The first twin distinction is their zygosity: the degree of identity in the genome. There are five common variations
- Fraternal (dizygotic) twins occur when two separate eggs are fertilized at around the same time:
- male-female (most common)
- female-female
- male-male
- Identical (monozygotic) twins occur when a single egg is fertilized forming a single zygote which splits:
- female-female
- male-male (least common)
Type | Description | Day |
---|---|---|
Dichorionic-Diamniotic | Twins share only the womb | Days 1-4 |
Monochorionic-Diamniotic | Twins share the same placenta | Days 4-8 |
Monochorionic-Monoamniotic | Twins share the same amnion | Days 9-12 |
Conjoined twins | Twins are physically connected | Days 12 or > |
Jamie is having DiDi twins. They share the same womb, but nothing else. The ultrasound picture below shows the separation between the twins.
Regardless of the type, twins are rare, estimated to be approximately 1.9% of the world's population, with identical twins making up only 0.2% of the total population-and 8% of all twins.
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