Monozygotic Twins

Monozygotic twins (MZ), often referred to as identical twins, are not simply twins who are similar in appearance. More specifically, the term relates to the biological relationship of the pair of siblings. MZ twinning occurs after the normal fertilization of one egg by a sperm that ultimately forms one zygote. Whereas during a typical [Page 1135]single childbirth the zygote continues to develop into one fetus, in this case, the zygote splits into two within the first 14 days of development, and continues to develop into two genetically identical twins. It is also possible for twins to remain partially conjoined, a result of improper separation, but this event is much more rare than that of separated twins (1 in every 200 twin pairs is conjoined). MZ twins may then share one placenta, referred to as monochorionic twins, or may have their own separate placentas, referred to as dichorionic twins. Timing of separation is the distinguishing factor in this case, as dichorionic twins separate within the first four days after fertilization.

With the ever-improving healthcare systems, twin pregnancies often have positive health outcomes. However, the age-old concerns of preterm delivery and low birth weight, and complications associated with a greater risk of morbidity and mortality compared to those of singleton births still exist.

Thus, healthcare providers are motivated to take proper precautions and care when supporting twin pregnancies so as to prevent adverse outcomes for the mother and/or the twins. Such precautions include the ability to perform emergency Caesarean section, and to provide fetal heart monitoring, ultrasound, and intensive care support.

Above all, it is necessary to ensure that the mother is adequately advised of the possibility of adverse events or complications. Normal vaginal delivery, Caesarean, or mixed methods are possible methods of delivery. Caesarean section, however, is often necessary in the event of malpositioned twins, cord entanglement, or twin-to-twin transfusion syndrome in which the fetuses share one placenta and its associated blood vessels resulting in disproportionate allotment of necessary nutrients.

Because the MZ twins share the same genes, the twins will always be the same sex and tend to develop similarly in appearance, as well as in other physical and mental features. This process is quite distinct from that which occurs with dizygotic twins (fraternal twins) where twins may or may not be of different sexes with or without similar physical appearances.

Development after birth, however, also relies on environmental factors and thus MZ twins may grow to become individuals who do not appear to be exactly “identical” in appearance and personality. Thus, questions of the relationships between environment and genetics in health, psychology, and development have led many researchers to conduct “twin studies” for clarification.

The sociological relationship of twins may also vary from being extremely close and dependent, to complete indifference. Often, parents and professionals note characteristic twin behaviors and some suggest the existence of idioglossia, twin language, although it is debated whether this language may actually be the result of delayed language development.

Contrary to popular belief, families are not genetically susceptible to having MZ twins. Researchers assert that MZ twin pregnancies occur only by chance, despite the recent increase in incidence of twin pregnancies. Recent estimates from the United States suggest the chance of having a MZ twin pregnancy is 3 in every 100 pregnancies.

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