You've likely heard of the 2 most frequent types of twins: identical, in which the fertilized egg splits and produces two identical babies with no genetic differences, and fraternal, in which two eggs are fertilized by two sperm and the babies are genetically distinct.
However, there are numerous other, less common twin subtypes. Let's take a closer look at the various types of twins.
1. Twins who are monozygotic
Monozygotic (identical) twins are born from a single fertilized egg, which divides into two embryos and develops into two babies. Because each baby is born from the same egg and sperm, their genetic material (referred to as chromosomes) is identical. They are born with the same sex and share physical characteristics such as eye and hair color.
If you've ever met identical twins and thought you noticed a slight difference between them, it could be due to prenatal environmental differences, such as their positioning and space in the uterus.
2. Dizygotic Twins
Dizygotic (fraternal) twins are created from their own egg (released at the same time by the birthing parent's body) and sperm. Fraternal twins have the same percentage of chromosomes as individual siblings born at different times (around 50%, according to the National Human Genome Research Institute), which explains why they don't look the same and can be assigned different sexes at birth.
3. Twins who are conjoined
Conjoined twins are identical twins who do not fully separate from each other because the fertilized ovum fails to divide completely. This means that when the babies are born, they are physically connected to each other in places like the abdomen (omphalopagus twins) or the chest (thoracopagus twins). Conjoined twins may share internal organs or limbs as well.
When a person is already pregnant, an egg may be released from the ovaries in very rare cases. This results in twins who are conceived at different times, a condition known as superfetation.
Rebecca Roberts, 39, of England, gave birth to "twins" three weeks apart in 2020. Roberts told Good Morning America that her doctors couldn't figure out what was wrong with her.
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