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Prenatal or antenatal development is the process in which a human embryo or fetus (or foetus) gestates during pregnancy, from 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.
When semen is deposited in the vagina, the spermatozoa travel through the cervix and body of the uterus and into the Fallopian tubes. Fertilization of the ovum (egg 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 egg then polarizes, repelling any additional sperm. The resulting combination is called a zygote, a new and genetically unique human organism. The term "conception" refers variably to either fertilization or to formation of the conceptus after uterine implantation, and this terminology is controversial.
Prior to fertilization, each ovum contains a complete human genome, including a single X but no Y chromosome. Likewise, each spermatozoon contains a complete set of autosomes and a single sex chromosome, either X or Y. The resulting human zygote is similar to the majority of somatic cells because it contains two copies of the genome in a diploid set of chromosomes. One set of chromosomes came from the nucleus of the ovum and the second set from the nucleus of the sperm.
If the spermatozoon contributes a Y chromosome then the zygote will develop as a male. Unlike the X chromosome, the Y chromosome contains very little genetic information. However it does contain a gene, SRY, which will switch on androgen production at a later stage, leading to the development of a male body type. In contrast, the mitochondrial genetic information of the zygote comes entirely from the mother via the ovum.
The embryonic period in humans begins at fertilization (penetration of the egg by the sperm) and continues until the end of the 10th week of gestation (8th week by embryonic age).
The embryo spends the next few days traveling down the Fallopian tube. It starts out as a single cell zygote and then 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, as it is confined within a glycoprotein shell, known as the zona pellucida. Instead, 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 occurs. This process is analogous to zona hatching, a term that refers to the emergence of the blastocyst from the zona pellucida, when incubated in vitro. This allows the trophectoderm cells of the blastocyst to come into contact with, and adhere to, the endometrial cells of the uterus. The trophectoderm will eventually give rise to extra-embryonic structures, such as the placenta and the membranes. The embryo becomes embedded in the endometrium in a process called implantation. In most successful pregnancies, the embryo implants 8 to 10 days after ovulation (Wilcox et al. 1999). 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 external features begin to take form. This process is called 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:
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 "ontogeny recapitulates phylogeny." For example, the backbone is a common structure among all vertebrates such as fish, reptiles and mammals, 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 brain, develops last. The concept of recapitulation is not absolute, but it is recognized as being partly applicable to development of the human embryo.
Gestational age is the time that has passed since the onset of the last menstruation, which generally or as standard occurs 2 weeks before the actual fertilization. Embryonic age, in contrast measures the actual age of the embryo or fetus from the time of fertilization. Nevertheless, menstruation has historically been the only means of estimating embryonal/fetal age, and is still the presumed measure if not else specified. However, the actual duration between last menstruation and fertilization may in fact differ from the standard 2 weeks by several days.
Thus, the first week of embryonic age is already week three counting with gestational age.
Furthermore, the number of the week is one more than the actual age of the embryo/fetus. For example, the embryo is 0 whole weeks old during the 1st week after fertilization.
The following table summarizes the various expression systems during week number x of gestation.
Gestational age: 0 weeks and 0 days until to 1 week and 6 days old. 1–14 days from last menstruation.
Embryonic age: -2 to -1 weeks old. (Week 1–2 of gestational age are theoretical extrapolations of embryonic age, since the fertilization hasn't actually occurred yet.)
Gestational age: 2 weeks and 0 days until 2 weeks and 6 days old. 15–21 days from last menstruation.
Embryonic age: Week nr 1. 0 (whole) weeks old. 1–7 days from fertilization.
Gestational age: 3 weeks and 0 days until 3 weeks and 6 days old. 22–28 days from last menstruation.
Embryonic age: Week nr 2. 1 week old. 8–14 days from fertilization.
Gestational age: 4 weeks and 0 days until 4 weeks and 6 days old. 29–35 days from last menstruation.
Embryonic age: Week nr 3. 2 weeks old. 15–21 days from fertilization.
Gestational age: 5 weeks and 0 days until 5 weeks and 6 days old. 36–42 days from last menstruation.
Embryonic age: Week nr 4. 3 weeks old. 22–28 days from fertilization.
Gestational age: 6 weeks and 0 days until 6 weeks and 6 days old. 43–49 days from last menstruation.
Embryonic age: Week nr 5. 4 weeks old. 29–35 days from fertilization.
Gestational age: 7 weeks and 0 days until 7 weeks and 6 days old. 50–56 days from last menstruation.
Embryonic age: Week nr 6. 5 weeks old. 36–42 days from fertilization.
Gestational age: 8 weeks and 0 days until 8 weeks and 6 days old. 57–63 days from last menstruation.
Embryonic age: Week nr 7. 6 weeks old. 43–49 days from fertilization.
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.
Each organ has its own 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.
Gestational age: 9 weeks and 0 days until 11 weeks and 6 days old.
Embryonic age: Weeks nr 8–10. 7–9 weeks old.
Gestational age: 12 weeks and 0 days until 15 weeks and 6 days old.
Embryonic age: Weeks nr 11–14. 10–13 weeks old.
Gestational age: 18 weeks old.
Embryonic age: Week nr 17. 16 weeks old.
Gestational age: 22 weeks old.
Embryonic age: Week nr 21. 20 weeks old.
Gestational age: 26 weeks old.
Embryonic age: Week nr 25. 24 weeks old.
Gestational age: 30 weeks old.
Embryonic age: Week nr 29. 28 weeks old.
Gestational age: 34 weeks old.
Embryonic age: Week nr 33. 32 weeks old.
Gestational age: 35 and 0 days until 39 weeks and 6 days old.
Embryonic age: Weeks nr 34–38. 33–37 weeks old.
The growth rate of an embryo and infant can be reflected as the weight per gestational 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 as appropriate for gestational age (AGA). An abnormally slow growth rate results in the infant being small for gestational age, and, on the other hand, an abnormally large growth rate results in the infant being large for gestational age. A slow growth rate and preterm birth are the two factors that can cause a low birth weight. Low birth weight (below 2000 grams) can ultimately increase the likelihood of schizophrenia by almost four times. 
|Wikimedia Commons has media related to: Embryology|
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