The probability of identical twins is equal all over the globe: around 3 in every 1000 births. The probability of fraternal twins depends on various factors. They are mainly prevalent in mothers over 35, and in fertilisation that includes IVF treatment. Fraternal twins transpire due to hyper-ovulation when the mother releases more than one egg per cycle. Women who previously had fraternal twins are four times more prone to have fraternal twins in their subsequent pregnancy. Certain drugs like clomiphene develop the possibility of fraternal twins to around 10% because they manage to bring about hyper-ovulation.