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妊娠 |
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胚胎與胎兒發育
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胎兒
術語胎兒
(fetus，自拉丁語：後代)定義為生命第3月致誕生之月的生命發育有機體。在這個時期，它從約2.54釐米（1英寸）長和一盎司重發育成約50.8釐米(20英寸)長與約3.18千克(7磅)重的寶寶。在這個發育時期的第1周裏，男女的內部性器官開始明顯出現分化。不久，外部性器官發育特徵顯現。大約於5個月時，胎兒活動變得如此強而有力，以至於翹首以待的媽媽也能夠感受得到。從前，人們認為這個稱之為胎動的現象為生命進入身體的時候。
在胎兒整個生長發育期間，他/她漂浮在羊水裏而得到了很好的保護。在6個月末，胎兒長約15.24釐米(6英寸)、重約0.68千克（1.5磅）。此時控制呼吸的腦中樞神經開始發育。這樣的胎兒早產（儘管可能性非常之小）後實際上要生存下來簡直是不可能的。因為未有呼吸而仍然導致腦損傷的可能性非常大。胎兒的最後數月發育為的是進一步的完善結構與功能，如腦的體溫控制機制的完善與皮下脂肪保護層的增厚等等。在男胎，睾丸下降到陰囊。如果未下降，出生後必須在合適時間裏做手術矯正。不然，將會導致男性不育。
出生前的最後數周，胎兒不僅在個頭上快速地生長，而且其體重也增加了許多。胎兒出生時體重小於2.27千克(5磅)者，則為早產兒。 |
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Pregnancy |
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Development of Embryo and Fetus
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The Fetus
The word fetus (Latin: offspring) is used to describe the growing organism from the beginning of the third month of its life to the moment of birth. During this time, it develops from a small growth of slightly over an inch weighing only a fraction of an ounce into a baby of about twenty inches in length weighing approximately seven pounds. In the first weeks of this development, the male female sex differentiation becomes apparent in the internal sex organs. A little later, the external sex organs develop their characteristic structure. Sometime around the fifth month, the fetal movements become strong enough to be felt by the expectant mother. This so called quickening was formerly believed to be the moment when life entered the new body.
Throughout its growth, the fetus is well protected from injuries as it floats almost weightlessly inside a fluid filled sac called the amniotic sac. At the end of the sixth month, the fetus measures about six inches in length and weighs about one and a half pounds. At this time, the centers of the brain which control breathing begin to develop. It is not entirely impossible (although very unlikely) that such a fetus could actually survive a premature birth. However, the probability of brain damage because of still ineffective breathing is great. The last months of fetal development bring further refinements, such as the temperature control mechanism in the brain and a protective layer of fat under the skin. In the case of a male fetus, the testicles descend into the scrotum. If this descent should fail to occur, corrective measures have to be taken sometime after birth. Otherwise, sterility will result.
During the final weeks before birth, the fetus not only grows rapidly in size, but also gains much of its weight. The birth of a fetus weighing less than five pounds is premature. |
