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Embryonic age: 10 weeks and 0 days until 13 weeks and 6 days old. The fetus reaches a length of about 15 cm (6 in). 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.
The next period is that of fetal development where many organs become fully developed. This fetal period is described both topically (by organ) and chronologically (by time) with major occurrences being listed by gestational age. The very early stages of embryonic development are the same in all mammals, but later stages of development, and the ...
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.
A woman pregnant for the first time (nulliparous) typically feels fetal movements at about 21 weeks, whereas a woman who has given birth before will typically feel movements by 20 weeks. [13] By the end of the fifth month, the fetus is about 20 cm (8 in) long.
Women who have previously given birth have more relaxed uterine muscles which are more sensitive to fetal motion during subsequent pregnancies. For them fetal motion can sometimes be felt as early as 14 weeks. [4] Quickening indicates the start of fetal movements, usually felt 14–26 weeks after conception, or between the fourth and sixth month.
The embryonic stage covers the first eight weeks of development; at the beginning of the ninth week the embryo is termed a fetus. The germinal stage refers to the time from fertilization through the development of the early embryo until implantation is completed in the uterus. The germinal stage takes around 10 days. [2]
The sex of the fetus may be discerned by ultrasound as early as 11 weeks' gestation. The accuracy is relatively imprecise when attempted early. [15] [16] [17] After 13 weeks' gestation, a high accuracy of between 99% and 100% is possible if the fetus does not display intersex external characteristics. [18]
This activity assists with proper joint development. [15] Women who have already given birth have more relaxed uterine muscles that are consequently more sensitive to fetal motion, and for them fetal motion can sometimes be felt as early as 14 weeks. [18] By about week 21, the fetus begins to develop a regular schedule of movement. [17]
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