It is a well known fact that there usually is only one egg ready to be fertilized in the fallopian tubes during ovulation. However, in some cases it can so happen that two or more eggs are released at the same time. When this happens and more than one of these eggs are fertilized and implant in the uterus, fraternal twins (or in some cases triplets, quadruplets, etc.) will result. It is important to note that with fraternal twins, one fertilized egg means one offspring. So where two eggs are present and fertilized, the result is fraternal twins, where three eggs are present and fertilized the result is triplets, and so on. It is important to note that, outside of fertilization therapies, cases where more than two eggs are present and fertilized are not very common.
Fraternal twins are essentially siblings who were conceived at the same time and occupied a shared space within the womb of their mother. This is because with fraternal twin pregnancies, each fetus originates from its own unique egg. The contrast between fraternal and identical twins is due to their origins: while fraternal twins each have their own fertilized egg, identical twins come from the same egg cell which splits into two separate embryos.
The origins of two eggs versus one egg separating into two explains easily why fraternal siblings can share many of the same physical traits, just as you and your brother or sister might, however they do not tend to look identical.
Due to the fact that the splitting of an egg (for identical twins) or the multiple egg fertilization (fraternal twins) process occurs within the mother during the first week after fertilization, many twins and their parents will not immediately know their “twin-type”. A test that can clarify the type of sibling connection (whether twins are fraternal or identical) is called a twin zygosity test.
There are many factors, including genetic predisposition on the part of the mother, which can contribute to the occurrence of fraternal twins. Additionally, with modern fertilization techniques such as multiple implantations of eggs, multiple pregnancy had become much more commonplace, recent fertility treatment has become more refined.