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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.
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 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.
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.
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.
Implantation is the first stage of gestation, and, when successful, the female is considered to be pregnant. [3] An implanted embryo is detected by the presence of increased levels of human chorionic gonadotropin (hCG) in a pregnancy test. [3] The implanted embryo will receive oxygen and nutrients in order to grow.
Fetal viability refers to a point in fetal development at which the fetus may survive outside the womb. The lower limit of viability is approximately 5 + 3 ⁄ 4 months gestational age and is usually later. [20] There is no sharp limit of development, age, or weight at which a fetus automatically becomes viable. [21]
Gestation is the period of development during the carrying of an embryo, and later fetus, inside viviparous animals (the embryo develops within the parent). [1] It is typical for mammals, but also occurs for some non-mammals. Mammals during pregnancy can have one or more gestations at the same time, for example in a multiple birth. [2]