What Triggers Twinning and How Likely is a Pregnancy to Result in Twins?

Twins are two individuals who shared the same womb. They are usually born on the same day, and rarely otherwise. Twins may be classified into two types- Fraternal and Identical twins. Around 1.2% of all human births have resulted from twin pregnancy. In 2001, 3% of all the births in the U.S have been of twins. The development of fraternal and identical twins inside the womb differs from each other at the formational stage.

Fraternal twins are commonly called non- identical twins. They are the result of two fertilized eggs being planted in the uterus simultaneously. Most often, they are planted during the same menstrual cycle and in rare cases in consecutive menstrual cycles. Since there are two eggs, two different zygotes are formed. (-A Zygote is a cell formed because of fertilization). Hence, fraternal twins are also known as dizygotic twins. Fraternal twins being non-identical have common genetic traits only to the extent found in ordinary siblings. The reason being that they have separate amnions (An amnion is a membranous sac that forms to protect the embryo) and placenta (an organ that develops from the same sperm and egg cells that form the fetus).

Identical twins occur when a single fertilized egg is planted in the uterus. They form one zygote (i.e. they are monozygotic). In turn, the zygote then divides to form two separate embryos. The two embryos develop in the same womb into two different fetuses. Identical twins may be monoamniotic (having a common amnion) or diamniotic (or having their own amnions). Diamniotic identical twins are described as monochorionic if they share the placenta and dichorionic if they do not. Monoamniotic twins are monochorionic i.e. having the same amnion and placenta. This can create a host of problems- there is a chance that umbilical cords of the twins tangling and interrupting the blood supply or of the blood supply to the twins being disproportionate.

Though identical twins are genetically meant to similar, they may not be so due to genetic mutation. They are usually of the same sex. At first glance, identical twins are hard to distinguish one from another, but their fingerprints are distinct. In addition, such twins resemble each other lesser as they grow up due to environment factors. Identical twin happen randomly and not due to genetic reasons. Fraternal twins may or not be of the same gender. Fraternal twinning, according to studies, occurs due to hereditary factors. Such births are likely if the mother is of African descent, if she is between 30-40 years of age, taller and heavier than the normal and if she has just stopped using oral contraceptives. Thus, the number of fertilized eggs planted in the uterus determines the type of twin.

See http://www.wonderquest.com/TwinsTrigger.htm for further reading.