Difference Between Identical and Fraternal Twins

I have a colleague who has a twin but her sister looks very different from her, whereas my aunt's twin children look a mirror image of each other. This is because of the two different types of twin pregnancy. One is identical and the other is fraternal. Identical and fraternal are totally different pregnancies. The only common thing between them is that they give birth to multiple kid at the same time. Twins make approximately 1.9% of the total world population. But it's not necessary that all the twins will look similar or will possess similar character and behavior. Let's find out what is the difference between identical and fraternal twins and how it takes place.

Identical vs Fraternal Twins

Identical and fraternal twins do not only possess different characteristics but also differ in the fertilization process. In the following paragraphs, you will find the difference between both the type of twins.

Identical Twins
Identical twins are also known as uniovular or monozygotic twins. Only 8% of the total twin population are identical twins. In identical twins only one ovum is fertilized with only one spermatozoid. Then this fertilized egg further divides into two embryos. Genetic research of identical twins shows that they have same sex, same blood group and have the same set of chromosomes. This is the similarity in the fertilization process in case of identical twins. But later the maturation and the development of identical twins can be in four different ways.

• In the first type there is only one placenta and two amniotic sacs. Placenta feeds the baby and the amniotic sac is a sac where the fetus develops. Hence, in this type both the babies share same placenta but have their own amniotic sacs. This type of pregnancy have a "mono-chorial" placenta and is "bi-amniotic" and is known as monochorionic-diamniotic identical pregnancy.
• Second type of identical pregnancy consists of only one placenta and only one amniotic sac. Which means that both the babies will share same placenta and same amniotic sac. This type of pregnancy has "mono-chorial" placenta and is "mono-amniotic" and is known as monochorionic-monoamniotic identical pregnancy.
• The third type of identical pregnancy is quite similar to the fraternal twin pregnancy. In this there are two placenta as well as two amniotic sac which means both the babies have their own placenta and amniotic sac. This type of pregnancy has "bi-chorial" placenta and is "bi-amniotic". It can be called dichorionic-diamniotic identical pregnancy.
• The fourth type of identical twins is of conjoined twins. In this there is only one placenta and one amniotic sac. But due to some complications the zygote is unable to divide properly into two which finally results in conjoined babies.

Fraternal Twins
Fraternal twins are known as biovular or dyzygotic twins or non-identical twins. So, what are fraternal twins. In this type of pregnancy, two different eggs are fertilized by two different spermatozoids. This may be possible when both the ovaries of a woman ovulate at the same time and get fertilized by different spermatozoids. Both the embryo will possess different characteristic as they will have different number of chromosomes, may be of the same or different sex and may or may not have the same blood group. It can be said that there are two different pregnancies which took place at the same time. The embryos will have different placentas and different amniotic sac. It can be called "bi-chorial" and "bi-amniotic". This is very similar to the third type of identical pregnancy. The growth of fraternal twins will differ from each other whereas the identical twins will follow the same growth pattern. Fraternal twins are more common than the identical twins. Although they are born together but they are almost like normal siblings as they rarely have any identical characters.

So, these were some of the points and facts which differentiate identical twins from the fraternal twins. Identical twin pregnancy is considered to be risky as there is a danger of tangling of umbilical cord of the babies which may even cause fetus death. Hence, it's very necessary to get a regular check up done if you are carrying identical twins. But it is believed that identical twins share a more stronger bond than the fraternal twins. Well, we hope this Buzzle article helped you to understand about the difference between both the types of twins biologically.