The majority of twins are fraternal twins, yet there is a lot of confusion about what that means. 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.
1. Fraternal twins form from two separate eggs.
Photo reprinted with permission of tabreedy.
Fraternal twins form from two separate eggs and two separate forms. They are also known as dizygotic
. The term describes how they form and distinguishes them from identical -- or monozygotic
-- twins. "Di" means two, and zygotic refers to zygote, the combination of egg and sperm that will develop into an embryo and grow into a baby. Monozygotic twins from from a single egg and sperm that splits into two after conception.
2. Fraternal twins can be either gender or a combination of boys and girls.Because fraternal twins originate from separate conceptions, they can be boys, girls, or one of each. Chromosomes from the father's sperm determine gender: XX for a girl, and XY for a boy, so the chances of fraternal twins being boys, girls, or a combination are the same as for any other babies. About one-third of fraternal twins are two girls, one-third are two boys, and one-third are boy/girl. (Monozygotic -- identical -- twins, on the other hand, are always the same gender, either two girls or two boys.)
3. Fraternal twins are as genetically similar as any other siblings.Just as any brothers and sisters, fraternal twins will have about half of their genetic makeup in common. They will share about 50% of their DNA, the same as any siblings. Each individual receives half of their DNA from Mom's egg and half from Dad's sperm, and so any two offspring will have some overlapping qualities.
4. Fraternal twins may or may not look or act alike.Monozygotics -- who share the same DNA because they originated from a single sperm/egg combination -- are called "identical" because they often have remarkably similar appearances and characteristics. Fraternal twins, on the other hand, are as alike as any two siblings. They may look alike -- sometimes even "identical" -- or they may look very different. They can have different hair color, different eye color, different stature, different personalities. Or, they may indeed be so similar that they are assumed to be identical, just as some siblings would be remarkably indistinguishable, if only they were the same age. Twins and multiples are also shaped by their environment, and some similarities are enhanced because they are raised in the same home, share the same experiences, and are educated in the same classroom.
5. Fraternal twins have two placentas.
During pregnancy, the placenta provides vital sustenance to the baby. In a multiple pregnancy with fraternal twins, a placenta develops for each baby. Sometimes, however, the two placentas fuse together and appear to be one single placenta. Since some monozygotic twins have one placenta
, this can make it difficult to determine zygosity in utero. Because they have their own placentas, fraternal twins are not at risk for some of the risky conditions that affect monozygotic twins, such as TTTS
or monomniotic twins
6. Fraternal twins can run in families or be influenced by heredity.
Fraternal twins occur when more than one egg is fertilized. Normally, one egg is released from the ovaries each month, but sometimes there is more than one. Some women release multiple eggs in every cycle, a condition called hyperovulation. Women who hyperovulate are more likely to have fraternal twins. A tendency towards hyperovulation can be a genetic trait. In this way, fraternal twinning can be hereditary
. A woman who has the gene for hyperovulation can pass it down to her daughter. Then the daughter's chances of having twins is increased. Because men carry both X (female) and Y (male) chromosomes, they can also hold the trait for hyperovulation and pass it along to their daughters, increasing their daughter's chances of having fraternal twins. However, having the gene for hyperovulation does not increase a man's chances of fathering fraternal twins. (Men carry the gene, but they don't ovulate!) That is why twins are sometimes assumed to "skip a generation."
7. Fraternal twins can be conceived at different times or by different fathers.
Usually one egg is released during ovulation. But in cases of hyperovulation, multiple eggs are released. Sometimes that happens with a few day's interval in between. After one egg is fertilized and begins to travel to the uterus for implantation, another egg is fertilized by sperm from a later incident of sexual intercourse. The result is fraternal twins who are actually conceived a few days apart. This phenomenon is known as superfetation
. There have even been instances of fraternal twins with different fathers. This occurs when a woman releases mulitple eggs, and has sexual relations with more than one partner. If an egg is fertilized by sperm from one man, and then another egg is fertilized by sperm from another man, the result is fraternal twins with different fathers. This phenomenon is termed superfecundation
8. Fraternal twinning rates vary across populations.
Not all races and cultures have the same rates of twinning. Population studies have shown that some groups of people have twins much more frequently, while twins are rare among other groups. A 2011 study
showed that the highest rates of twinning were found in Central African populations, with the country of Benin producing the most twins. Asia and Latin America had the lowest rates of twinning.
9. Fraternal twins can be the result of fertility treatments.
As medical technology made fertility-enhancing treatments more accessible, the twin birth rate skyrocketed in the late twentieth century. Fertility treatments, whether drugs like Clomid
or procedures like in-vitro fertilization (IVF
), sometimes produce twins or multiples, and in the majority of these births, the multiples are dizygotic. Some instances of monozygotic twinning do occur in IVF.
10. Fraternal twins can be caused by other factors.
Many of the factors that influence multiple birth
only impact fraternal twinning. Heredity, maternal age, birth control pills, and diet can all contribute to a woman's chances of producing multiples, but only dizygotic multiples. That's because all of these factors can encourage hyperovulation, prompting the release of more than one egg per cycle and increasing the chances of having twins