Multiple Pregnancies

怀孕

多胞胎

在一些例子,受精卵可能经历外分裂,分成两个分离的细胞。这两个新细胞然后以各自的卵裂继续经历独立的发育。最终,它们俩都到达子宫,在那里也分别地完成植入。这样的结果就是双胞胎妊娠。因为双胞胎来自于相同的卵子与精子,他们会完全相同,也就是它们将是同样的性别,当然他们的长相在各个方面也相同。
实际上,同卵双生的过程并不是必定在输卵管就开始发生,而是在受精后的2-4天之间的任何时间里发生。因而,甚至在植入期间,胚泡仍然可能分裂并发育成两个分离的胚胎。

还有一些非常罕见的多排卵现象发生,也就是在同一时期内,有数个卵子释放出来。如果这些卵子都受精,当然就可能产生多胞胎。这样所导致的妊娠双胞胎就叫异卵双胞胎。如果有3个孩子诞生,就叫三胞胎,四个就叫四胞胎,五个就是五胞胎,如此等等。因为他们中每一个是不同的卵子与不同的精子受精的结果,他们可能有不同的性别,并且不会比任何其他的兄弟姊妹更相像。

两类双胞胎举例
A.同卵双胞胎性别相同,并共享一个胎盘。 两个羊膜囊由一个外膜层包裹。
B.异卵双胞胎可能性别不同。有两个胎盘和两个分隔的羊膜囊,各自有自己的外膜层包裹。

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.

Conception

Multiple Pregnancies

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.

[Course 2] [Description] [How to use it] [Introduction] [Conception] [Historical Notes] [Beginning of Life?] [No definite answer] [Egg and Sperm] [Coitus] [Fertilization] [Segmentation] [Implantation] [Multiple Pregnancies] [Pregnancy] [Birth] [Infertility] [Contraception] [Abortion] [Additional Reading] [Examination]