Multiple births have become more common with modern medicine. Women are having children later than in previous generations, and advances in assisted and reproductive technologies enable more women to carry multiple babies. Twins account for 90% of all multiple births.
There are several types of twins, including monozygotic, dizygotic and the extremely rare sesquizygotic twins. Their differences are explained below.
Typical and atypical twins
Twin pregnancies are usually divided into two categories: fraternal (dizygotic) and identical (monozygotic).
Fraternal twins result when two different sperm fertilize two different eggs. Identical twins form when one egg is fertilized by one sperm and then divides into two embryos. Higher-order multiples, such as triplets and quadruplets, may be fraternal, identical or a combination. Doctors refer to this as typical twinning.
In rare cases, twin embryos may form in the uterus through a process that differs from the traditionally recognized mechanisms.
As doctors perform more assisted reproductive technology procedures, such as in vitro fertilization, they encounter more unconventional forms of twin conception. These are called atypical twinning.
The term covers twins conceived through fertilization processes that differ slightly from the usual mechanisms.
Atypically conceived twins may have genetic differences. In some cases, these differences may prevent the fetuses from surviving the pregnancy. Because such cases are extremely rare, experts do not yet fully understand them and continue to study the phenomenon and its possible causes.
What are monozygotic twins?
Monozygotic twins are commonly called identical twins. About one-third of all twins are identical. They form when one egg is fertilized by one sperm and divides into two during the first few weeks after fertilization, producing two genetically identical babies. They share 100% of their DNA.
Identical twins may share an amniotic sac, the thin-walled sac surrounding a baby during pregnancy. Whether they share the same sac depends on when the egg divides. These twins are always the same sex: either two girls or two boys.
Research suggests that about 1 in 4 identical twin pairs are mirror-image twins, meaning the right side of one twin corresponds to the left side of the other.
Experts still do not know exactly how or why a single fertilized egg divides in two.
What are dizygotic twins?
Dizygotic, or fraternal, twins are the most common type and account for two-thirds of all twins.
They form when two eggs are fertilized by two different sperm during the same pregnancy. Fraternal twins generally do not share a placenta because they arise from separate eggs and sperm. They also usually develop separate amniotic sacs and supporting structures. They share about half of their DNA.
Unlike monozygotic twins, they may not look exactly alike and do not have to be the same sex, so they may be a girl and a boy.
Genetically, fraternal twins are like other siblings because they share about half their genes. They may look alike, but genetically they are no different from siblings born at different times.
Are there other types of twins?
Sesquizygotic twins. Although extremely rare, a third type is known as sesquizygotic, or “semi-identical,” twins. Experts believe this may occur when one egg is fertilized by two sperm. The babies may share the same placenta and between half and all of their DNA.
Even when sesquizygotic twins form in the same amniotic sac, they do not have to be the same sex. When the fertilized egg divides, the twins may be a boy and a girl, which is not possible with identical twins.
Sesquizygotic twins are exceptionally rare. Fertilization of one egg by two different sperm produces three sets of chromosomes—the threadlike structures that contain DNA—and studies suggest these embryos usually do not survive.
Only a few cases of sesquizygotic twins have been identified, and experts say more research is needed.
Quaternary twins. This rare and little-studied phenomenon occurs when one pair of identical twins has children with another pair of identical twins within a short period, usually less than nine months.
The two children do not share parents or identical DNA. They are cousins, but genetically resemble siblings born to the same parents more closely and may look very similar. The phenomenon raises interesting questions about twin genetics and family resemblance.
Genetic and environmental factors in twinning
Both genetics and environmental factors are important. Some families have a hereditary tendency toward twins, especially fraternal twins. A woman with a family history of twins may be more likely to have twins. Identical twinning, however, appears to occur randomly and is unrelated to family history.
Environmental factors such as maternal age, use of assisted reproductive technology and overall maternal health may also affect the likelihood of twins. As women age, they may be more likely to release multiple eggs in one menstrual cycle, which can result in fraternal twins.
Summary
Twin birth is a complex and fascinating process involving genetics, chance and sometimes modern medicine. Identical, fraternal and rare sesquizygotic twins each have distinct characteristics and formation mechanisms. As science and technology advance, our understanding of these phenomena continues to deepen.
Insights | Types of twins: What you need to know
Types of twins: What you need to know
Multiple births have become more common with modern medicine. Women are having children later than in previous generations, and advances in assisted and reproductive technologies enable more women to carry multiple babies. Twins account for 90% of all multiple births.
There are several types of twins, including monozygotic, dizygotic and the extremely rare sesquizygotic twins. Their differences are explained below.
Typical and atypical twins
Twin pregnancies are usually divided into two categories: fraternal (dizygotic) and identical (monozygotic).
Fraternal twins result when two different sperm fertilize two different eggs. Identical twins form when one egg is fertilized by one sperm and then divides into two embryos. Higher-order multiples, such as triplets and quadruplets, may be fraternal, identical or a combination. Doctors refer to this as typical twinning.
In rare cases, twin embryos may form in the uterus through a process that differs from the traditionally recognized mechanisms.
As doctors perform more assisted reproductive technology procedures, such as in vitro fertilization, they encounter more unconventional forms of twin conception. These are called atypical twinning.
The term covers twins conceived through fertilization processes that differ slightly from the usual mechanisms.
Atypically conceived twins may have genetic differences. In some cases, these differences may prevent the fetuses from surviving the pregnancy. Because such cases are extremely rare, experts do not yet fully understand them and continue to study the phenomenon and its possible causes.
What are monozygotic twins?
Monozygotic twins are commonly called identical twins. About one-third of all twins are identical. They form when one egg is fertilized by one sperm and divides into two during the first few weeks after fertilization, producing two genetically identical babies. They share 100% of their DNA.
Identical twins may share an amniotic sac, the thin-walled sac surrounding a baby during pregnancy. Whether they share the same sac depends on when the egg divides. These twins are always the same sex: either two girls or two boys.
Research suggests that about 1 in 4 identical twin pairs are mirror-image twins, meaning the right side of one twin corresponds to the left side of the other.
Experts still do not know exactly how or why a single fertilized egg divides in two.
What are dizygotic twins?
Dizygotic, or fraternal, twins are the most common type and account for two-thirds of all twins.
They form when two eggs are fertilized by two different sperm during the same pregnancy. Fraternal twins generally do not share a placenta because they arise from separate eggs and sperm. They also usually develop separate amniotic sacs and supporting structures. They share about half of their DNA.
Unlike monozygotic twins, they may not look exactly alike and do not have to be the same sex, so they may be a girl and a boy.
Genetically, fraternal twins are like other siblings because they share about half their genes. They may look alike, but genetically they are no different from siblings born at different times.
Are there other types of twins?
Sesquizygotic twins. Although extremely rare, a third type is known as sesquizygotic, or “semi-identical,” twins. Experts believe this may occur when one egg is fertilized by two sperm. The babies may share the same placenta and between half and all of their DNA.
Even when sesquizygotic twins form in the same amniotic sac, they do not have to be the same sex. When the fertilized egg divides, the twins may be a boy and a girl, which is not possible with identical twins.
Sesquizygotic twins are exceptionally rare. Fertilization of one egg by two different sperm produces three sets of chromosomes—the threadlike structures that contain DNA—and studies suggest these embryos usually do not survive.
Only a few cases of sesquizygotic twins have been identified, and experts say more research is needed.
Quaternary twins. This rare and little-studied phenomenon occurs when one pair of identical twins has children with another pair of identical twins within a short period, usually less than nine months.
The two children do not share parents or identical DNA. They are cousins, but genetically resemble siblings born to the same parents more closely and may look very similar. The phenomenon raises interesting questions about twin genetics and family resemblance.
Genetic and environmental factors in twinning
Both genetics and environmental factors are important. Some families have a hereditary tendency toward twins, especially fraternal twins. A woman with a family history of twins may be more likely to have twins. Identical twinning, however, appears to occur randomly and is unrelated to family history.
Environmental factors such as maternal age, use of assisted reproductive technology and overall maternal health may also affect the likelihood of twins. As women age, they may be more likely to release multiple eggs in one menstrual cycle, which can result in fraternal twins.
Summary
Twin birth is a complex and fascinating process involving genetics, chance and sometimes modern medicine. Identical, fraternal and rare sesquizygotic twins each have distinct characteristics and formation mechanisms. As science and technology advance, our understanding of these phenomena continues to deepen.