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Human embryonic development covers the first eight weeks of development, which have 23 stages, called Carnegie stages. At the beginning of the ninth week, the embryo is termed a fetus (spelled "foetus" in British English). In comparison to the embryo, the fetus has more recognizable external features and a more complete set of developing organs.
The fetus reaches a length of about 40–48 cm (16–19 in). The fetus weighs about 2.5 to 3 kg (6 lb 10 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. Fetus at 38 weeks after fertilization.
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.
The development of the human embryo follows fertilization, and continues as fetal development. By the end of the tenth week of gestational age, the embryo has acquired its basic form and is referred to as a fetus. The next period is that of fetal development where many organs become fully developed.
Maternal to zygotic transition (MZT), also known as embryonic genome activation, is the stage in embryonic development during which development comes under the exclusive control of the zygotic genome rather than the maternal (egg) genome. The egg contains stored maternal genetic material mRNA which controls embryo development until the onset of ...
The stages are delineated through the development of structures, not by size or the number of days of development, and so the chronology can vary between species, and to a certain extent between embryos. In the human being, only the first 60 days of development are covered; at that point, the term embryo is usually replaced with the term fetus.
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]
This leads to extreme amounts of pain and discomfort and may result in an irregular menstrual cycle. [35] Teratogenesis is a condition in which the mother of a developing offspring has defects form in the embryo or fetus they are carrying. This can inhibit the growth of the offspring physiologically, physically, and cause a degeneration in the ...