Monozygotic and Dizygotic Twins
Identical twins are often referred to as monozygotic; they begin life as a single fertilized egg, and shortly afterward split into two (well before they can be seen clearly by ultrasound). Contrary to popular belief, there are no hereditary factors at play in the incidence of identical twin births. Families that have identical twins in multiple generations are simply lucky. They account for approximately one-third of all live twin births.
The chances are also not affected by IVF; there is no increase in chances of a single egg splitting no matter the method of fertilization. However, the time it takes for an egg to split into twins after fertilization does affect the similarities between the children. An embryo that splits before day four will result in "less identical" twins than an egg that doesn't split until day ten. Also, approximately 99.9% of identical twins are the same sex.
Fraternal twins are referred to as dizygotic-they begin as two separate eggs fertilized at different (but close) times. Dizygotic twins are the more prominent type-occuring in the other two-thirds of twin births. IVF is a factor here, as the medications lead to the production of egg release-thus producing greater chances of multiple egg fertilization. Fraternal twins actually can be hereditary, as a predisposition to releasing more than one egg (hyperovulation) can be passed on through a maternal gene.
Several statistics show to be a factor in conceiving fraternal twins. Women above average height, as well as those with a BMI (Body mass index) over 30 show an increased frequency of multiples. Other factors include a previous twin pregnancy, IVF medications (because of the stimulation of the ovaries), and certain food items. There is actually a tribe in Africa (the Yorubas) with the highest rate of twins in the world. A study concluded that the frequent consumption of a cassava (a type of sweet potato) led to an increased stimulation of their ovaries!
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