Multiples

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Multiples are two or more children of a mother and father who have grown up within the same pregnancy and are usually born during the same birth process. The term is usually only used if the special case of a twin pregnancy is not present.

Origin
The formation of triplets and higher grade multiples corresponds to the formation of twins:
Identical twins (monozygotic)
Fraternal twins (dizygotic)

In multiple pregnancies with identical (= monozygotic) twins, depending on the time of division of the embryo blast, four forms of how the twins share placenta and amniotic sac:
The dichorial-diamniote twin pregnancy until the third day after fertilization is by far the most common form with about 70 out of 100 pregnancies with identical twins. Each twin has its own mother cake (placenta) and fruit cave and is enveloped by two eggs (amnion and chorion), so that the partition between them consists of four eggs.
The monochorial-diamniotic twin pregnancy between the 3rd and 7th. The day after fertilization occurs in about 30 out of 100 pregnancies with identical twins. The twins share a mother cake (placenta), but each child has its own fruit cavity surrounded by an egg skin, so the partition between them consists of two egg skins, namely the amnion of each twin.
The monochorial-monoamniotic twin pregnancy between 9 and 12. The day after fertilization occurs in about 1 in 100 pregnancies with identical twins. The twins share a placenta and a fruit cavity, so there is no partition of eggs between them.
Monochorial monoamniotic Siamese twin pregnancy occurs in about 1 in 300 pregnancies with identical twins. Due to an incomplete division of the embryo blast, the children have grown together on one or more parts of the body. The twins share a placenta and a fruit cave.

Triplets occur naturally when division occurs again after twin formation. All forms of the second twin formation occur, but accumulate succinctly in certain forms.

In vitro fertilisation usually involves the implantation of several fertilised egg cells. Since they settle separately, their own eggs and mother cakes are formed if the fetuses survive for a sufficiently long time.

In very rare cases, one or even several fetuses of the identical multiples may develop with severe atrophy within a normally developing fetus. This foetus in foeto usually develops in the abdomen and causes early discomfort there. However, the multiple fetus can also be enclosed in other organs and, for example, considered a brain tumor