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The Alphabet Soup of Twin Biology

by Geoffrey Machin, M.D., Ph.D.
  Reproduced with permission from Twin Services Reporter, Spring 1997

During a twin pregnancy, the main business is to make sure the mother and her babies are well nourished; that the pregnancy goes as near to term as possible; that common complications are anticipated (increased risk of hypertension and hemorrhage); and, that support systems are in place when the twins come home. Chorionicity of the placenta is the most important prenatal issue.

Although it is well known that twin pregnancies carry higher risks than single pregnancies, it is not always realized that some types of twins are more likely to have very high risks; this is the group who have both twins connected to a single (monochorionic or MC) placenta. For MC twins, the risks of serious, life-threatening complications are up to 10 times higher than for those twins who have one placenta each (dichorionic or DC). The distinction between MC and DC twins can be made by ultrasound exam in the first and early second trimester.

The dangers in MC twins are caused by the fact that the circulations of the twin pair are usually connected with each other via the placenta. The dangers include: twin-twin transfusion (TTF); twin reversed arterial perffusion (TRAP); unequal sharing of the placenta by the twins, leading to growth discordance; brain damage to one twin if the other should die during fetal life; monoamniotic (MA) twins sharing a single gestational sac with danger of umbilical cord entwinement; conjoined twins. In addition, the rate of major malformation is higher in these twins than in other twins or singletons. Usually, one twin is malformed and the other is not. The malformed twin fetus may threaten the life of the co-twin, and consideration is sometimes given to selective termination; this is highly dangerous in MC twins because of the shared circulations.

Although the distinction between MC and DC twins can be made early in pregnancy, in practice this is not always done. And, if the first ultrasound exam is not done until 16 weeks, some of these disorders will already be in an advanced stage, and it may be too late to plan proper management. There are particular risks in MC twins that are over and above the risks of twin pregnancies in general. The great majority of adverse outcomes in twin pregnancy occur in the MC group. They can be anticipated and diagnosed if there is sufficiently intense prenatal care. Outcomes can be improved, although some of the problems are very challenging and are presently the subject of intense debate among obstetricians.

Zygosity becomes the most important issue after birth. There are two major causes of confusion about zygosity:

1.There is no simple relationship between zygosity and placental status. All MC twins are MZ, but some MZ twins are DC, so not all DC twins are DZ. Many parents are misinformed and believe that their twins are DZ because there were two placentas. I have found that many parents continue in this belief even when their twins look so much alike that everyone else assumes they are MZ, and even after this has been proved by the best genetic testing available. What this telis me is that zygosity is deeply important to twins and their parents, that it is an issue that they want settled in their minds, and that they will stick to what they have been told even when the information is clearly wrong.

2.I have never met a pair of "identical" twins and I do not believe that they exist. The use of so-called "identical" and "fraternal" (for girls?) is inaccurate and sometimes disastrously misleading; I would stop it today if I could. The use of "identical" to describe MZ twins is a major cause of confusion. Parents distinguish between their MZ babies by identifying their differences. Once they have found these differences, however slight, parents may believe that, since their twins are not "identical" in the strict sense of the word, they are not MZ.

Sometimes parents who are able see differences between their MZ twins which are not apparent to others invoke the theory of "polar body" twins (PBT) to explain the differences between their twins. The theory of PBT is based on the hypothesis that the polar body of a fertilized egg can be fertilized by a different sperm, thus creating twins which are "half identical". This theory is extremely doubtful and is not needed to account for differences in MZ twins. In fact MZ twins are never absolutely "identical", as this misleading label suggests. Some common differences in MZ twins include variations in the shape of their faces, their height, and/or weight They may have different birthmarks; and be discordant for illnesses such as downs syndrome and cerebral palsy.

I strongly advocate that parents be given technically correct information about the zygosity of their multiples.

Apart from the fact that parents have the right to know such a basic piece of information about their twins - as do the twins about themselves - there are other important medical and biological reasons which support routine determination of zygosity:

1.MZ twins may well have more difficulty in individuating, and require special social and educational considerations.

2.There are high concordance rates in MZ twins for many chronic disorders with a genetic background, e.g., allergies, asthma, heart disease, Alzheimer's, schizophrenia, diabetes, neuroses, cancer. These disorders may not actually happen simultaneously, but they can be anticipated in the second twin if they occur in the first twin.

3.MZ twins can serve as rejection-free organ transplant donors for their co-twins.

4.Medical research needs MZ and DZ twins to sort out the relative importance of environmental and genetic factors in the development of normal people as well as their disorders and diseases. Many twins are eager to participate in these studies for the good of all as well as to learn more about twinning. Not everybody wants to participate, but the studies are non-harmful and non-interventional. They contribute greatly to the cause of disease prevention.

Zygosity can be determined as follows:

1.Monochorionic (MC) twins are all monozygotic (MZ).

2.Unlike-sexed twins are dichodonic (DC) and dizygotic (DZ).

3.Not all dichorionic (DC) twins are dizygotic (DZ).

4.Like-sexed dichorionic (DC) twins need testing for zygosity.

The zygosity test of choice is DNA fingerprinting, a non-invasive procedure which currently costs about $100. This is not at present considered a standard part of the care of twins. I believe that it should be and that parents and twins and others should lobby for it.

Geoffrey Machin. M.D., Ph.D., is the fetal pathologist for the Northern California region of Kaiser Permanente Medical Centers and co author with Louis Keith of An Atlas of Multiple Pregnancy, Biology and Pathologyy and Pathology. 1998. Parthenon Publishing.

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