TYPES OF TWINS

There are two different types of twins – monozygotic or identical (MZ) and dizygotic, fraternal or non-identical (DZ). 

Monozygotic twins develop when a single egg is fertilised by a single sperm and, at some stage in the first two weeks after conception, the developing embryo splits into two. As a result, two, genetically identical babies develop.

Dizygotic twins occur when two eggs are released at a single ovulation and are fertilised by two different sperm.  These two fertilised eggs then implant independently in the uterus.  Dizygotic twins share the same type of genetic relationship as non-twin siblings, hence the term fraternal.

FREQUENCY OF TWIN BIRTHS

There are approximately 4,300 pregnancies in Australia annually, with multiples making up about 1.66% of them. Birth rates for MZ twins are consistent among all races (about 4 per 1000); but the incidence of DZ twinning varies among races.  The rate among Caucasians is approximately 8 per 1000, while it is highest among people of African descent, about 16 per 1000, and lowest among Asians, about 4 per 1000.  A genetic predisposition or inherited characteristic for DZ twinning exists in some families, but the consistency of MZ twinning among all populations suggests that identical twinning is a random occurrence that is not influenced by genes.  Overall, about one in every eighty births in our country is a twin and of these about 30% are MZ. 

A dramatic increase in the number of DZ twin, triplet and quadruplet births occurred when new treatments for infertility, most involving the use of hormones to stimulate the ovulation of more than one egg, were developed in the late 1970s.  In treatments where the mature eggs are harvested and fertilized outside of the woman’s body, as is the case with IVF (In Vitro Fertilisation), two or more embryos have routinely been transferred back into the uterus in order to better the odds that at least one will implant successfully.  Surprisingly, treatment with ART (Assisted Reproduction Technologies) also seems to boost the rate of MZ twinning, but at this stage researchers don’t understand why. Multiple pregnancies are high risk, especially where supertwins (more than two) are involved.  Many leading infertility specialists now advocate the transfer of only one embryo at a time, and certainly never more than two, since advanced techniques have improved the chances of a viable pregnancy resulting from the transfer of just one high quality embryo.  As this becomes accepted practice, there is likely to be a decline in the rate of ART related twin and supertwin births.

MZ or DZ?

Opposite sex twin pairs, which make up approximately 1/3 of all twin births, are obviously dizygotic.  Determining the zygosity of same sex twin pairs can be more problematic.  A diagnosis is often made at the birth based on an examination of the placenta and fetal membranes.  If there is only one placenta, the pair is monozygotic.  If a pair is dizygotic, each twin will have its own placenta, outer membrane (chorion), and inner membrane (amnion) (fig. A).  This is also the case for one third of MZ pairs, however, so the appearance of two of placentas and two sets of membranes does not enable a definitive assessment to be made regarding twin type.

MZ twins can be categorised into four types based on when the division of the embryo occurrs.  If the cleavage happens before the sixth day after conception, there will be two placentas, two chorions, and two amnions (fig. A).  If it takes place between approximately the sixth and tenth day, there will be one placenta, one chorion, and two amnions (fig. C).  About 64% of MZ twins are of this type.  If the embryo splits between the tenth and the fourteenth day, the result will be twins sharing the one placenta, one chorion, and one amnion (fig. D).  This type is less common, accounting for only 4% of MZ twins.  If cleavage of the embryo occurs sometime after the fourteenth day, there is an increased risk that the division will be incomplete and the twins will be conjoined or what is often called “Siamese”.

Twins with separate placentas and sets of membranes can be implanted so closely together in the womb that the individual placentas appear to fuse (fig. B).  To the naked eye it looks as if there is only a single placenta.  This happens in 42% of DZ twin pregnancies and 13% of MZ, meaning that for approximately 49 out of 100 pairs, (or more to the point 70% of same sex pairs), an examination of the placenta and fetal membranes will not yield conclusive information about zygosity. 

About 30% of same-sex pairs will be MZ twins resulting from an embryo that split more than six days after conception.  These pairs will be monochorionic, sharing a single placenta and chorion.  Monochorionic twins have an increased obstetric risk of complications such as Twin-to-Twin Transfusion Syndrome (TTS).  This is a life-threatening prenatal condition for both twins in which abnormal, interconnecting blood vessels create an imbalanced blood flow that passes through one twin to the other.  The “recipient” twin grows much larger because of the extra blood it receives and can develop significant cardiovascular problems as its system tries to cope.  The “donor” twin receives much less blood and nutrients, so remains smaller, and may develop severe anemia.

DNA fingerprinting is the most objective way to assess zygosity.  The twins’ blood or another form of physical sample, such as their cheek cells or placentas, can be tested for a range of genetic markers.  Matches and differences between the samples are then identified and test results are usually reported as a likelihood ratio of the twins being MZ versus DZ.  At the moment, DNA zygosity testing is specialised and expensive.  There are a limited number of places in Australia which offer DNA testing as a service to twins.  If you would like to know more about having a test done, contact the Australian Twin Registry information on how to contact is outlined in our next section Useful links.

Research shows that in most cases (about 95%), it is possible to tell whether twins are MZ or DZ by simply comparing them for similarities in colouring, body build, and facial features as they mature.  Parents and very close friends and family members will no doubt be attuned to subtle differences between the twins, but if they are frequently confused by teachers and friends it is highly likely that they are monozygotic.