Types of twins

After sharing my news I have come to realize the general public has no basic knowledge of different twins types.

So I am going to explain the three types as best possible.

  

There are two main types of twins fraternal and identical. From these two types there are three different types in utero.

Fraternal twins are created by two different eggs and two different sperm. Genetically fraternal twins are related in the same way siblings are, they are just in utero and the same time and share a birthday! 

Fraternal twins are the most common type of twins. They most often run in families because hyper ovulation (the release of more than one egg during a menstrual cycle) is said to be a genetic trait. 

There has also been a rise in the fraternal twin rate due the increase of fertility medicines. 

In utero fraternal twins are called di/di twins because they do not share a sac or placenta. 

The second type of twins is identical twins. In utero they can be called mo/mo twins, mo/di twins, or di/di twins. It is important to know what time of twins you are pregnant with because there is different risks with each type. 

Identical twins are said to be completely random and it is generally believe that these type of twins do not run in families. (I have seen a few debates over this.) The identical twinning rate has not changed over time since it is a random act. Identical twins are formed from 1 egg and 1 sperm, meaning they share the same DNA. During cell division the cell splits an extra time creating an extra identical embryo.  This is actually an error in cell division! A beautiful error! 

Depending on when in cell division the embryo split determines the type of twins you have in utero. 

mo/mo twins - most risky, they completely share a sac and there is no membrane to separate them, they also share a placenta. Rare - 1% -5% of identical twin pregnancies  

mo/di twins - still high risk, they share a sac but there is a membrane to separate them, they share a placenta. about 70% of identical twin pregnancies

di/di twins -  least risky, do not share a sac or placenta (but can fuse as seen in picture). about 30% of identical pregnancies, in utero, there is no way to tell if di/di are fraternal or identical (unless a boy and girl) this has to be confirmed through DNA testing after birth. 

When I had my first ultrasound it was easy to see that my twins share a sac. It was not clear if there was a membrane to separate them. My doctor believes she saw one, but because it was so early (7 weeks), I will go back for another ultrasound at 12 weeks to check again on the type of twins I am carrying.

The doctor seemed pretty certain these twins are MO/DI. 

Now that you may understand the different type of twins a little better... perhaps you understand our shock when we found out we were having twins again... except this time identical!!