Stages of the Unborn Fetus
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The Human Fetus
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The human fetus is defined as the unborn human during the fetal stage of development. This stage begins 9 weeks after fertilization and ends with birth. The fetus develops organs and grows to 425 times its starting size during this time.
The Beginning of the Fetal Stage
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The fetal stage is reached during the week 11 of gestation. The fetal stage can also be said to begin 9 weeks after fertilization. However, for the remainder of this article, weeks will be counted from the date of gestation.
The fetus begins at 1.2 inches long weighing 8 grams (.28 ounces). Many of the organs are already present--including the heart, brain, hands, feet and lungs--but they exist in rudimentary forms and do not function as they do when fully developed. Movement, including the appearance of twitches and breathing, can occur as muscles and nerves develop, but volitional movement and pain sensitivity does not occur until the third trimester.
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16 to 25 weeks
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During this time--ending around five months after fertilization--the fetus grows to eight inches in length. Organ development continues. The mother will be able to detect fetal movement at around 21 weeks post-fertilization.
26 to 35 Weeks
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The fetus grows rapidly, gaining full-length fingernails, hair and increasing body fat. By 35 weeks, the bones of the fetus are fully developed, but they're still soft. At 35 weeks, the fetus is developed to the point that it could exist outside the uterus and is called "full-term." Most pregnancies last closer to 40 weeks, however.
35 to 40 weeks
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The fetus will have grown to 19 to 21 inches in length before birth. Although the thalamic neural connections are now relatively well-developed, coordination is still crude. It will continue to develop through puberty.
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