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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:
- 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.
- 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: