This refers to the genetic relationship between your twins. It stems from “zygote” which refers to the single fertilized egg. There are two options:
- Monozygotic twins, also known as identical twins, form when a single fertilized egg splits, resulting in the development of two individual embryos. Because they originate from the same set of cells, these individuals have the same DNA and often have remarkably similar physical appearances.
- Dizygotic twins, also known as fraternal twins, form when multiple eggs are fertilized and develop. They may look alike or they may look completely different, the same as any siblings would.
Opposite-sex twins (male and female) are always dizygotic. However, in the case of same-sex twins, determining whether they are identical (monozygotic) or non-identical (dizygotic) may require additional information including ultrasounds findings regarding chorionicity (as described below), blood type or DNA analysis.
This refers to whether your babies share a placenta. It’s important to find out early on during your pregnancy as babies who share a placenta put you and your babies at a higher risk of complications. Dichorionic means each baby has a separate placenta and is inside a separate sac which has its own outer membrane. Monochorionic means the babies share a placenta and outer membrane.
Relationship between zygosity and chorionicity
Dizygotic (fraternal) twins would always be dichorionic, since each twin originates from a separate fertilized egg, each developing into a fetus with its own membranes and placenta. In contrast, monozygotic (identical) twins may be either dichorionic (2 placentas) or monochorionic (single placenta), depending on when the egg splits.
When the egg splits at a very early stage (usually within first three days following fertilization, and before the placenta begins to form), there is the possibility of each baby having its own membranes and placenta, which results in dichorionic twins. However, when the egg splits at a later stage (after day three, when the placenta has already began to from), the resulting twin will ‘need’ to share the placenta and membranes that were already formed prior to the switch, resulting in monochorionic twins.
Rarely, the egg splits at an even later stage, after the fetal body started to develop, in which case the two twins will be sharing the body parts that were already formed at the time of split, resulting in conjoined twins. This is illustrated in the figure below.
A common mistake is that dichorionic twins, where there are two placentas, are necessarily non-identical twins. This is not always the case, as about a third of the identical (monozygotic) twins will be dichorionic, as illustrated in the figure below.