What Causes Twins, Triplets or More – FREE Zygosity Questionnaire

What Causes Twins, Triplets or More

Identical and non-identical twinning

Some introduction to the biology of twinning is important for the appreciation of multiples in school. There are two types of twins:

Non-Identical, who stem from the fertilization of two eggs and are no more alike genetically than ordinary brothers and sisters. The release of two or more eggs every month runs in families to some extent and also changes with age, as well as differing between ethnic groups. Such twins have separate placentas although sometimes these may be fused so closely they appear as one until there is a detailed examination.

Identical, where the single fertilized egg splits sometime in the first 12 days after conception. If it splits in the first 3 ­ 4 days, the twins have separate placentas. In the most common situation, the split occurs 4 ­ 8 days after conception and the twins share the one placenta. A later split can lead to more complications, and Siamese (conjoined) twins are those where the splitting process begins after 12 days.

Identical and non-identical twinning


(From D.A. Hay (1985) Essentials of Behaviour Genetics. Oxford: Blackwells, p218) – Get it Here

It is important to note that identical twins are only identical genetically and there are many physical and behavioral characteristics less determined by heredity where they may differ considerably and where non-identical twins may be very similar. Thus we shall use the more neutral terms DZ (for dizygotic or two-egg twins) instead of non-identical and MZ (for monozygotic or one-egg twins) instead of identical.

Higher multiples can be any combination of MZ and DZ. For example, two eggs may be fertilized (making DZ twins) and then one developing embryo splits to form a set of MZ twins. It would be possible to have identical triplets where one early embryo splits and then one of these splits again.

We have such a family in Australia which interestingly resulted from fertility treatment. The doctors and initially the parents assumed the girls must be DZ, resulting from three eggs being fertilized. By the time they were three, the mother was pretty convinced they were MZ and blood tests confirmed this. There has been some discussion that the process of in vitro fertilization may make splitting of the embryo and the formation of MZ twins more likely.



Contributing factors

Maternal age

Between the age of 20 and 37-38, there is a consistent increase in a woman’s chances of releasing more than one egg in each cycle and thus a greater chance of DZ twins or higher multiples. There is a separate increase associated with the number of children a woman has already had. Thus multiples are more likely to be born to older women and to be the later-born children in the family.


While there are some families with an extensive history of MZ or of both MZ and DZ multiples, most studies have focussed on a history of DZ to MZ or of both MZ and DZ multiples, most studies have focussed on a history of DZ twins on the mother’s side of the family. The evidence is less consistent for a role for males, either directly through a greater capacity for some men’s sperm to fertilize two or more eggs or through a history of twinning in female relatives being passed on by men to their daughters. However, it is fair to say the genetics of multiple births are not clear because of two things:

(i) the mortality of multiples at birth is still higher than that of singletons, but the survival rate is much better now than a generation or two ago.

“I said to my Mum that our boys would be the first twins in the family, but she said that was not true. My aunt (Mum’s sister) was the surviving member of a twins, but it was something that has never been spoken about.”

(ii) Even if a family history of multiples does include those who died, as well as those who did survive, it is still not complete. Genetics is involved in the release of multiple eggs and not in whether these result in babies many months later. Ultrasound studies are showing that there are a considerable number of women who are carrying multiples 8-10 weeks into pregnancy at the time of a first scan. By the time of the next scan, there is a single fetus, and so this is appropriately known as the “vanishing twin” syndrome. There may be no signs when the other multiple(s) are lost or at the birth of the singleton and thus many people may be surviving multiples and not know it.

Assisted Reproduction

While much of the publicity surrounding higher multiple births, in particular, has focussed on in vitro fertilization and its many developments, the majority of fertility treatments that result in multiples births are associated with the use of fertility drugs such as Clomid (Clomiphene). Many places are now setting strict guidelines on the number of fertilized eggs that can be implanted within vitro fertilization, but it can be more difficult to control the number of eggs released by ovarian stimulation with drugs like Clomid.

It is really no business of the school to know whether a particular set of multiples resulted from assisted reproduction, except for two things:

(i) In an Australian study where we followed expectant couples from the time of diagnosis of twins, we found higher rates of stress and depression in those where there had been assisted reproduction, and this depression was present well before the birth. Obviously, they were happy to finally be pregnant, but knowing some of the risks associated with multiples, especially in what could be their only chance at children was a heavy burden to bear. As discussed in one of the Preschool post “Parenting Multiples” anxiety and depression can limit the natural patterns of interaction that develop between parents and their newborn multiples.

(ii) Every parent wants their child(ren) to succeed at school, but this may be especially so for parents who have used assisted reproduction. Not only may their multiples be their only children, but they have also spent much longer dreaming of how their children would develop and invested much time and effort in achieving this dream.



Identical or not?

Ways of determining zygosity

“Of course my twins are not identical-their socks are different colors.”

There certainly can be much confusion in the early years, as DZ twins may look and behave as similarly as MZ twins. But by the age of 3-4, then questionnaires such as this one can help. The ‘gold standard” is, of course, genetic testing, done in the past by blood-typing but increasingly by directly examining the DNA. Previously a blood sample was needed, and much medical staff is unwilling to take blood from young children, just to resolve uncertainty over zygosity. Now DNA can usually be obtained successfully from cells gathered by brushing inside the cheek, and more and more laboratories are offering DNA zygosity testing. It is still expensive and for many families, completing a questionnaire will resolve things in their minds.



