Causes of Identical and Fraternal Twins

How and why do twins form?

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Causes of Twins. Nino H Photography/Moment / Getty Images

There is a mystique associated with multiples. Twins and multiples are the subject of much interest and attention. Although they have become more common in recent generations, they are still a relatively rare phenomenon. For every one hundred people you encounter, only three will be a twin, and much fewer can claim to be a triplet or higher order multiple. Because of their scarcity, there are many misunderstandings about them.

One of the most common is the cause of twins and multiples. 

It’s human nature to be inquisitive about the cause of a phenomenon. When something happens, we wonder “why? For families with multiples, the questions arise from the first moment they discover that they are expecting twins or more. How did this happen? Why did this happen? Others are curious about the causes of twins because they want to have twins, or perhaps they want to avoid having twins. 

Not all twinning can be attributed to the same cause. In order to understand the causes of twins, it is important to understand that there are two types of twins, classified by zygosity. Monozygotic multiples form from a single zygote (fertilized egg) that splits after fertilization. Dizygotic or multizygotic twins form from two or more zygotes, separate eggs fertilized by separate sperm that share the same period of gestation. Dizygotic or multizygotic twins are more commonly known as fraternal twins.

With their own unique combination of genes, they have the same genetic connection as any other siblings but share a womb during gestation. Dizygotic twins can be boys, girls, or a combination. Monozygotic twins are more commonly known as identical twins; because they originate from a single egg/sperm combination and share the same genetic background, they often look alike and share many of the same characteristics.

Monozygotic twins are always the same gender (with very few, very rare exceptions). 

The Process of Reproduction

During a normal cycle of ovulation, a single egg (or oocyte) is released from a woman’s ovaries. If the egg is fertilized by sperm from a man during sexual intercourse, the resulting zygote travels to the woman’s uterus, dividing and duplicating through the process of mitosis, where it will implant and grow into an embryo and eventually a fetus.

How Dizygotic (Fraternal) Twins Form

Sometimes, more than one egg is released during ovulation. If two eggs are fertilized during sexual intercourse and both successfully implant in the uterus, the result is a multiple pregnancy. If more than two eggs are released, fertilized and implant, the result is multizygotic multiples, higher order multiples such as triplets (3), quadruplets (4), quintuplets (5), sextuplets (6), septuplets (7), octuplets (8) or even more, although no multiples beyond octuplets have ever been known to survive. 

The Causes of Dizygotic (Fraternal) Twins

Dizygotic or fraternal twins form in the same way that all humans do, by the union of sperm and egg.

The explanation for dizygotic twinning lies in the cause of hyperovulation, or the release of more than one egg in a cycle of ovulation. There are numerous reasons for hyperovulation, and any one or combination of factors can be attributed as the cause for fraternal twins. 

Hormones control the process of ovulation. Usually, they signal the body to release one egg in a cycle. But sometimes they trigger the release of two or more eggs. Some of the factors that have an effect on hormones and influence this process are:

  • Discontinuing the use of birth control pills, or irregular usage of birth control pills
  • Breastfeeding
  • Using fertility drugs such as Clomid
  • Obesity, defined as women with a BMI over 30, as extra fat stores produce increased levels of estrogen

Some other factors are thought to generate hyperovulation in women and cause twins, such as:

Genetics: it’s thought that some women just hyperovulate regularly, and that there is a gene that causes them to do so. Although both men and women carry the gene, only women ovulate. So a woman with the gene who hyperovulates may have fraternal twins. A man with the gene wouldn’t be more likely to have twins, but he may pass the trait to his daughter, and maybe she’d be a candidate to have twins.

Family History: It’s thought that women who have had conceived and borne several children already are more likely to have twins. While the previous pregnancies don’t cause hyperovulation, or cause twins, it’s possible that the mother’s womb is just more hospitable to sustaining a twin pregnancy. 

Maternal Age: As a woman grows older, she is more likely to hyperovulate; perhaps it is the body’s accelerated attempt to reproduce before time runs out. Women over the age of 30 are more likely to have twins, and the rate increases even more after the age of 35. 

Height: Taller women have a higher than average rate of twin pregnancies. The reasons aren’t particularly specific, but perhaps it is because increased height is associated with better nutrition, or the additional height provides more “room in the womb,” making a multiple pregnancy more likely to thrive.

Race: Women of African descent produce a higher number of twins than women of Asian descent. 

Diet: High rates of twins have been found in cultures where the diet is rich in a type of yams that contain phytoestrogen. One example is the Yoruba tribe in Nigeria, which has the highest rate of twins of any population in the world. 

What Causes Monozygotic (Identical) Twins?

The causes of identical twinning are much more vague. No clear explanation is offered by science. Data about monozygotic twins in populations indicates that the rate generally remains stable across populations and time periods. No specific theory has been confirmed as to why a fertilized egg splits and develops as two embryos. 

As technology improves, scientists are getting closer to finding answers. One 2007 study used specialized computer software to capture photos of embryos developing and found that the embryo “essentially collapses,” splitting the progenitor cells in half and divided into two sets of genetic material that form as two separate fetuses. 

While the discovery was important, it still didn’t pinpoint the reason for the split or explain exactly why identical twins happen. No genetic link has been identified. Some theories have been proposed but not confirmed. These include: 

  • an enzyme in sperm
  • a mutation in the cells of the blastocyst 
  • age of the egg after ovulation

Identical twins are generally considered to be random and unexplained. The mystery is part of their magic and mystique! 

Do Fertility Treatments Cause Twins?

Dizygotic Twins: Increased utilization of fertility treatments have definitely resulted in an increase in twin births. Fertility enhancing drugs and injections contribute to hyperovulation and can cause dizygotic twins. Artificial insemination (IUI Treatment) does not necessarily increase the rate of twins, but is usually accompanied by a routine of fertility enhancing drugs that do. IVF treatment (in vitro fertilization) may cause twins. This treatment involves transferring embryo(s) (or fertilized eggs) to the mother’s womb; often two or more embryos are transferred to increase the chances of a successful outcome. 

Monozygotic Twins: Generally, fertility treatments are not considered a cause of identical twins. However, the rate of monozygotic twinning is slightly higher among pregnancies produced by reproductive assistance, particularly in IVF situations where an embryo is fertilized outside the womb and transferred to the mother. However, as with natural conceptions of monozygotic twins, the reasons aren’t expressly understood. 

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