| What is a multiple pregnancy?
A multiple pregnancy is one in which there is more than one baby in the uterus. Multiple births are a relatively rare phenomena and only about 1 in 36 babies born is a pair of twins. Triplets or multiplets are even rarer. The recent increase in fertility treatments has led to an increase in the incidence of multiple births.
Are there different types of twins?
Twins are of two basic types – identical and fraternal or non-identical. Identical twins are formed when one fertilised egg splits into two at some time during early pregnancy. Fraternal twins are born when two separate eggs are fertilised by two separate sperms. Identical twins are also called monozygotic while fraternal twins are known as dizygotic. Fraternal twins usually develop in separate amniotic sacs and have separate placentas. Identical twins share the same amniotic sac but have separate placentas.
Identical twins are born when the fertilised egg splits into two distinct entities in the early stages of cell division. The DNA and other genetic material is shared by the twins. Identical twins have the same genotype and phenotype which means that they not only have a similar genetic make up but also look similar from the outside. Identical twins are always of the same sex.
Siamese or conjoined twins:
A variation of identical twins, these twins are joined at one or the other body part. This is because of incomplete splitting of the fertilised egg during pregnancy leading to incompletely separated individuals. Siamese twins, like identical twins look alike and are of the same sex. However, conjoined twins are joined at some point in their bodies and may also share some organs. Conjoined twins have been known to be joined from the head, chest, stomach and hips.
Another variation of identical twinning, these twins are mirror images of each other. They are extremely rare. Such twins have organs on the opposite sides, for eg., one twin will have the heart on the right side instead of the left and will be opposite handed to the other twin.
The most commonly occurring twins, fraternal twins are no more alike – genetically and phenotypically, than any regular pair of siblings. Fraternal twins are formed when two separate eggs are fertilised by two different sperms. They share only the uterine space and the intra-uterine environment. Fraternals may not be of the same sex and they may also not look alike.