**2. Defining gestational age**

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].


**Table 1.** How the chorionicity and amnionicity are differentiated by the timing of the embryo splitting in monozygotic twins (Table is modified from Simpson L, 2015 [6]).

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].

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].

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.
