View source for Prenatal development

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==Overview==
'''Prenatal development''' is the process in which an [[human embryo|embryo]] or [[human fetus|fetus]] (or ''foetus'') [[gestation|gestates]] during [[pregnancy]], from [[human fertilisation|fertilization]] until [[birth]].  Often, the terms '''fetal development''', '''foetal development''', or '''[[embryology]]''' are used in a similar sense.

After fertilization the embryogenesis starts. In humans, when embryogenesis finishes, by the end of the 10th week of [[gestational age]], the precursors of all the major organs of the body have been created. Therefore, the following period, the fetal period, is described both topically on one hand, i.e. by organ, and strictly chronologically on the other, by a list of major occurrences by weeks of gestational age.  

==Fertilization==
{{main|Human fertilization}} [[Image:Sperm-egg.jpg|thumb|230px|left|A sperm fertilizing an ovum]]
When [[semen]] is deposited in the [[vagina]], the [[spermatozoa]] travel through the [[cervix]] and body of the uterus and into the [[Fallopian tube]]s. [[Fertilization]] of the [[ovum]] (egg [[cell (biology)|cell]]) usually takes place in the Fallopian tube.  Many sperm must cooperate to penetrate the thick protective shell-like barrier that surrounds the ovum.  The first sperm that penetrates fully into the egg donates its [[genetic material]] ([[DNA]]). The resulting combination is called a [[zygote]].  The term "conception" refers variably to either fertilization or to formation of the [[conceptus]] after uterine implantation, and [[beginning_of_pregnancy_controversy#Beginning_of_the_controversy|this terminology is controversial]].

Like every cell in the body, the zygote contains all of the genetic information unique to an individual. Half of the genetic information residing in the zygote's [[Cell nucleus|nucleus]] comes from the mother's egg nucleus, and the other half from the nucleus of a single [[spermatozoon|sperm]]. However, the [[mitochondria]]l genetic information of the zygote is in its totality contributed by the mother's egg.

==Embryonic period==
{{Main|Human embryogenesis}}

The embryonic period in humans begins at [[fertilization]] (2nd week of gestation) and continues until the end of the 10th week of gestation (8th week of development).  

The zygote spends the next few days traveling down the Fallopian tube.  Meanwhile it divides several times to form a ball of cells called a [[morula]]. Further [[cellular division]] is accompanied by the formation of a small cavity between the cells. This stage is called a [[blastocyst]].  Up to this point there is no growth in the overall size of the [[embryo]], so each division produces successively smaller cells.

The [[blastocyst]] reaches the [[uterus]] at roughly the fifth day after fertilization. It is here that lysis of the [[zona pellucida]], a glycoprotein shell, occurs. This is required so that the trophectoderm cells of the blastocyst can come into contact with the luminal epithelial cells of the endometrium. (Contrast this with [[zona hatching]], an event that occurs ''[[in vitro]]'' by a different mechanism, but with a similar result).  It then adheres to the uterine lining and becomes embedded in the [[endometrium|endometrial]] cell layer. This process is also called [[implantation]]. In most successful pregnancies, the conceptus implants 8 to 10 days after ovulation (Wilcox et al 1999). The inner cell mass forms the embryo, while the outer cell layers form the membranes and [[placenta]].  Together, the embryo and its membranes are referred to as a conceptus, or the "products of conception".  

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

* [[Alcohol]], certain [[Psychoactive drug|drug]]s, and other [[toxins]] that cause [[birth defect]]s, such as [[Fetal alcohol syndrome]]
* [[Infection]] (such as [[rubella]] or [[cytomegalovirus]])
* [[Radiation]] from [[x-ray]]s or [[radiation therapy]]
* [[Nutritional deficiencies]] such as lack of [[folate]] which contributes to [[spina bifida]]

Generally, if a structure pre-dates another structure in evolutionary terms, then it often appears earlier than the other in an embryo; this general observation is sometimes summarized by the phrase "[[recapitulation theory|ontogeny recapitulates phylogeny]]."<ref name="Gould">{{cite book |author=Stephen Jay Gould, |title=Ontogeny and Phylogeny | url= http://www.sjgarchive.org/library/ontogeny.html|publisher=Belknap Press |location=Cambridge, Mass |year= |pages= 206|isbn=0-674-63941-3 |oclc= |doi=}}</ref>  For example, the backbone is a common structure among all vertebrates such as fish, reptiles and [[mammal]]s, and the backbone also appears as one of the earliest structures laid out in all vertebrate embryos. The [[cerebrum]] in humans, which is the most sophisticated part of the [[human brain|brain]], develops last. The concept of recapitulation is not absolute, but it is recognized as being partly applicable to development of the human embryo.<ref name="Gould" />

===Changes by weeks of gestational age===
{{seealso|embryo|Human embryogenesis}}
[[Image:Tubal Pregnancy with embryo.jpg|thumb|200px|left|A 10mm embryo from an [[ectopic pregnancy]], still in the oviduct. This embryo is about five weeks old (or from the seventh week of menstrual age).]]
[[Image:9-Week Human Embryo from Ectopic Pregnancy.jpg|thumb|200px|left|This embryo is also from an
[[ectopic pregnancy]], this one in the cornu (the part of the uterus to which
the [[uterine_tubes|Fallopian tube]] is attached). The features are consistent with
a developmental age of seven weeks (reckoned as the ninth week of pregnancy).]]

