How is twins formed

Twins are relatively uncommon. However, the birth of twins has increased significantly alongside the rise in the use of fertility treatments and with more women choosing to have children at a later age. Since , the birth rate of twins has increased by more than 75 percent. A woman is more likely to have fraternal twins than identical twins. Identical twins account for only about one-third of multiple pregnancies.

A woman has a slightly higher chance of having twins if she has a family history of twins. However, this only applies if conception takes place without the use of fertility treatments. According to the ASRM , women who are themselves nonidentical twins produce twins in around 1 in every 60 births. For men who are nonidentical twins, the chance of conceiving twins is only 1 in every births. Some people believe that twins can skip a generation, meaning that a person will potentially have twins if one of their grandparents did.

However, there is little evidence to support this theory. The ASRM note that the main factor that increases the chance of having twins is the use of fertility treatments. The various types of fertility treatment available increase the likelihood of twins in different ways. If sperm fertilizes both of these eggs, this can result in twins.

To increase the likelihood of success, the healthcare professional may place more than one embryo into the womb. Twins can occur if both embryos implant and develop successfully. The reason for this is that women of this age are more likely than younger women to release more than one egg during their reproductive cycle.

If sperm fertilizes two separate eggs, a twin pregnancy can occur. The ASRM report that nonidentical twins are slightly more common in taller or heavier women than they are in small women.

The reasons for this are not clear, but it may be due to better nutrition. Women with a higher body weight may have more resources available for a developing fetus. In the U. There are many unproven claims about how to increase the likelihood of conceiving twins.

Some people advise following specific diets or using certain alternative therapies, but there is no scientific evidence to support these methods. Fertility treatments, particularly IVF and ovary stimulants, do increase the chances of having twins. However, a twin pregnancy is riskier for both the woman and the developing fetuses. Measure content performance. Develop and improve products. List of Partners vendors. There is a mystique associated with multiples.

Twins and multiples are the subjects 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 misunderstandings is the cause of twins and multiples. Not all twinning can be attributed to the same cause.

In order to understand the causes of twins, it's important to understand that there are two types of twins, classified by zygosity. They include:. 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.

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, which is 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 may have an effect on hormones and influence this process are:. Some other factors are thought to generate hyperovulation in women and cause twins, such as:. The causes of identical twinning are much vaguer. No clear explanation is offered by science.

No specific theory has been confirmed as to why a fertilized egg splits and develops into two embryos. Data about monozygotic twins in populations indicates that the rate generally remains stable across populations and time periods. The two embryos have the same genetic components to start, but slight differences develop over time.

When twins are born with genetically different sexes one twin is genetically male and the other is genetically female they are usually dizygotic. Monozygotic twins are typically born as the same genetic sex. However, while it's extremely rare, it is possible for monozygotic twins to be two different sexes.

This goes back to the idea that genetic mutations can occur from the very first split. If an egg carries two X chromosomes when a normal egg should carry just one X chromosome and is fertilized with a Y sperm, you can get an XXY embryo. This is also known as Klinefelter syndrome. However, what happens if that XXY embryo splits into monozygotic twins?

You can end up with one twin with XX expression female , and the other with XY male. This situation is so rare that it has only been described in medical literature four times.

There is another rare way to have identical twins of different sexes. If you have an egg with one X chromosome, and sperm with a Y chromosome, you would typically get a boy XY. Thus, one twin will be a girl with the congenital disorder known as Turner syndrome , and the other twin will be a boy. All this said, these situations are so rare that it's safe to assume that A rare form of monozygotic twins is semi-identical or half-twins, which occurs when two separate sperm fertilize one egg.

The first case was only identified by researchers in , and another was found in At first glance, it appeared that the twins would be identical, but they were born as genetically different sexes. Conjoined twins are another rare form of monozygotic twinning, where the twins do not fully separate when the zygote splits. They may share multiple organs. Most conjoined twins die in utero or are stillborn. Most of the time, monozygotic twins have separate amniotic sacs but share one placenta.

Sharing one placenta increases risks to the pregnancy, due to the possibility of twin-to-twin transfusion syndrome. The pregnancy must be watched very carefully if Mo-Di twins are diagnosed. Another possibility is for the twins to each have their own placenta and amniotic sac. This is known as dichorionic-diamniotic or Di-Di twins. The risks in a Di-Di pregnancy are lower than with a Mo-Di pregnancy.

There is a misconception that Di-Di twins are always fraternal non-identical , but this is not true. The riskiest combination is when twins share one amniotic sac and one placenta. This only occurs in monozygotic twins and never with non-identical twins.

With Mo-Mo twins, the biggest risk is that the babies can become entangled in the umbilical cords. There is also a risk of twin-to-twin transfusion syndrome and a higher risk of prematurity. The only way to be sure about zygosity is with a DNA test.

If the twins have the same DNA markers, they are most likely monozygotic. If they have different markers, they are dizygotic. The genetic similarities in the DNA of monozygotic twins explain why they look remarkably similar and can even share similar traits, interests, behaviors, and personalities. Monozygotic twins will also have the same blood type. Sometimes, zygosity can be determined during pregnancy , but it depends on the timing of the split.

Some monozygotic twins develop with a single, shared placenta and are enclosed in a single chorion or amnion. Healthcare providers can look for these telltale signs on an ultrasound, but it's not definitive of monozygotic twins. Amniocentesis can also be used to determine twin zygosity before birth, but it is an invasive procedure and is not generally part of routine prenatal care.

There is some research looking into using less-invasive prenatal DNA testing to determine twin zygosity. DNA tests can be used to compare the twins' genetic markers and confirm that they are monozygotic. However, genetics do not determine everything about a person.

For example, research has shown that monozygotic twins do not necessarily have the same genetic risk factors for disease. Furthermore, identical twins do not have the same fingerprints. While the patterns are similar, they are not exactly alike. A set of identical twins is still made up of two separate people. Environmental influences, epigenetic differences, and life experiences make each twin a unique individual.

Get diet and wellness tips to help your kids stay healthy and happy. Krasnewich D. Identical twins. National Vital Statistics Reports. Published November 27, Twin births.