There are two types of twins: identical (also called monozygotic, abbreviated MZ) and fraternal (also called dizygotic, abbreviated DZ). A zygote is the fertilized egg formed when a sperm combines with (fertilizes) an egg at conception. The zygote then starts to divide and forms an embryo. The embryo is called a fetus in later stages of prenatal (before birth) development. Monozygotic means “one zygote.” MZ twins develop when one egg is fertilized by one sperm to produce a single zygote. The zygote starts to develop into an embryo, but at some time during the first two weeks after conception, the developing embryo splits into two identical parts. Each part develops into a baby. The two babies are genetically identical: they share 100% of their genetic material (genes, made of DNA) and are always the same sex. Dizygotic means “two zygotes.” DZ twins develop when two eggs are released at the same time, and are fertilized by two different sperm. These two zygotes are each implanted in the womb (uterus) and develop at the same time. On average, DZ twins share half their genetic material, which is the same amount shared between any pair of siblings. In fact, it has been jokingly said that the only difference between DZ twins and non-twin siblings is that DZ twins have been “womb-mates” for nine months. DZ twins may be of the same sex or opposite sexes.
The likelihood of conceiving twins is a complex trait. It is probably affected by multiple genetic and environmental factors, depending on the type of twins. The two types of twins are classified as monozygotic and dizygotic. Monozygotic (MZ) twins, also called identical twins, occur when a single egg cell is fertilized by a single sperm cell. The resulting zygote splits into two very early in development, leading to the formation of two separate embryos. MZ twins occur in 3 to 4 per 1,000 births worldwide. Research suggests that most cases of MZ twinning are not caused by genetic factors. However, a few families with a larger-than-expected number of MZ twins have been reported, which indicates that genetics may play a role. It is possible that genes involved in sticking cells together (cell adhesion) may contribute to MZ twinning, although this hypothesis has not been confirmed. Most of the time, the cause of MZ twinning is unknown. Dizygotic (DZ) twins, also called fraternal twins, occur when two egg cells are each fertilized by a different sperm cell in the same menstrual cycle. DZ twins are about twice as common as MZ twins, and they are much more likely to run in families. Compared with the general population, women with a mother or sister who have had DZ twins are about twice as likely to have DZ twins themselves. DZ twinning is thought to be a result of hyperovulation, which is the release of more than one egg in a single menstrual cycle. To explain how DZ twinning can run in families, researchers have looked for genetic factors that increase the chance of hyperovulation. However, studies examining the contributions of specific genes have had mixed and conflicting results. Few specific genes in humans have been definitively linked with hyperovulation or an increased probability of DZ twinning. Other factors known to influence the chance of having DZ twins include the mother’s age, ethnic background, diet, body composition, and number of other children. Assisted reproductive technologies such as in vitro fertilization (IVF) are also associated with an increased frequency of DZ twins. Scientific journal articles for further readingHoekstra C, Zhao ZZ, Lambalk CB, Willemsen G, Martin NG, Boomsma DI, Montgomery GW. Dizygotic twinning. Hum Reprod Update. 2008 Jan-Feb;14(1):37-47. Epub 2007 Nov 16. Review. PubMed: 18024802. Machin G. Familial monozygotic twinning: a report of seven pedigrees. Am J Med Genet C Semin Med Genet. 2009 May 15;151C(2):152-4. doi: 10.1002/ajmg.c.30211. PubMed: 19363801. Mbarek H, Steinberg S, Nyholt DR, Gordon SD, Miller MB, McRae AF, Hottenga JJ, Day FR, Willemsen G, de Geus EJ, Davies GE, Martin HC, Penninx BW, Jansen R, McAloney K, Vink JM, Kaprio J, Plomin R, Spector TD, Magnusson PK, Reversade B, Harris RA, Aagaard K, Kristjansson RP, Olafsson I, Eyjolfsson GI, Sigurdardottir O, Iacono WG, Lambalk CB, Montgomery GW, McGue M, Ong KK, Perry JR, Martin NG, Stefánsson H, Stefánsson K, Boomsma DI. Identification of Common Genetic Variants Influencing Spontaneous Dizygotic Twinning and Female Fertility. Am J Hum Genet. 2016 May 5;98(5):898-908. doi: 10.1016/j.ajhg.2016.03.008. Epub 2016 Apr 28. Pubmed: 27132594. Painter JN, Willemsen G, Nyholt D, Hoekstra C, Duffy DL, Henders AK, Wallace L, Healey S, Cannon-Albright LA, Skolnick M, Martin NG, Boomsma DI, Montgomery GW. A genome wide linkage scan for dizygotic twinning in 525 families of mothers of dizygotic twins. Hum Reprod. 2010 Jun;25(6):1569-80. doi: 10.1093/humrep/deq084. Epub 2010 Apr 8. PubMed: 20378614. Free full-text available from PubMed Central: PMC2912534. Shur N. The genetics of twinning: from splitting eggs to breaking paradigms. Am J Med Genet C Semin Med Genet. 2009 May 15;151C(2):105-9. doi: 10.1002/ajmg.c.30204. PubMed: 19363800.
