**1. Introduction**

It is a well-established fact that multiple pregnancies occur more commonly nowadays than a few decades ago. The progress of reproductive technologies and *in vitro* fertilization has played a major role in this increase. In fact, twins comprise about 3% of all live births in the United States [1]. As we speak about history, the vast majority of multiple pregnancies that occurred

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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

In the first trimester—before the 14th week of gestation—crown-rump length (CRL) is the parameter that is used in order to estimate gestational age with 5–7 days of deviation [7–9]. If there is a doubt about the reliability of the menstrual cycle or if the woman is administrated late for care, a repeat scan in 3–4 weeks can be helpful to determine pregnancy dating [10].

Early Pregnancy Ultrasound Assessment of Multiple Pregnancy

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Modest size discordance is very common in multiple pregnancies [4]. Some studies suggest that pregnancy dating must be defined by using the mean of the fetuses [11]. However, more recent studies agreed that if the gestational age is based on the CRL of the larger twin, the possibility of missing a fetus that might develop intrauterine fetal growth restriction (IUGR) is decreased [12]. Salomon et al. [13] suggested that the CRL of the smallest fetus can estimate more accurately the gestational age, if the intertwin CRL discrepancy is less than the 95th percentile, using charts from studies. An interesting finding is that if the intertwin discordance in CRL is higher than 10%, the

possibility of pregnancy loss, aneuploidy, or congenital anomalies is increased [3, 14, 15].

**3. Defining chorionicity and amnionicity**

week 10 to week 14.

**3.1. Before 10 weeks of gestation**

(3) the number of yolk sacs [4].

*3.1.1. Number of observable gestational sacs*

intervention can optimize the outcome of the pregnancy [4].

In the second trimester, a combination of parameters is used to define pregnancy dating such as abdominal circumference, femur length, and biparietal diameter [8]. Further discussion about calculating gestational age in second trimester is beyond the scope of this chapter.

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

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

Three ultrasound findings can help in the detection of chorionicity: These are (1) the number of observable gestational sacs, (2) the number of amniotic sacs within the chorionic cavity, and

The number of the gestational sacs and the number of fetal heartbeats in early multiple pregnancy scan are strongly related with chorionicity: each gestational sac will form a distinct placenta and chorion. Therefore, visualization of a single gestational sac with two visible heart beats indicates a monochorionic twin pregnancy, while the presentation of two distinctive

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 exceptions despite being infrequent [5].

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 each type of a monozygotic pregnancy [3].
