Fraternal Twins

Introduction

Fraternal twins usually occur when 2 fertilized eggs are implanted in the uterus wall.

Unlike identical twins, fraternal twins have 2 placentas (which provide vital substances to the baby during pregnancy). As they have their own placentas, fraternal twins are not at risk for some of the risky conditions that affect identical twins, such as TTTS or monoamniotic twins.

Fraternal twins usually do not look alike as they have a very small chance of having the same chromosome profile. It is quite common to have twins of different genders. Some people may even regard fraternal twins as, simply, siblings of the same age.


 How are they formed? 

The 2 most common ways fraternal twins develop are superfecundation and superfetation.
                                                                                 
Superfecundation:
  • A female ovulates more than 1 egg during her cycle (a process called hyper ovulation). Different sperms fertilize each of the 2 eggs.
  • The female may have inherited a gene that causes hyperovulation, thus the common conception that "twins are hereditary". Many fertility drugs cause a female to hyperovulate too.

Superfetation:
  • A female ovulates and releases another egg when she is already pregnant. A different sperm then fertilizes the second egg.
  • While the twins have different conception dates the babies will most likely be delivered at the same time, although they may be of different size and at different developmental stages.

In-vitro fertilization - where fertilized eggs are implanted in the female - is a form of superfetation. Fertility treatments, whether by drugs like Clmid or by procedures like in-vitro fertilization, sometimes produce twins or multiples, and in majority of these births, the twins are fraternal.

<br><a href="http://www.freecodesource.com" target="_blank"><img src=""></a><br>





      Here's a self-made video showing the formation of 
     fraternal twins.

Sources