We usually describe twins according to:
The main types of twins are fraternal twins and identical twins. Most twins are born healthy, but a twin pregnancy can mean a higher chance of health complications for you and your babies. If you're pregnant with twins, you and your babies will need extra check-ups. Fraternal twinsAll pregnancies start when a sperm fertilises an egg. This fertilised egg is called a zygote. Sometimes a woman’s ovaries release 2 eggs, and a separate sperm fertilises each egg. This forms twins. These twins are called fraternal twins, dizygotic twins (meaning two zygotes) or non-identical twins.  During pregnancy, the developing babies get oxygen and food from their mother through the placentas and umbilical cords. Fraternal twins have separate placentas and umbilical cords. The technical name for this is dichorionic. Fraternal twins can be the same or opposite sex and their genes are as different as any other brother and sister. Often, same-sex fraternal twins look different. For example, they might have different hair or eye colour. Occasionally they look quite similar. Fraternal twins happen in about 70% of twin births in Australia. Very rarely, fraternal twins share a placenta. These types of twins are called chimaeric twins. Identical twinsSometimes a fertilised egg splits within a few days of conception to produce genetically identical twins. Because these twins come from one zygote, they’re also known as monozygotic. Identical twins are the same sex. There are 3 types of identical twins. About one-third of identical twins split soon after fertilisation and form completely separate twins. Like fraternal twins, these twins have separate placentas. The other two-thirds split after they attach to the wall of the womb. As a result, they share a placenta. The technical name for this is monochorionic. In a very small number of identical twins, splitting might happen even later. In this case, both twins share an inner sac, called the amnion, in addition to sharing a placenta. The technical name for this is monoamniotic twins. They’re often called MoMo twins. Identical twins happen in around 1 in 250 pregnancies in Australia. Although identical twins have the same genes, they don’t always look the same. This is because children’s health and development are shaped not only by genes but also by experiences in the womb and after birth. For example, a twin who gets less blood from a shared placenta might weigh less at birth. Sharing a placenta or inner sac: why it’s important to knowSharing a placenta means that twins share a blood supply during pregnancy. Sometimes the blood supply is shared unequally, which can cause health problems for both twins. Women who are pregnant with twins sharing a placenta need to be checked more often than women who are pregnant with twins with separate placentas. Frequent checks help to pick up any complications early. Twins sharing an inner sac (monoamniotic) are also at a higher risk of complications during pregnancy because of the chance that their umbilical cords might tangle and cut off their blood supply. These twins are checked even more closely. Medical professionals often recommend that these twins are born at 32-34 weeks. This is earlier than other types of twins, who are generally born at 37-38 weeks. Medical professionals use ultrasound to work out how many placentas twins have. The earlier the ultrasound, the more accurately it can say how many placentas there are. It gets harder to work out later in pregnancy. After the birth, medical staff will look at the placentas to check what type of twins they are. Fraternal or identical: why it’s important to knowSame-sex twins with separate placentas can be fraternal or identical. For health reasons, it’s good to know whether your twins are fraternal or identical. To find out whether twins are identical or fraternal, you can ask for a genetic test after your babies are born. This is called a zygosity test. The test doesn’t hurt and involves collecting a sample of cheek cells by rubbing the inside of your babies’ cheeks with a soft applicator (like a cotton bud). The cost of this test starts at $199 (for both twins) in Australia. Identical twins are more likely than fraternal twins to get the same illness. If one of a pair of identical twins is diagnosed with a particular disease or health condition, like high blood pressure, the other twin should be checked often for early symptoms. Because of their genetic make-up, identical twins will always be compatible for organ transplantation, if they ever need it. Fraternal twins are compatible only sometimes. Other reasons why twins and parents of twins might want to know whether they’re fraternal or identical include:
Some identical twins are mirror twins – for example, their hair parts on opposite sides, they are oppositely handed, or they have birthmarks on opposite sides of their body. In some rare cases, their internal organs can be mirror images of each other. It’s not known why some twins are like this. In very rare cases, twins can be born physically joined together in different ways. These twins, called conjoined twins (previously called Siamese twins), can happen if the fertilised egg splits quite late after fertilisation. Triplets (1 in 5000 births) and quadruplets (less than 1 in 100 000 births) can develop as a result of combinations of fraternal and identical twinning. There are no reliable figures for quintuplets (5 babies) and sextuplets (6 babies). More about twinsIn Australia, twins happen in 1 in every 80 births. This means that 1 in 40 Australians is a twin. The birth rate of identical twins is the same around the world and doesn’t vary with the mother’s age. In contrast, the birth rate of fraternal twins varies widely across countries and can be influenced by the mother’s age. Women aged over 35 years are the most likely to have fraternal twins, because their ovaries are more likely to release more than one egg at a time. More twins were born from the 1990s to the mid-2000s. This is because it was common to transfer more than one embryo during in-vitro fertilisation (IVF) treatments. The rate of twins from IVF is now lower, because it’s now usual practice to transfer only a single embryo. |
Fraternal twins are referred to as monozygotic twins because they come from two zygotes.
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