**3. Defining chorionicity and amnionicity**

in the past were diagnosed during the intrapartum period [2]. Today, as the use of ultrasound has become a routine in daily medical practice, multiple pregnancies are diagnosed in the initial ultrasound scan [3]. Beyond the diagnosis of early multiple pregnancy, ultrasound scan

In this chapter, we will present the ultrasound figures that help us determine gestational age, chorionicity, and amnionicity, focused on the 14 first weeks of gestation in multiple pregnancies. We will also focalize the discussion on twin pregnancies, as they comprise >98% of multiple pregnancies and the vast majority of studies today include twin pregnancies [4]. Nonetheless, we will review some cases from the literature that show that situations can be a little more complicated and may lead to a false diagnosis of chorionicity and amnionicity, in order to highlight that when we manage multiple pregnancies, we have to be alert about

A twin pregnancy can be either dizygotic (two-third of twin pregnancies), in which two different eggs are fertilized by two different sperms, and in this case, the pregnancy is always dichorionic-diamniotic or monozygotic. A monozygotic pregnancy occurs when an egg is fertilized by one sperm, producing one embryo, which can split any time, more commonly between day 2 and day 13 after fertilization. Chorionicity and amnionicity are differentiated by the timing of embryo splitting. **Table 1** presents this differentiation and the frequency of

The accurate determination of gestational age is critical for pregnancy management as it shows wherever the measurements of the fetus are in line for the estimate gestational age [4]. In addition, a correct pregnancy dating is necessary not only for the appropriate timing for screening and diagnostic testing but also for optimal scheduling of delivery [6]. For women with regular cycles, the date of the last menstrual period is used to estimate gestational age, taking into account the biological variability and correct the cycle length. For IVF pregnancies, the date of the embryo transfer has been used to define pregnancy dating. The vast majority of authors embraced with multiple pregnancies agree that during the second trimester the evaluation of gestational age is more accurate and it is statistically superior to the second trimester [4]. Moreover, there is an agreement that the parameters and formulas that have been used for dating singleton pregnancies are also accurate for dating multiple pregnancies, since studies in this area include a combination of singleton and multiple pregnancies [7–9].

**Time of embryo splitting (in days) Chorionicity Amnionicity Frequency (%)**

**Table 1.** How the chorionicity and amnionicity are differentiated by the timing of the embryo splitting in monozygotic

2–3 Dichorionic Diamniotic 30 3–8 Monochorionic Diamniotic 70 8–13 Monochorionic Monoamniotic <1

is more than necessary to define chorionicity, amnionicity, and gestational age [4].

exceptions despite being infrequent [5].

64 Multiple Pregnancy - New Challenges

each type of a monozygotic pregnancy [3].

**2. Defining gestational age**

twins (Table is modified from Simpson L, 2015 [6]).

Early and accurate definition of chorionicity and amnionicity has an undeniably determinant role in the management of multiple pregnancies, since chorionicity plays a key role in the appearance of complications: monochorionic-monoamniotic twins present the highest mortality and morbidity. There is no doubt that the continuous surveillance and the timely intervention can optimize the outcome of the pregnancy [4].

The determination of chorionicity and amnionicity is better to be done in the first trimester [4]. If chorionicity is defined in the first trimester, accuracy is extremely close to 100% and if the definition is carried out in the second trimester, correct assignment decreases to 90% [16, 17].

At this point, we will classify the determination based on gestational age, separated in two periods: the first before the 10th week of gestation and the second that includes the period from week 10 to week 14.
