The differences between identical and fraternal twins.

Based on the French version at

Roughly translated, reproduced and enhanced by kind permission.  

Identical Twins (also known as Uniovular or Monozygotic twins). 

During the fertilization process, only one ovum is impregnated by only one spermatozoid, but the egg divides in 2 embryos afterwards. They have exactly the same chromosomes, they have a perfect resemblance and they have the same sex.

There are 3 different ways that identical twins can develop and be carried: 

• With the first case there is only one placenta that feeds the babies, but there can be two amniotic sacs (as in the diagram at the side).  When there is 1 placenta and 2 amniotic sacs then the pregnancy is referred to as having a "mono-chorial" placenta and is "bi-amniotic".

• In the second scenario, there is only one amniotic sac.  In the case of 1 placenta and 1 amniotic sac then the pregnancy is referred to as having a "mono-chorial" placenta and is "mono-amniotic".

• The third scenario is where there are two placentae and each embryo has its own amniotic pocket. When there are two placentas and two amniotic pockets like this, then one speaks of a "bi-chorial" pregnancy that is "bi-amniotic". (This last case is an identical situation to fraternal twins as shown below).

Non-Identical Twins (also known as Fraternal, Binovular or Dyzygotic twins). 

When twins are non-identical, then two seperate eggs have been impregnated by 2 separate spermatozoa.

They each have a different chromosome make-up, they don't look alike and they can be either the same or different sex.  In fact, this is simply two separate pregnancies that just happened to occur at the same time.

Each embryo is in its own amniotic sac and has its own placenta.  This type of pregnancy is referred to as "bi-chorial" and "bi-amniotic".