Early imaging is key to detecting anomalies, some of which are unique to multiple gestations and some that also occur with singletons.
Any anomaly that occurs in singletons can occur in 1 fetus in a multifetal pregnancy. Part 2 of this article discusses the conditions that are unique to multifetal pregnancies.
The distinction between monochorionic and dichorionic twin pregnancies is critical to defining risk of morbidity and mortality and all efforts should be undertaken to determine chorionicity as early in gestation as possible.
The division of 1 fertilized zygote into 2 is a teratogenic event, which can produce discordant malformations and discordant growth.
Zygosity and Chorionicity
Essentially all monochorionic pregnancies are monozygotic, with rare exceptions. In naturally occurring dichorionic pregnancies, approximately 70% are dizygotic, 30% monozygotic. Bajoria and Kingdom provide a more detailed explanation of the difference between zygosity and chorionicity.1
Counseling at diagnosis
When ultrasound is performed in the office to establish pregnancy and a multifetal pregnancy is diagnosed (usually early in the first trimester), it is important that 2 things occur at that moment: determination of chorionicity and discussion of the unique dangers of multifetal pregnancy.
Determining chorionicity is best done as early in gestation as possible. The dichorionic intertwin membrane is very thick and easy to identify in the first trimester (Figure 1), whereas the monochorionic (MC) intertwin membrane is difficult to identify in the late first trimester (Figure 2) or even invisible to ultrasound in the early first trimester and may appear as a monoamniotic (MA) twin pregnancy. We recommend waiting until 10 weeks’ gestation and using vaginal ultrasound in an attempt to identify the intertwin membrane before making a diagnosis of MA pregnancy. Demonstration of the “twin peak” sign, a triangular projection of chorionic tissue between the layers of amnion at the point where the intertwin membrane meets the placental chorionic surface, demonstrates dichorionicity (Figures 3-5). The finding of the “T-sign,” the lack of a triangular projection of chorionic tissue at the same junction, demonstrates monochorionicity (Figures 6 and 7). The “ipsilon sign,” the finding of 3 thick intertwin membranes at the juncture of all 3 in the middle of the uterine cavity, demonstrates trichorionicity in a triplet pregnancy (Figure 8).
Discussion of the unique dangers of multifetal pregnancy includes the following: For dichorionic pregnancies—increased risk of vanishing twin, preterm birth, discordant anomalies, stillbirth, and selective fetal growth restriction (sFGR or sIUGR). For monochorionic pregnancies, all of the above and twin-to-twin transfusion syndrome (TTTS), twin reverse arterial perfusion syndrome (TRAP), MA twins, and conjoined twins.
Pregnancies of any chorionicity
Vanishing twin occurs in 21% to 63% of first-trimester twin pregnancies. It is important to prepare the couple for this possibility so that they are not confused or devastated if later in gestation they must be told there is a singleton pregnancy. Preterm birth occurs in the majority of multifetal gestations and this fact needs to be included in the counseling early in gestation. Discordant anomalies occur and are more frequent in monochorionic pregnancies. A search for anomalies by an experienced ultrasound unit is important. First-trimester anomaly scanning is an option in some centers. Stillbirth is more frequent in multifetal pregnancies, particularly in monochorionic pregnancies. Selective fetal growth restriction complicates up to 15% of twin pregnancies. Current recommendations include monthly ultrasound for fetal growth throughout pregnancy for all multifetal gestations.
Table 1 shows the complications of monochorionic multifetal gestations, along with the diagnostic criteria and best time in gestation for diagnosis.
Twin-to-Twin Transfusion Syndrome happens in 10% of MC twins and can happen in higher-order multifetal gestations that include a MC pair.2 Tis occurs at 14 to 26 weeks gestation. Clues to the diagnosis are amniotic fluid disparity between the sacs, 1 fetus with a large bladder and the other fetus with a small or absent bladder, inability to find the dividing membrane, and fetal size difference. Diagnostic criteria are polyhydramnios and a large bladder in 1 sac and oligohydramnios and a small or absent bladder in the other sac. Inability to find the dividing membrane (intertwin membrane) may also mean there is a MA pregnancy. If there is a suspicion of MA pregnancy or TTTS, refer to an advanced ultrasound center. Figures 9, 9b, and 9c show a typical ultrasound presentation of TTTS.
Monoamniotic pregnancy has the highest risk of all of complications of multifetal gestations. A clue to the diagnosis is inability to find the intertwin membrane. Demonstration of umbilical cord entanglement cements the diagnosis (Figure 10). Tracing the umbilical cord from the abdominal insertion of each fetus to the tangled knot of cord is important.
Conjoined twins are a subset of MA pregnancies and are extremely rare. The diagnosis is much easier in the first trimester and sometimes impossible in the third trimester. The diagnosis can be suspected if the fetuses do not separate throughout the ultrasound examination, especially when gentle pressure with the transducer does not produce separation. Referral to an advanced ultrasound center is prudent if conjoined twins are suspected. Figures 11 and 11b represent the first-trimester appearance of conjoined twins.
Twin reversed arterial perfusion sequence (TRAP) is a unique anomaly where an acardiac monozygous twin, usually with other severe anomalies, is perfused through placental anastomoses by the normal co-twin, leading to heart failure of the normal co-twin (pump twin), fetal demise, or severely preterm birth. The diagnosis includes demonstration of pulsatile blood flow in the demised twin’s umbilical circulation. Any occurrence of fetal demise of 1 twin in the second trimester necessitates Doppler assessment of the umbilical cord going to the twin with demise to rule out TRAP. Referral to an advanced ultrasound center is appropriate. Figures 12 and 12b show the appearance of the TRAP sequence.
Ultrasound imaging early in pregnancy is key to identifying chorionicity, which helps determine the risks for each woman. Early imaging is also key to the diagnosis of the many anomalies that are unique to multifetal gestations. With any suspicion of abnormal findings, referral to an advanced ultrasound unit is appropriate.
1. Bajoria R, Kingdom J. The case for routine determination of chorionicity and zygosity in multiple pregnancy. Prenat Diagn. 1997;17:1207-25.
2. Opekes D, Sueters M. Antenatal fetal surveillance in multiple pregnancies. Best Pract Research: Clin Obstet Gynecol. 2017;38:59-70.