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懷孕 |
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多胞胎
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在一些例子，受精卵可能經歷外分裂，分成兩個分離的細胞。這兩個新細胞然後以各自的卵裂繼續經歷獨立的發育。最終，它們倆都到達子宮，在那裏也分別地完成植入。這樣的結果就是雙胞胎妊娠。因為雙胞胎來自于相同的卵子與精子，他們會完全相同，也就是它們將是同樣的性別，當然他們的長相在各個方面也相同。
實際上，同卵雙生的過程並不是必定在輸卵管就開始發生，而是在受精後的2-4天之間的任何時間裏發生。因而，甚至在植入期間，胚泡仍然可能分裂並發育成兩個分離的胚胎。 |
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還有一些非常罕見的多排卵現象發生，也就是在同一時期內，有數個卵子釋放出來。如果這些卵子都受精，當然就可能產生多胞胎。這樣所導致的妊娠雙胞胎就叫異卵雙胞胎。如果有3個孩子誕生，就叫三胞胎，四個就叫四胞胎，五個就是五胞胎，如此等等。因為他們中每一個是不同的卵子與不同的精子受精的結果，他們可能有不同的性別，並且不會比任何其他的兄弟姊妹更相像。 |
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兩類雙胞胎舉例
A.同卵雙胞胎性別相同，並共用一個胎盤。
兩個羊膜囊由一個外膜層包裹。
B.異卵雙胞胎可能性別不同。有兩個胎盤和兩個分隔的羊膜囊，各自有自己的外膜層包裹。
Two Examples of Twinning
A. Identical
Twins Fetuses are
of the same sex and share one placenta. One outer membrane envelops both amniotic sacs.
B. Fraternal Twins Fetuses may be of different sex. There are two placentas and two separate amniotic sacs, each with its own membrane.
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Conception |
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Multiple Pregnancies
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In some cases, the zygote may proceed to an external division and split up into two separate cells. These two new cells then continue their development independently as they undergo their own segmentation. Eventually, they both arrive in the uterus where they also implant separately. The result is a twin pregnancy. Since both came from the same egg and the same sperm, the twins will be identical, i.e., they will be of the same sex and, of course, also look alike in every other respect.
Actually, identical twinning does not necessarily have to begin in the Fallopian tube, but can occur at any time between the second and fourteenth day after fertilization. Thus, even during implantation the blastocyst may still divide and grow into two separate embryos.There are also some very rare cases of multiple ovulation, i.e., the release of several eggs at the same time. If these eggs are then fertilized, they may, of course, produce multiple pregnancies. Twins born as a result of such a pregnancy are known as fraternal twins. If three of these children are born, they are called triplets, four are quadruplets, five are quintuplets, and so on. Since each of them comes from a different egg fertilized by a different sperm cell, they may be of different sex and will not resemble each other any more than other brothers and sisters.
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