Chorionicity refers to the number of placentas in a multiple gestation; amnionicity refers to the number of amniotic cavities. Prenatal determination of chorionicity and amnionicity is essential in the clinical management of multiple gestations. In the first trimester, chorionicity can be determined by counting the number of gestational sacs, and the number of yolk sacs can be used to predict amnionicity. In the late first trimester and early second trimester, systematic evaluation of placental number, fetal gender, and insertion of the intertwin membrane into the placenta allows accurate prenatal diagnosis of chorionicity. In dichorionic pregnancies, the intertwin membrane remains thick with a triangular projection of placenta known as the twin peak or lambda sign, which is visible between the layers of the dividing membrane. In monochorionic pregnancies, the intertwin membrane inserts directly into the placenta forming a characteristic “T” sign.
Keywordschorionicity, twin, multiple gestation
In 2014, the twin birth rate in the United States rose to an all-time high of 33.9 per 1000 live births. Multiple gestations result from either the fertilization of multiple ova or the division of a single fertilized ovum into more than one fetus. The terms monozygotic and dizygotic refer to the number of ova leading to a multifetal gestation. A monozygotic pregnancy results from a single fertilized ovum that divides into more than one fetus. In contrast, dizygotic pregnancies originate from the fertilization of two separate ova and are genetically dissimilar.
In contrast to zygosity, which refers to the genetic constitution of a twin pregnancy, the terms chorionicity and amnionicity describe the placentation and membrane composition of a pregnancy. More so than zygosity, the determination of chorionicity and amnionicity is essential in the clinical management of multiple gestations because monochorionic multiples, whether diamniotic or monoamniotic, are at increased risk of adverse outcomes.
General Anatomic Descriptions
Chorionicity refers to the number of placentas in a multiple gestation; amnionicity refers to the number of amniotic cavities. The term dichorionic refers to a multiple gestation with two distinct placental disks (or two chorions), whereas the term monochorionic refers to a pregnancy with a single placental disk (or one chorion). Similarly, a pregnancy with two distinct amniotic cavities is described as diamniotic, and a pregnancy with a single amniotic cavity is monoamniotic . Depending on the number of chorions and amnions, twin pregnancies can be described as dichorionic diamniotic, monochorionic diamniotic, or monochorionic monoamniotic .
Detailed Description of Specific Areas
Monochorionic twins occur spontaneously in 0.4% of the general population. Studies have reported that the frequency of monozygotic twinning may be more than 10 times higher in pregnancies after fertility treatment.
In contrast, the frequency of dizygotic twins varies by maternal age, parity, family history, maternal weight, nutritional state, and race. Some of these risk factors lead to an increased frequency of dizygotic twinning through increases in serum concentrations of follicle-stimulating hormone and luteinizing hormone, which increase the likelihood of multiple ovulations in a single menstrual cycle. Because gonadotropin levels are increased with advancing maternal age and use of infertility medications, both delayed childbearing and assisted reproductive technologies are associated with an increased frequency of dizygotic twinning. In addition, the incidence of spontaneous dizygotic twins varies by race, with highest rates among certain populations in Africa and relatively lower rates in whites and Asians. Other factors associated with an increased likelihood of dizygotic twinning include maternal family history of twin gestations and increasing maternal height and weight.
Dizygotic pregnancies are almost always dichorionic diamniotic, with each fetus having its own placenta and amniotic cavity. In contrast, chorionicity of monozygotic gestations is determined by the time at which division of the fertilized ovum occurs, and monozygotic gestations can be dichorionic diamniotic, monochorionic diamniotic, or monochorionic monoamniotic. If twinning occurs during the first 2 to 3 days, it precedes the separation of cells that eventually become the chorion and results in a monozygotic dichorionic diamniotic pregnancy. After approximately 3 days, twinning cannot split the chorionic cavity, and from that time forward, a monochorionic placenta results. If the split occurs between the third and eighth days, a monochorionic diamniotic pregnancy develops. Between the eighth and twelfth days, the amnion has already formed, and the pregnancy is monochorionic monoamniotic if twinning occurs. Embryonic cleavage between the 13th and 15th days results in conjoined twins within a single amnion and chorion; beyond that point, twinning does not occur. Among monozygotic twin gestations, approximately one-third are dichorionic diamniotic, whereas almost two-thirds are monochorionic diamniotic, and less than 1% are monochorionic monoamniotic.
By definition, essentially all monochorionic twins are monozygotic. In contrast, among spontaneously conceived same-sex dichorionic twins, approximately 18% are monozygotic. There have been several case reports of twin pregnancies conceived using assisted reproductive technology that resulted in dizygotic monochorionic gestations ; the mechanism of this phenomenon is not yet understood.
Compared with singletons, multiples face an increased risk of preterm delivery, growth disorders, and maternal complications. In addition to these general risks, multiples also face increased risk depending on the zygosity, chorionicity, and amnionicity of the pregnancy.
Monochorionic twins have a worse prognosis than dichorionic twins because of numerous complications unique to the twinning process and to monochorionic placentation. Monozygotic twins, whether monochorionic or dichorionic, have a significantly higher incidence of congenital anomalies than singletons or dizygotic twins. A primary concern in twin gestations with monochorionic placentation is twin transfusion syndrome, characterized by an unequal distribution of the blood flow across the shared placenta of two fetuses. Although all monochorionic twins share a portion of their vasculature, only approximately 15% to 20% develop this condition. Untreated, twin transfusion syndrome is associated with a 60% to 100% mortality rate for both twins.
There is an increased risk of fetal loss with monochorionic twins across all gestational ages and an increased risk of complications secondary to fetal demise. Specifically, owing to vascular anastomoses within the monochorionic placenta, hemodynamic changes associated with the death of one fetus result in an approximately 20% risk of multicystic encephalomalacia in the surviving twin, and an increased risk of preterm delivery. The literature has suggested that the risk of intrauterine death of one or both twins is higher in monochorionic than dichorionic pregnancies. In a study of 1000 consecutive twin pairs, monochorionic diamniotic twins had a higher risk of stillbirth compared with dichorionic diamniotic twins overall and at each gestational age after 24 weeks ; this increased risk of fetal loss persisted in “apparently normal” monochorionic diamniotic twins unaffected by growth abnormalities, congenital anomalies, or twin transfusion syndrome.
Other unique but rare problems that occur in monochorionic pregnancies include cord entanglement in monoamniotic twins, conjoined twins, and twin reversed arterial perfusion sequence, also known as acardiac twinning ( Chapter 163 ). Monoamniotic gestations are associated with increased perinatal mortality secondary to cord entanglement. Previous studies reported a fetal mortality rate of greater than 50%, but more recent studies indicate a perinatal mortality rate ranging from 10% to 21%.
Pertinent Imaging Considerations
Given the impact that chorionicity and amnionicity have on pregnancy outcome, determination of placentation and membrane composition is vital to guide prenatal care of a twin pregnancy. Although chorionicity and amnionicity can be definitively determined postnatally by gross and histologic evaluation of the placenta and fetal membranes, prenatal diagnosis is preferable to allow appropriate prenatal evaluation and intervention. Prenatal determination of chorionicity and amnionicity is possible using ultrasound (US) ( Table 158.1 ).
|EARLY FIRST TRIMESTER||LATER GESTATION|
|Gestational Sacs||Intertwin Membrane||Yolk Sacs||Gender||Placentas||Intertwin Membrane||Membrane Insertion||Membrane Layers|
|Dichorionic-diamniotic||2||Thick||2||Discordant or concordant||2||Thick||Twin peak or lambda sign||4|