The majority of twins are fraternal twins twins that look different from one another. Learn more about fraternal twinning, from how they form to how they relate to each other, as well as how they differ from identical twins. Zygotic refers to zygote, the egg fertilized by the sperm that will develop into an embryo and grow into a baby.
All pregnancies start when a sperm fertilises an egg. This forms twins. These twins are called fraternal twins, dizygotic twins meaning two zygotes or non-identical twins.
Twins are two offspring produced by the same pregnancy. In contrast, a fetus that develops alone in the womb is called a singletonand the general term for one offspring of a multiple birth is multiple. The Yoruba peoples have the highest rate of twinning in the world, at 45—50 twin sets or 90— twins per 1, live births,    possibly because of high consumption of a specific type of yam containing a natural phytoestrogen which may stimulate the ovaries to release an egg from each side.
Their mum Beata Bienias, 36, had been trying to have a baby for a decade and was about to have an embryro transfer. But she had no idea that when the embryo, which went on to become Amelia, was implanted, she was already pregnant with twins Matylda and Borys. Before falling pregnant, Beata had begun to give up hope of ever becoming a mother due to issues with her health, including polycystic ovaries.
Superfecundation is the fertilization of two or more ova from the same cycle by sperm from separate acts of sexual intercoursewhich can lead to twin babies from two separate biological fathers. Homopaternal superfecundation refers to the fertilization of two separate ova from the same father, leading to fraternal twins ;  while heteropaternal superfecundation is referred to as a form of atypical twinning where, genetically, the twins are half siblings. Superfecundation, while rare, can occur through either separate occurrences of sexual intercourse or through artificial insemination.
By Martha Cliff for MailOnline. The year-old siblings from Perth, Australia, also revealed that they are both present while one is making love with their partner of eight years, Ben Byrne. The sisters, who live with Ben and their mother said on the show: 'We sleep in a king size bed, bigger than a king size bed - about six inches more.
Back to Pregnancy. It depends on the type of twins. There's no evidence that identical twins run in families.
A pair of Australian siblings are some of the rarest humans ever documented, according to their doctors. The children are thought to be only the second case of semi-identical twins discovered, and the first to be identified during pregnancy. According to the authors, the siblings are a class of twins called sesquizygotic, something of a midpoint between fraternal and identical twins. Fraternal twins happen when two different sperm cells fertilize two different eggs, creating two different zygotes, both of which end up implanted in the uterus as developing embryos.
Having twins has its advantages. Of course, there is no denying the fact that having two babies together is a lot more burdensome than having one at a time. But imagine how you can avoid going through the pain of pregnancy one more time.
Despite this, they have both been documented in medical records. Both of these processes result in fraternal, or dizygotic, twins, since identical monozygotic twins can only come from a single fertilized egg. Superfecundation or superfetation can both occur naturally, but they can also be the result of certain fertility treatments.