In utero in the context of "Conjoined twins"

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⭐ Core Definition: In utero

The uterus (from Latin uterus, pl.: uteri or uteruses) or womb (/wm/) is the organ in the reproductive system of most female mammals, including humans, that accommodates the embryonic and fetal development of one or more fertilized eggs until birth. The uterus is a hormone-responsive sex organ that contains glands in its lining that secrete uterine milk for embryonic nourishment. (The term uterus is also applied to analogous structures in some non-mammalian animals.)

In humans, the lower end of the uterus is a narrow part known as the isthmus that connects to the cervix, the anterior gateway leading to the vagina. The upper end, the body of the uterus, is connected to the fallopian tubes at the uterine horns; the rounded part, the fundus, is above the openings to the fallopian tubes. The connection of the uterine cavity with a fallopian tube is called the uterotubal junction. The fertilized egg is carried to the uterus along the fallopian tube. It will have divided on its journey to form a blastocyst that will implant itself into the lining of the uterus – the endometrium, where it will receive nutrients and develop into the embryo proper, and later fetus, for the duration of the pregnancy.

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👉 In utero in the context of Conjoined twins

Conjoined twins, popularly referred to as Siamese twins, are twins joined in utero. It is a very rare phenomenon, estimated to occur in anywhere between one in 50,000 births to one in 200,000 births, with a somewhat higher incidence in southwest Asia and Africa. Approximately half are stillborn, and an additional one-third die within 24 hours. Most live births are female, with a ratio of 3:1.

Two possible explanations of the cause of conjoined twins have been proposed. The one that is generally accepted is fission, in which the fertilized egg splits partially. The other explanation, no longer believed to be accurate, is fusion, in which the fertilized egg completely separates, but stem cells (that search for similar cells) find similar stem cells on the other twin and fuse the twins together. Conjoined twins and some monozygotic, but not conjoined, twins share a single common chorion, placenta, and amniotic sac in utero.

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