Fraternal Twins
The term ‘Fraternal Twins’ is more commonly used to replace the medical term, ‘Dizygotic Twins’ (DZ), that essentially means ‘non–identical’ twins. This is opposed to ‘Mono Zygotic Twins’ (MZ), which are identical twins. Twin sets can be either boy/boy, boy/girl or girl/girl sets.
Fraternal twins can occur during pregnancy as a result of two eggs being separately fertilized during conception, by two independent sperm cells. The term ‘bi Ovular’ is often applied, as two eggs would have been released during ovulation. These two eggs form two Zygotes and then attach themselves to the uterine wall. The pregnancy progresses as normal, with the babies growing independently, with their own placentas. Fraternal twins are essentially siblings that share the womb at the same time.
Fraternal twins occur not just as a result of hyper ovulation (Super fecundation), but can also occur when the female is already pregnant, but releases another egg, which is then fertilized (Superfetation). It is estimated that the difference in conception time of Superfetation twins can be as long as twenty four days! Mono zygotic twins occur during pregnancy as a result of the a single sperm fertilising a single egg, forming one zygote, but then splitting into two embryos that share the same placenta for the duration of the pregnancy.
Being a twin yourself will increase that changes of having fraternal twins. Also, fraternal twins occur more commonly in slightly older women, who are above average weight and height and have already experienced several pregnancies. Fraternal twins most commonly occur in Africa, and least commonly occur in Asia. Fraternal superfetation twins can even be fathered by different men!