* '''Week 2 (week of fertilization)'''
** Fertilization of the ovum to form a zygote which undergoes [[mitotic]] [[cellular division]], but does not increase in size. This mitosis is also known as [[cleavage (embryo)|cleavage]]. A hollow cavity forms marking the [[blastocyst]] stage.  
** The blastocyst contains only a thin rim of [[trophoblast]] cells and a clump of cells at one end known as the "embryonic pole" which include [[embryonic stem cells]].
**The blastocyst hatches from its protein shell ([[zona pellucida]]) and implants onto the [[endometrial]] lining of the mother's [[uterus]].
**If separation into [[identical twins]] occurs, 1/3 of the time it will happen before day 5.<ref>{{cite book |author=Scott F. Gilbert; with a chapter on plant development by Susan R. Singer |title=Developmental biology |url= http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=dbio.box.2669  |publisher=Sinauer Associates |location=Sunderland, Mass |year=2000 |pages= |isbn=0-87893-243-7 |oclc= |doi=}}</ref>

* '''Week 3 (1 week following fertilization)'''
** Trophoblast cells surrounding the embryonic cells proliferate and invade deeper into the uterine lining. They will eventually form the [[placenta]] and embryonic membranes.
** Formation of the yolk sac.  
** The embryonic cells flatten into a disk, two-cells thick.
** If separation into identical twins occurs, 2/3 of the time it will happen between days 5 and 9.  If it happens after day 9, there is a significant risk of the twins being [[conjoined twins|conjoined]].
**[[Primitive streak]] develops. <ref name=larsen>{{cite book |author=William J. Larsen |title=Human embryology |publisher=Churchill Livingstone |location=Edinburgh |year=2001 |pages= |isbn=0-443-06583-7 |oclc= |doi=}}</ref>
**[[Primary stem villi]] appear. <ref name=larsen/>

* '''Week 4 (2 weeks from fertilization - first missed menstrual period)'''
**A [[notochord]] forms in the center of the embryonic disk.
** A neural groove (future [[spinal cord]]) forms over the notochord with a [[brain]] bulge at one end. [[Neuromeres]] appear.
**[[Heart]] tubes begin to fuse.
**[[Gastrulation]] commences. <ref name=larsen/>
** [[Somite]]s, the divisions of the future [[vertebra]], form. <ref name=larsen/>
**Primitive heart tube is forming. Vasculature begins to develop in embryonic disc.  <ref name=larsen/>

* '''Week 5 (3 weeks from fertilization)'''
** The embryo measures 4 mm (1/8 inch) in length and begins to curve into a C shape.
** The heart bulges, further develops, and begins to beat in a regular rhythm. [[Septum primum]] appear.  <ref name=larsen/>
** [[Branchial arches]], grooves which will form structures of the [[face]] and [[neck]], form.
** The [[neural tube]] closes.
** The [[ear]]s begin to form as [[otic pit]]s.
** [[Arm]] buds and a tail are visible.
** Pulmonary primordium, the first traits of the [[lung]] appear. <ref name=larsen/>
**Hepatic plate, the first traits of the [[liver]] appear.  <ref name=larsen/>
**[[Buccopharyngeal membrane]] ruptures. This is the future mouth.  <ref name=larsen/>
**Cystic diverticulum, which will become the [[gallbladder]], and [[dorsal pancreatic bud]], which will become the [[pancreas]] appear.  <ref name=larsen/>
** [[Urorectal septum]] begins to form. Thus, the [[rectum|rectal]] and urinary passageways become separated. <ref name=larsen/>
** [[Anterior horn|Anterior]] and [[posterior horn]]s differentiate in the [[spinal cord]]  <ref name=larsen/>
**[[Spleen]] appears.  <ref name=larsen/>
**[[Ureteric bud]]s appear. <ref name=larsen/>

[[Image:6 weeks pregnant.jpg|thumb|left|Embryo at 4 weeks after fertilization.<ref>[http://www.3dpregnancy.com/static/pregnancy-week-6.html 3D Pregnancy] (large image of fetus at 4 weeks after fertilization).  Retrieved 2007-08-28.  A rotatable 3D version of this photo is available [http://www.3dpregnancy.com/rotatable/6-weeks-pregnant.html here], and a sketch is available [http://www.3dpregnancy.com/pictures/pregnancy-week-6.html here].</ref>]]
* '''Week 6 (4th week of development)'''
** The embryo measures 8 mm (1/4 inch) in length.
** Lens pits and [[optic cup]]s form the start of the developing [[eye]].
** Nasal pits form.
** The brain divides into 5 [[vesicle]]s, including the early [[telencephalon]].
** [[Leg]] buds form and [[hands]] form as flat paddles on the arms.
** Rudimentary [[blood]] moves through primitive vessels connecting to the [[yolk sac]] and chorionic membranes.

