being stillborn or premature. Cocaine use also results in low birthweight, damage to the central nervous system, and motor dysfunction. The vasoconstriction of the effects of cocaine lead to a decrease in placental blood flow to the fetus that results in fetal hypoxia (oxygen deficiency) and decreased fetal nutrition; these vasoconstrictive effects on the placenta have been linked to the number of complications in malformations that are evident in the newborn.[37]

Methamphetamine

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Prenatal methamphetamine exposure has shown to negatively impact brain development and behavioral functioning. A 2019 study further investigated neurocognitive and neurodevelopmental effects of prenatal methamphetamine exposure. This study had two groups, one containing children who were prenatally exposed to methamphetamine but no other illicit drugs and one containing children who met diagnosis criteria for ADHD but were not prenatally exposed to any illicit substance. Both groups of children completed intelligence measures to compute an IQ. Study results showed that the prenatally exposed children performed lower on the intelligence measures than their non-exposed peers with ADHD. The study results also suggest that prenatal exposure to methamphetamine may negatively impact processing speed as children develop.[38]

Alcohol

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Main article:Fetal alcohol spectrum disorder

Maternal alcohol use leads to disruptions of the fetus' brain development, interferes with the fetus' cell development and organization, and affects the maturation of the central nervous system. Even small amounts of alcohol use can cause lower height, weight and head size at birth and higher aggressiveness and lower intelligence during childhood.[39]Fetal alcohol spectrum disorderis a developmental disorder that is a consequence of heavy alcohol intake by the mother during pregnancy. Children with FASD have a variety of distinctive facial features, heart problems, and cognitive problems such as developmental disabilities, attention difficulties, and memory deficits.[39]

Tobacco use

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Main article:Smoking and pregnancy

Tobacco smokingduring pregnancy exposes the fetus to nicotine, tar, andcarbon monoxide.Nicotine results in less blood flow to the fetus because it constricts the blood vessels. Carbon monoxide reduces the oxygen flow to the fetus. The reduction of blood and oxygen flow may result in miscarriage, stillbirth, low birth weight, and premature births.[40]Exposure to secondhand smoke leads to higher risks of low birth weight and childhood cancer.[41]

Infections

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If a mother isinfectedwith a disease, the placenta cannot always filter out thepathogens.Virusessuch asrubella,chicken pox,mumps,herpes,andhuman immunodeficiency virus(HIV) are associated with an increased risk ofmiscarriage,low birth weight,prematurity,physical malformations,and intellectual disabilities.[42]HIV can lead toacquired immune deficiency syndrome(AIDS). Untreated HIV carries a risk of between 10 and 20 per cent of beingpassed on to the fetus.[43]Bacterial or parasitic diseases may also be passed on to the fetus, and includechlamydia,syphilis,tuberculosis,malaria,and commonlytoxoplasmosis.[44]Toxoplasmosis can be acquired through eating infected undercooked meat or contaminated food, and by drinking contaminated water.[45]The risk of fetal infection is lowest during early pregnancy, and highest during the third trimester. However, in early pregnancy the outcome is worse, and can be fatal.[45]

Maternal nutrition

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Adequate nutrition is needed for a healthy fetus. Mothers who gain less than 20 pounds during pregnancy are at increased risk for having a preterm or low birth weight infant.[46]Iron and iodine are especially important during prenatal development. Mothers who are deficient in iron are at risk for having a preterm or low birth weight infant.[47]Iodine deficiencies increase the risk of miscarriage, stillbirth, and fetal brain abnormalities. Adequateprenatal caregives an improved result in thenewborn.[48]

Low birth weight

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Low birth weightincreases an infants risk of long-term growth and cognitive and language deficits. It also results in a shortened gestational period and can lead to prenatal complications.

Stress

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Stress during pregnancycan have an impact the development of the embryo. Reilly (2017) states that stress can come from many forms of life events such as community, family, financial issues, and natural causes. While a woman is pregnant, stress from outside sources can take a toll on the growth in the womb that may affect the child's learning and relationships when born. For instance, they may have behavioral problems and might be antisocial. The stress that the mother experiences affects the fetus and the fetus' growth which can include the fetus' nervous system (Reilly, 2017). Stress can also lead to low birth weight. Even after avoiding other factors like alcohol, drugs, and being healthy, stress can have its impacts whether families know it or not. Many women who deal with maternal stress do not seek treatment. Similar to stress, Reilly stated that in recent studies, researchers have found that pregnant women who show depressive symptoms are not as attached and bonded to their child while it is in the womb (2017).[49]

Environmental toxins

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Main article:Environmental toxicants and fetal development

Exposure toenvironmental toxins in pregnancylead to higher rates of miscarriage, sterility, and birth defects. Toxins include fetal exposure to lead, mercury, and ethanol or hazardous environments. Prenatal exposure to mercury may lead to physical deformation, difficulty in chewing and swallowing, and poor motoric coordination.[50]Exposure to high levels of lead prenatally is related to prematurity, low birth weight, brain damage, and a variety of physical defects.[50]Exposure to persistentair pollutionfrom traffic andsmogmay lead to reduced infant head size, low birth weight, increased infant death rates, impaired lung and immune system development.[51]

See also

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References

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