Common misconceptions about zygosity

The most common myth is that only DZ twins have separate placentas. Firstly many DZ twins may have fused placentas that cannot be distinguished in the rush of the delivery room after the arrival of multiples. More significantly, there are a significant number of MZ twins who have separate placentas. The old myth about one placenta = identical twins persists. Where there are separate placentas, the babies will be separated by two membranes, the inner amnion, and the outer chorion. While an experienced ultrasonographer will be able to identify the number of membranes around the unborn multiples, we have all heard stories like

“He said they were going to be identical and that’s what we told everyone. It took a bit of explaining when Alison and Peter were born!”



Not all MZ twins are identical – the twin-twin transfusion syndrome

Some MZ twins share the one placenta, and the outer membrane (chorion) and so are called monochorionic (MC). A few share the inner membrane as well, and these monoamniotic twins need careful monitoring during pregnancy. MC twins may be much more different from each other than are DZ twins because of a one-way circulation in the placenta whereby one twin shunts blood to the other. Such twins are very different in birthweight (often 500 g or more) and typically the larger one (the donor) is red, while the smaller one is pale. In fact, the larger one is more at risk because the blood is so thick. The size difference may persist throughout school, and we have seen 15-year-old MZ twins where one is 15cm taller than the other. There is growing evidence that some differences in abilities between the twins may be associated with this condition, called the twin-twin transfusion syndrome, but studies are not yet conclusive

Sometimes this is associated with the common MZ situation of mirror-image twins, where one is right and one left-handed. There can be other aspects of mirror-imaging such as the direction of hair whorls, or on which side of the mouth teeth erupts. So being different in such aspects of laterality does not mean multiples are DZ. It also does not mean the opposite-if twins differ in handedness; you cannot assume they are mirror-image MZ twins.



Does it matter?

“I get so embarrassed sometimes. People ask if our girls are identical and I have to say we are not sure. They look at our girls are identical, and I have to say we are not sure. They look at me like this was some kind of child abuse. ‘You have twins, and you have not had the responsibility to find out if they are genetically the same.’ I try to explain it does not matter. Kate and Melissa have the right to develop as individuals, irrespective of their genetics. But you can tell this does not convince people.”

When families ask if they need to definitely know the zygosity of their multiples, the best answer is “only if you want to know.” Obviously there are exceptions as in the case of transfusions or transplants, but fortunately for most families, such issues will not arise. MZ twins have the right to be treated as individuals, irrespective of their genetic similarity. DZ twinning is more likely to run in families and in individual couples and so knowing if your first twins have been DZ does make it more likely that future pregnancies will also result in multiples.

There has been some debate in both the UK and Australia about routine zygosity testing being available for all same-sex multiples at no or minimal cost to the family, but demands on the health budget make that seem unlikely. It would certainly eliminate the embarrassment the mother above describes but is this enough for health authorities to make this a priority? If significant differences do appear between multiples in their achievements at school, then zygosity determination may be one thing to consider.



Using the Zygosity Questionnaire

If you ask adult twins if they are MZ or DZ or if they are ” as alike as two peas in a pod,” their answers will almost always agree with formal genetic tests. When you are getting data from parents, it is quite another matter, and more extensive questioning is needed. For example, many parents will say the twins are so similar on all the questions about physical appearance and that they are mistaken by almost everyone (except the mother who can usually tell them apart). But when it comes to the question about being MZ or DZ, they say the twins are DZ.

The lower part of the table gives some Australian results. Using a statistical technique which takes all the various items into account, we estimated there were 726 sets of twins who were MZ, while the parents said only 604 were MZ. (these twins were born between 1980 and 1988 when our estimate of the proportion of MZ twins is just about right.) Similarly, of 134 where the parents were unsure, our analysis indicated 103 were really MZ. What does it mean?

(i) Parents may think the smallest difference may mean the twins are DZ or at least that they are not sure of zygosity

(ii) some parents want their twins to be individuals and would prefer not to think they are MZ. Of course, some parents think the opposite, that MZ twins are really special and are “sure” they are MZ despite quite marked differences.

The questions about placentation and blood groups are not put in to be definitive but to flag areas of confusion. The fact that “Two placentas does not mean DZ” has already been mentioned. There can be similar confusion over blood groups. Many people know their status on the two most common blood systems, ABO and Rhesus and a lot of people are A+ while not being twins. Although such blood typing is being replaced more and more by DNA analysis, many may not realize that blood-typing for zygosity involves not just ABO and Rhesus but many lesser-known blood types. Otherwise, an awful lot of strangers would be MZ twins!

Where uncertainty remains, then formal genetic testing can resolve the issue. But we have found that for many parents who approached us about zygosity testing, just filling-in this questionnaire was enough to resolve the doubts in their minds and they decided genetic testing was unnecessary.


Click Here To Download Zygosity Questionnaire (Word Document) or (PDF Document)



References: See here





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