The twin birth rate has significantly increased in most regions of the world during the last decades. In the United States, it rose around 75% in 35 years, from 19/1000 births in 1980 to 334/1000 births in 2016.³ The main reasons for this are delayed childbirth, advanced maternal age at conception, and the widespread use of assisted reproduction techniques (ART).

The incidence of monozygotic twins is around 1 in 250 pregnancies, and it is quite constant across the world,⁴ showing a slight increase in cases of ART. The incidence of dizygotic twins varies with race (it is more frequent in African ethnicity, intermediate in white European, and lower in East Asian), with maternal age (it increases with maternal age), and with family history (specially with maternal history), and it is highly influenced by the use of ART.⁴

ART plays an important role in twins, especially in dizygotic twins. In 2016, among infants conceived by ART, 30.4% were twins and 1.1% were triplets and higher order infants, with the consequent increase in adverse maternal and perinatal outcomes.⁵

Multifetal pregnancies can be classified according to the number of fetuses, zygosity, or chorionicity. According to the number of fetuses, multiple pregnancies can be twins, triplets, quadruplets, etc. In clinical practice, most are twins. According to zygosity, multiple pregnancies can be classified as dizygotic or monozygotic. Dizygotic twins result from the fertilization of two oocytes by two sperms that produce two genetically different embryos. Monozygotic twins result from the fertilization of one oocyte and one sperm and the early splitting of the resulting embryo in two genetically identical twins.⁶

According to chorionicity and amnionicity, twin pregnancies can be dichorionic (DC) or monochorionic (MC). In DC twins, each fetus has its own placenta and its own amniotic sac (diamniotic [DA]), whereas in MC, both fetuses shared the same placenta and can be DA (˜99%) or monoamniotic (MA; ˜1%).⁷

The relationship between zygosity, chorionicity, and amnionicity is described in Algorithm 5.1. Around 70% of twins are dizygotic, and all of them are DCDA. Monozygotic twins are the result of the cleavage of a single inner cell mass. If this cleavage occurs before the fourth day after conception, the twin pregnancy will be DCDA (˜30% of all monozygotic twins). If the cleavage occurs after the third day, the pregnancy will be MC (˜70% of monozygotic twins). In 99% of those MC twins, the cleavage of the inner cell mass will take place between the fourth and eighth day, and it will result in MCDA twins. Only in 1% of MC pregnancies the cleavage will occur after the eighth day resulting in MCMA
twins. In rare cases, the twinning process will take place after the 12th day producing an incomplete inner cell mass cleavage that results in conjoined twins.⁷

From an obstetrical standpoint, the most important distinction is if the twin pregnancy is DC or MC. MC twins have a significantly higher morbidity and mortality, and they also have complications exclusive for MC placentation. Knowledge of the chorionicity is crucial for prenatal screening and diagnosis of fetal anomalies, for choosing the optimal strategy of surveillance, for the appropriate management of complications, and for perinatal decision-making. In clinical practice, it is no longer acceptable to talk about “twin pregnancy”—it must be classified as MC or DC.

Before 10 weeks of gestation, determining chorionicity is usually easy. The first structures that can be seen at 5 to 6 weeks are the gestational sacs. In a panoramic view of the uterus, if one gestational sac with two embryos is observed (Figure 5.2A-D), the diagnosis of MC twin pregnancy is established.
Conversely, if two gestational sacs are identified with one embryo in each sac (Figure 5.2C and D), DC twin pregnancy is diagnosed. An ultrasound image showing the chorionicity should be documented and kept in the patient’s records for future reference.⁸

All DC twin pregnancies are DA. However, if an MC twin pregnancy is diagnosed, the next step is establishing amnionicity. Of note, more than 99% of MC twins are DA. To establish amnionicity in MC twins before 10 weeks, the amniotic membranes have to be identified. In early pregnancies, the amnion is seen as a very thin membrane that surrounds the embryo. From 8 to 9 weeks onward, a thin intertwin membrane dividing both amniotic sacs can be observed (Figure 5.3).

If the amniotic membranes cannot be identified, amnionicity can be inferred through the assessment of the yolk sacs. Usually, the number of yolk sacs correlates with the number of amniotic cavities. Therefore, if two yolk sacs are observed, it is more likely to be an MCDA (Figure 5.4). However, this concept has been challenged because several exceptions have been reported^11,12 (Figure 5.5A). If monoamnionicity is suspected, confirmation can be obtained by the observing entanglement of the cords by color Doppler (Figure 5.5B).

At the 11- to 14-week scan, determining chorionicity and amnionicity is based on the assessment of the number of placental masses, and the characteristics of the membranes at the placental insertion sites. When two chorionic masses are identified, a DC twin pregnancy is diagnosed (Figure 5.6). If a single mass is observed, it could mean either an MC twin or a DC twin with fused placentas (Figure 5.7). In this circumstance, chorionicity is established by exploring the intertwin membranes at the site of the placental insertion. In DC twins, the intertwin membrane is thick and easily observed, and the placental insertion is thicker and shows a typical feature known as the “λ” (lambda) sign (Figure 5.8) produced due to the presence of chorionic tissue between membranes. Membrane insertion in MC twins shows a very thin membrane that

ends in a “T” form (Figures 5.7B and 5.8). Use of lambda sign predicts chorionicity with a high degree of accuracy before 14 weeks.¹³ Moreover, the combination of number of placentas, “T” sign, and “lambda” sign have shown more than 99% of sensitivity and specificity.¹⁴

Other ultrasound features related to chorionicity are the thickness and the number of layers of the intertwin membrane (Figure 5.8). In MC twins, the membrane is made of two layers of amnion, is thin, and is sometimes even difficult to identify by ultrasound. In DC twins, the membranes have four layers—two chorionic and two amniotic—that are thicker and usually easily identifiable by ultrasound.

If chorionicity cannot be determined by abdominal ultrasound, a transvaginal approach should be attempted. If chorionicity cannot be determined during a first-trimester scan in a routine setting, a second opinion should be sought from a specialist. If MCMA is diagnosed, the patient should be referred to a tertiary center.⁸

Occasionally, there are pitfalls in determining chorionicity.¹⁵ For instance, a single bilobulated
placenta or different phenotypic sex in heterokaryotypic fetuses could be a source of a misdiagnosis.

The performance in determining chorionicity decreases with increasing gestational age. Sometimes it becomes extremely difficult to establish a definite diagnosis. It has to be assessed using the number of placental masses and concordance or discordance of twins’ sex. If chorionicity cannot be established (DA with a single placental mass, twins of the same sex, and lack of first-trimester images), the pregnancy should be managed as MC.⁸