**Articles written by Cheryl are not intended to be authoritative, but rather to help others in their quest for information on various topics of interest.

The majority of twin births result from the release and subsequent fertilization of two ova (zygotes) by two different sperms. These fraternal twins may be of either sex and are no more genetically alike than ordinary siblings. Scientifically they are termed dizygotic twins, indicating their origin from two zygotes.

Twins births may also originate from the division of a single zygote at some stage during the development of the embryo. Fetal membranes and placenta(s) formed will depend on the stage at which the division occurs. These twins are genetically identical and of the same sex. They are known as monozygotic twins, indicating their origin from one zygote.

Ultrasound is the best modality for early diagnosis of a twin or multiple gestation. At six weeks, if more than one embryo of fetus is identified, each with a visible heartbeat, a diagnosis of a multiple gestation can be made.

Ultrasound also plays an important role in determining the fetal placentation type and any related complications. Twins, compared in singletons, are at increased risk of fetal anomalies, complications and mortality,. Information obtained from ultrasound examinations aids in the management of these high risk pregnancies and can lead to an increased rate of survival.

PLACENTATION OF MONZYGOTIC TWINS

As with dizygotic twins, monozygotic twins that are dichorionic and diamnoitic, will have separate placentas that may fuse if they implant close to one another. The thick membrane dividing them will consist of four layers, two layers of amnion and two layers of chorion.

Monochorionic, diamniotic twins will share a common placenta and the membrane separting them will be thin, consisting of two layers of amnion.

Monochorionic, monoamniotic twins share a common placenta in which no membrane divides them. The significance of monochorial placentae is its effect on fetal circulation. In approximately 90% of monochorical placentas there is some sort of connection between the fetal circulation of twins.

The placenta is responsible for a number of functions essential to the growth and development of the fetus. One main function of the placenta is respiration. Oxygen from the maternal blood diffuses across the placental membrane into the fetal blood and carbon dioxide from the fetal blood passes in the other direction. Nutrients required by the fetus also diffuse from the maternal to the fetal blood through the membrane. Other functions include excretion of fetal waste, storage of carbohydrates, proteins, calcium and iron, hormonal production and protection.

Twin gestations place an increased demand on placental functions and maternal blood supply and therefore are susceptible to a number of problems. Placentation type plays a significant role in the type of complications that can occur.

RISK AND COMPLICATIONS

Increased risks common to all varieties of twins result from the high frequency of premature delivery and low birth weight. Prematurity, with its complications of immature lung development and cerebral haemorrhage, is an important contributor to the high morality rate of twins. Intrauterine growth retardation adds to this risk

The majority of monozygotic twins are monchorial and frequently encounter additional complications. Whenever twins shared common placenta there is the danger that one twin will recive more of the maternal blood supply than the other. This twin-twin transfusion results from vascular communication between the twins, with one twin growing at the expense of the other. This syndrome is associated with the donor twin at risk from anaemia and growth retardation and the recipient twin at risk from increased cardiac and pulmonary output.

With monozygotic, monoamniotic twins there is often cord entanglement because the twins are not separated by a dividing membrane. These pregnancies most often end prematurely because the entwined umbilical cords cut off the blood supply.

On the rare occasion, when separation of the embryonic disc is incomplete, (in monozygotic, monoamniotic twins) conjoined twins will result. Separation and survival of these twins depends on where they are joined and the organs they share.

SUMMARY

Ultrasound is useful in determining the zygosity of twins. This is important as complications and mortality rates vary considerably with placentation type.

Identification of two placentas and a difference in the sex of the twins is indicative of dizygosity.

When a single placenta is seen with a thick membrane separating the twins, dichorionicity can be assumed. Dichorionic twins can be either monozygotic or dizygotic. If the twins are of different sexes they are known to be dizygotic however, if the are of the same sex, zygosity by ultrasound cannot be determined.

A thin membrane suggests monochorionicity and all monochorionic twins are monozygotic.

Monochorionic twins can be either monoamniotic or diamniotic. If a dividing membrane is seen they are diamniotic, but nonvisualization of the membrane does not mean they are monoamniotic, the membrane may be present but not visualized. Entwining of the umbilical cords indicates monoamniocity.

As a sonographer, it is important to have a clear understanding of placentation, fetal membranes and zygosity, with its associated complications, so that proper obstetrical management and informed decisions concerning the pregnancy can be made.

Article written by Cheryl Mech

References:

Beck, F., Moffat, D.C., Lloyd, GB (1973). Human Embryology and Genetics, : Blackwall Scientific Publications.

Benacerraf, B.R. (Ed.) (1990). The Radiologic Clinics of North America. : Fetal Ultrasound (Volume 28 / Number 1). W.A. Sounders Company

Bulmer, M.G. (1970). The Biology of Twinning in Man. Clarendon Press.

Moore, K.L (1998). Essential of Human Embryology. B.C. Decker Inc.

Moore, K.L (1982). The Developing Human. (3rd ed.). W.B. Saunders Company

Potter, E.L., Craig, J.M. (1975). Pathology of the fetus and the Infant. (3rd ed.). Year Book Medical Publishers Inc.

Sanders, R.C. (1991). Clinical Sonography, A Practical Guide. (2nd ed.). Little, Brown and Company