* '''Week 7 (5th week of development)'''
** The embryo measures 13 mm (1/2 inch) in length.
** [[Lung]]s begin to form.
** The brain continues to develop.
** Arms and legs have lengthened with [[foot]] and [[hand]] areas distinguishable.
** The hands and feet have digits, but may still be webbed.
** The [[gonadal ridge]] begins to be perceptible

* '''Week 8 (6th week of development)'''
** The embryo measures 18 mm (3/4 inch) in length.
** [[Nipple]]s and [[hair follicle]]s begin to form.
** Location of the [[Elbow-joint|elbow]]s and [[toe]]s are visible.
** Spontaneous limb movements may be detected by ultrasound.
** All essential [[Organ (anatomy)|organ]]s have at least begun formation.

{{clear}}

==Fetal period==
{{See also|Human fetal development}}
The fetal period begins at the end of the 10th week of gestation (8th week of development). Since the precursors of all the major organs are created by this time, the fetal period is described both by organ and by a list of changes by weeks of gestational age. 

Because the precursors of the organs are formed, fetus also is not as sensitive to damage from environmental exposures as the embryo. Instead, toxic exposures often cause physiological abnormalities or minor congenital malformation.

===By organ===
Each organ has its own development.
*[[Development of circulatory system]]
**[[Heart development]]
*[[Development of digestive system]]
**[[Tooth development]]
*[[Development of endocrine system]]
*[[Development of integumentary system]]
*[[Development of lymphatic system]] 
*[[Development of muscular system]]
*[[Development of nervous system]]
*[[Development of the urinary and reproductive system]] 
**[[Development of the reproductive system]]
***[[Development of the gonads]]
***[[Development of the suspensory ligament of the ovary]]
*[[Development of respiratory system]]

===Changes by weeks of gestational age===
{{Main|Human fetal development}}

From the 8th week until birth (around 38 weeks), the developing organism is called a fetus.  The fetus is not as sensitive to damage from environmental exposures as the embryo, and toxic exposures often cause physiological abnormalities or minor congenital malformation.  All major structures are already formed in the fetus, but they continue to grow and develop.

[[Image:10 weeks pregnant.jpg|thumb|left|Fetus at 8 weeks after fertilization.<ref>[http://www.3dpregnancy.com/static/pregnancy-week-10.html 3D Pregnancy] (large image of fetus at 10 weeks after fertilization).  Retrieved 2007-08-28.  A rotatable 3D version of this photo is available [http://www.3dpregnancy.com/rotatable/10-weeks-pregnant.html here], and a sketch is available [http://www.3dpregnancy.com/pictures/pregnancy-week-10.html here].</ref>]]
* '''Week 9 (7th week of development)'''
** Embryo measures 30 mm (1.2 inches) in length.
** [[Intestine]]s rotate.
** Facial features continue to develop.
** the [[eyelid]]s are more developed.
** the external features of the ear begin to take their final shape.

*'''Weeks 10 to 13 (8th to 11th week of development)'''
** The fetus reaches a length of 8 cm (3.2 inches).
** The head comprises nearly half of the fetus' size.
** The face is well formed
** The eyelids close and will not reopen until about the 28th week.
** [[Tooth]] buds, which will form the baby teeth, appear.
** The [[limb]]s are long and thin.
** The fetus can make a fist with its [[finger]]s.
** Genitals appear well differentiated.
** [[Red blood cell]]s are produced in the [[liver]].

* '''Weeks 14 to 17 (12th to 15th week of development)'''
** The fetus reaches a length of about 15 cm (6 inches).
** A fine hair called [[lanugo]] develops on the head.
** Fetal [[skin]] is almost transparent.
** More muscle tissue and bones have developed, and the bones become harder.
** The fetus makes active movements.
** Sucking motions are made with the mouth.
** [[Meconium]] is made in the intestinal tract.
** The liver and [[pancreas]] produce fluid secretions.

[[Image:20 weeks pregnant.jpg|thumb|left|Fetus at 18 weeks after fertilization.<ref>[http://www.3dpregnancy.com/static/pregnancy-week-20.html 3D Pregnancy] (large image of fetus at 18 weeks after fertilization).  Retrieved 2007-08-28.  A rotatable 3D version of this photo is available [http://www.3dpregnancy.com/rotatable/20-weeks-pregnant.html here], and a sketch is available [http://www.3dpregnancy.com/pictures/pregnancy-week-20.html here].</ref>]]
* '''Week 20 (18th week of development)'''
** The fetus reaches a length of 20 cm (8 inches).
** [[Lanugo]] covers the entire body.
** Eyebrows and eyelashes appear.
** Nails appear on fingers and toes.
** The fetus is more active with increased muscle development.
** "Quickening" usually occurs (the mother can feel the fetus moving).
** The fetal heartbeat can be heard with a [[stethoscope]].

* '''Week 24 (22nd week of development)'''
** The fetus reaches a length of 28 cm (11.2 inches).
** The fetus weighs about 725 g (1 lb 10 oz).
** Eyebrows and eyelashes are well formed.
** All of the eye components are developed.
** The fetus has a hand and [[startle reflex]].
** [[Footprint]]s and [[fingerprint]]s continue forming.
** [[Alveoli]] (air sacs) are forming in lungs.

* '''Week 28 (26th week of development)'''
** The fetus reaches a length of 38 cm (15 inches).
** The fetus weighs about 1.2 kg (2 lb 11 oz).
** The brain develops rapidly.
** The [[nervous system]] develops enough to control some body functions.
** The eyelids open and close.
** The cochleae are now developed, though the myelin sheaths in neural portion of the auditory system will continue to develop until 18 months after birth. 
** The respiratory system, while immature, has developed to the point where gas exchange is possible.
** A baby [[premature birth|born prematurely]] at this time may survive, but the possibilities for complications and death remain high.

* '''Weeks 32 (30th week of development)'''
** The fetus reaches a length of about 38-43 cm (15-17 inches).
** The fetus weighs about 2 kg (4 lb 6 oz).
** The amount of body fat rapidly increases.
** Rhythmic breathing movements occur, but lungs are not fully mature.
** [[thalamus|Thalamic]] brain connections, which mediate sensory input, form.
** Bones are fully developed, but are still soft and pliable.
** The fetus begins storing [[iron]], [[calcium]], and [[phosphorus]].

* '''Week 36 (34th week of development)'''
** The fetus reaches a length of about 40-48 cm (16-19 inches).
** The fetus weighs about 2.5 to 3 kg (5 lb 12 oz to 6 lb 12 oz).
** Lanugo begins to disappear.
** Body fat increases.
** Fingernails reach the end of the fingertips.
** a baby born at 36 weeks has a high chance of survival, but may require medical interventions.

[[Image:40 weeks pregnant.jpg|thumb|left|Fetus at 38 weeks after fertilization.<ref>[http://www.3dpregnancy.com/static/pregnancy-week-40.html 3D Pregnancy] (large image of fetus at 38 weeks after fertilization).  Retrieved 2007-08-28.  A rotatable 3D version of this photo is available [http://www.3dpregnancy.com/rotatable/40-weeks-pregnant.html here], and a sketch is available [http://www.3dpregnancy.com/pictures/pregnancy-week-40.html here].</ref>]]
* '''Weeks 37 to 40 (35th to 38th week of development)'''
** The fetus is considered full-term at the 37th week of gestational age.
** It may be 48 to 53 cm (19 to 21 inches) in length.
** The lanugo is gone except on the upper arms and shoulders.
** Fingernails extend beyond fingertips.
** Small [[breast bud]]s are present on both sexes.
** Head hair is now coarse and thickest.

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==References==
{{reflist|2}}

* [http://www.nlm.nih.gov/medlineplus/ency/article/002398.htm "MedlinePlus Medical Encyclopedia"]
* Moore, Keith L. ''The Developing Human: 3rd Edition''. W.B. Saunders Company, Philadelphia PA
* Wilcox AJ, Baird DD, Weinberg CR. Time of implantation of the conceptus and loss of pregnancy. 1999 N Engl J Med. 340(23):1796-9. [http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=10362823&query_hl=15&itool=pubmed_docsum PMID 10362823]
* Ljunger, E, Cnattingius, S, Lundin, C, & Annerén, G. 2005 Chromosomal anomalies in first-trimester miscarriages. Acta Obstetricia et Gynecologica Scandinavica 84(11):1103-1107. [http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=16232180&query_hl=6&itool=pubmed_docsum PMID 10362823]

{{SIB}}
{{Embryology}}
{{Development of circulatory system}}
{{Development of digestive system}}
{{Development of nervous system}}
{{Development of urinary and reproductive systems}}
{{Reproductive physiology}}

[[Category:Embryology]]
[[Category:Obstetrics]]

[[pl:Okres prenatalny]]

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