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| Multiple Gestations By: Lacy Jordan |
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Abstract
A multiple pregnancy refers to a pregnancy involving more than one gestation. This article discusses the occurence and different types of multiple gestations, along with their sonographic appearances. The role of ultrasound in the management of theses high-risk pregnancies is also discussed. |
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Keywords
twins, multiple pregnancy, twin gestations, fraternal twins, identical twins
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Introduction
Naturally, twins occur about every 1 in 80 births. The era of assisted reproductive technology, such as in vitro fertilization, and the trend towards delayed childbirth has dramatically increased multiple births so that the incidence of twins is now 1 in 40 births. In the United States, the number of twin births has risen more than 50% over the last two decades.[1] By definition, a pregnancy involving multiple gestations is high risk, both for the fetus and the mother. Multifetal gestations have significant increases in morbidity and mortality rates in relation to singleton pregnancies. Overall, twin gestations have a 7 to 10 times greater mortality rate than singletons.[2] |
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| Sonogram demonstrating a diamniotic-dichorionic pregnancy. Note the thick membrane separating the two gestational sacs. |
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Body
Twin gestations result from the fertilization of either two separate ova (dizygotic, or fraternal) or a single ovum which subsequently divides (monozygotic, or identical). Each dizygotic twin develops embryologically similar to a single gestation. Each ovum implants separately in the uterus and develops its own placenta and amniotic sac. Therefore all dizygotic twins are referred to as dichorionic-diamniotic. Sonographically, dichorionic-diamniotic twins appear as two separate gestational sacs with individual trophoblastic tissue, which allows for the appearance of a thick dividing membrane. This membrane is four layers thick, with two amnions and two chorions. If twin fetuses are of opposite gender they are always dizygotic dichorionic-diamniotic.[3] Dizygotic twin pregnancies comprise 70% of all twins.[2] A maternal family history of dizygotic twinning is associated with a higher incidence of twins. One study states that the incidence of dizygotic twin births in women who themselves were a dizygotic twin is 1 per 58.[4] |
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| Monozygotic twins arise from a single fertilized egg, which divides, resulting in two genetically identical fetuses.[2] There are three types of monozygotic twins: dichorionic-diamniotic, monochorionic-diamniotic, and monochorionic-monoamniotic. These three different patterns of chorionicity and amnionicity depend on the stage at which the fertilized ovum splits.[5] |
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| Division of a fertilized ovum occurring prior to day four, before blastocyst formation, will result in two amnions and two chorions. Each embryo will have an individual placenta and amniotic sac. This is referred to as a monozygotic dichorionic-diamniotic gestation. This type of gestation is identical in appearance to that of a dizygotic gestation. Both involve two complete embryos, two complete sacs, and two separate placentas. Dichorionic-diamniotic twins compromise eighteen to thirty-six percent of monozygotic gestations.[4] |
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| Sonogram demonstrating an early monochorionic-diamniotic pregnancy. Note two yolk sacs and two embryos with a single chorion. |
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Division between four and eight days results in a monochorionic-diamniotic gestation. The embryos will share a common placenta but have their own amniotic sac.[3] Sonographically, the membrane seen dividing the fetuses is two layers thick, consisting of two amnions only, one from each fetus. Monochorionic-diamniotic twins constitute seventy-five percent of monozygotic pregnancies.[5] |
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| Sonogram demonstrating a monoamniotic-monochorionic pregnancy. Note two yolk sacs with no separating amniotic membrane. |
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The most crucial diagnosis for the sonographic observer to make is that of a monochorionic-monoamniotic gestation, which results from division occurring beyond eight days. In this type of pregnancy, two embryos share a single amnion and a single chorion.[3] Sonographically, one would identify one gestational sac with two embryos enclosed in a single amniotic membrane. Monochorionic-monoamniotic gestations have a mortality rate of approximately 50%. This type of twinning is rare and accounts for two percent of monozygotic gestations.[5] |
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If division occurs beyond thirteen days post-fertilization, the division may be incomplete and conjoined or “Siamese” twins may result. All conjoined twins are monozygotic and associated with one chorion and one amnion. Twins may be joined at a variety of sites, including the head, thorax, abdomen, and pelvis.[2] The most common type of conjoined twins share an anterior union of the upper half of the trunk and almost always share a heart. This type of twinning is termed thoracopagus. It has been estimated that the prevalence of conjoined twins is approximately 1 per 50,000 to 1 per 100,000 births.[4]
A multiple gestation is often diagnosed by an ultrasound examination. Ultrasound is then used to monitor fetal growth and development as the pregnancy advances.[6] The appeal of ultrasound in managing high-risk pregnancies is that it is a safe procedure that offers an abundance of information, which directly impacts maternal and fetal management.[3] With its use, less than 10% of twin gestations are undiagnosed before labor and delivery. Ultrasound provides a vital function as the only technique that allows assessment of each fetus individually.[7] |
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| Sonography plays an important role in the management of multifetal gestations in several key areas: (1) diagnosing multiple gestations and determining fetal number; (2) determining placentation; (3) diagnosing complications; (4) guiding procedures; and (5) identifying fetal lie late in pregnancy, which helps determine the safest route of delivery. The information obtained from single or serial ultrasound examinations can lead to improved obstetric decision-making and thereby to improved outcome in these high-risk pregnancies.[8] |
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| Sonography is of great value in assessing the relative risk for morbidity and mortality in multifetal gestations by determining the type of twinning, as well as the membrane and placental arrangements.[9] Complications in multiple gestations parallel those of single gestations but occur with higher frequency, including preterm birth, intrauterine growth restriction, and fetal anomalies.[3] Ultrasound can accurately diagnose the majority of fetal complications and anomalies associated with multiple gestations, thus contributing significantly to medical management.[9] |
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Conclusion
In conclusion, twin gestations may either be dizygotic or monozygotic. The type of monozygotic twinning depends on the point at which division of a single ovum takes place. Before ultrasound was used routinely, as many as 60% of twins were not diagnosed before delivery.[2] Today, ultrasound has proved to be a powerful tool in our understanding of twinning. Ultrasound can not only detect multiple fetuses but also provide valuable information throughout the multiple pregnancy. As new clinical management procedures evolve for handling high-risk pregnancies, ultrasound will continue to play a critical role in the development of new approaches.[3] |
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Works Consulted
1. Womens Health Channel. Multiple Pregnancies. September 2004. (http://www.womenshealthchannel.com/index.shtml)
Accessed on May 9, 2005.
2. Hagan-Ansert, Sandra L. Textbook of Diagnostic Ultrasonography. 5th ed. Vol.2. 2001; 687-695.
3. Curry, Reva Arnez; Tempkin, Betty Bates. Sonography: Introduction to Normal Structure and Function. 2nd ed. 2004; 360-362.
4. M.D., Levi, Clifford S. Sonography in the Diagnosis and Management of Multifetal Pregnancy. Vol.2. 2003; 1036-1046.
5. Hickey, Janice; Goldberg, Franklin. Ultrasound Review of Obstetrics and Gynecology. 1996. 130-132.
6. Web MD. Multiple Births. 2002.(http://www.webmd/IH/multiplebirths.com) Accessed on May 16, 2005.
7. Womens Health. Multiple Pregnancy Program. 2004. (http://www.muschealth.com/women/services/multipreg)
Accessed on May 3, 2005.
8. Pubmed. Sonography of Multiple Gestations. February, 2000. (http://www.ncbi.nlm.nih.gov/entrez/query.) Accessed on May 17, 2005.
9. Barth, Richard A. Ultrasonography in Obstetrics and Gynecology. 4th ed. 2000; 171-173.
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