Multiple pregnancy

Multiple pregnancy occurs when there is more than 1 fetus develop in the uterus (womb) during a single pregnancy. Different names have been used to describe multiple pregnancy depending on the number of the fetuses. For example two fetuses is called twins, three fetuses is called triplets, four is called quadruplets, five has been called as quintuplets, and so on.

Multiple pregnancy can be classified into several types based on: 1) The initial number of embryo produced 2)The number of placenta 3)The number of amniotic sac. 

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1) The number of initial embryo produced.

Multiple pregnancy can be resulted from the fertilization between 1 ovum and 1 sperm result in 1 embryo, which then divided into two or more embryos. It also can be a result of the fertilization between two or more ova and two or more sperms result in two or more embryos. The first way of fertilization will result in monozygotic twins / triplets / so on. These offspring is called identical twins / triplets / so on, which mean they are genetically identical since they are the result of the same one ovum and sperm. Meanwhile, the other way of fertilization will result in dizygotic, for twins, or polyzygotic triplets / so on. These offspring are called nonidentical twins / triplets / so on, which mean they are genetically not identical since they resulted from different sets of ovum and sperm.

2) The number of placenta

Monozygotic fetuses may share placenta or each of them has their own placenta. Twins who shared placenta is called monochorionic twins. These fetuses who shared placenta are at risk of having twin-to-twin transfusion syndrome. Meanwhile monozygotic twins which each one has their own placenta, thus also has separate amniotic sac, is called dichorionic diamniotic twins (DCDA)

3) The number of amniotic sac

Monochorionic fetuses may also share the same amniotic sac, or each of them has their own amniotic sac. Twins who shared amniotic sac is called monochorionic monoamniotic twins (MCMA), while the monochorionic twins which each of them has their own amniotic sac is called monochorionic diamniotic twins (MCDA). Only monochorionic monoamniotic twins have the risk to become conjoined twins. 

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Specific types of twins

DCDA twins – dichorionic diamniotic
MCDA twins – monochorionic diamniotic
MCMA twins – monochorionic monoamniotic