*4.1.1. Ultrasound*

Fetal ultrasound was developed as A-mode in the late 1950s, then modified to B-mode in the 1970s, followed by real-time imaging in the 1980s, and 3D imaging [32, 33] in the 1990s. Currently, ultrasonography is applied throughout pregnancy. Transvaginal ultrasonography is useful for examining the gestational sac at approximately 5 weeks, the yolk sac at 5.5 weeks, flickering cardiac motion at 6 weeks, etc. Embryos and early fetuses within 12 weeks of gestation are usually examined by transvaginal ultrasonography, whereas those beyond 12 weeks of gestation are examined by transabdominal ultrasonography. Ultrasonography is used for examining embryos and fetuses for several reasons, one of them is to determine the gestational age and estimate the fetal weight. A formula for estimating the latter was first suggested in the late 1970s [34]. Since then, a number of formulae have been proposed and accepted [35–39], while new formulae for the estimation have also been frequently promoted [40, 41]. Another purpose of ultrasonography is to detect (and occasionally, to assess) congenital fetal anomalies. Ultrasonography was first applied to evaluate anencephaly [42], but now it is able to detect a wide range of anomalies. The definition of optimal fetal anatomy survey has been published as guidelines by the International Society of Ultrasound in Obstetrics and Gynecology (ISUOG) [43] for performing effective screening of morphological anomalies. Meanwhile, studies conducted during the 1980s–1990s made it clear that soft markers in ultrasonography indicate an elevated risk of chromosomal abnormalities [44–46], even though they may not be directly harmful by themselves. Soft markers combined with maternal serum is capable of detecting aneuploidy with high precision [47].

#### *4.1.2. Magnetic resonance imaging*

comprises of ectodermal dysplasia and cleft lip, occasionally accompanied by cleft palate, due to an autosomal dominant inheritance [27]. The prevalence is approximately 1 in 18,000 births [28], and can be observed as early as CS 18, at the stage when the finger rays develop. There is no difference between females and males in terms of occurrence, and surgical treatment is

Sirenomelia, named after the Sirens (half-bird women in Greek mythology, often confused with mermaids), is an extremely rare form of malformation of the extremities, characterized by fused lower limbs hence resembling "merpeople" (**Figure 4D** and **E**) [29]. With only a single umbilical artery and vein, it is difficult for both limbs to develop, resulting in the formation of fused lower limbs [30]. It can be divided into categories, based on the degree of development of the lower limb: sympus apus, sympus monopus, and sympus dipus, referring to absence of feet, short feet, and a pair of feet, respectively. Anomalies of the kidneys, large intestine, and external genitalia are commonly observed as accompanying complications.

This lethal congenital anomaly begins to show up at CS 13, with an occurrence rate of approximately 1.5–4.2 in 100,000 births, with more than half of them born dead. All such cases lead to death within 5 h after birth [29]. Although chromosomal aneuploidy is not associated, maternal diabetes mellitus and monozygotic twins are considered important factors for increasing

Fetal ultrasound was developed as A-mode in the late 1950s, then modified to B-mode in the 1970s, followed by real-time imaging in the 1980s, and 3D imaging [32, 33] in the 1990s. Currently, ultrasonography is applied throughout pregnancy. Transvaginal ultrasonography is useful for examining the gestational sac at approximately 5 weeks, the yolk sac at 5.5 weeks, flickering cardiac motion at 6 weeks, etc. Embryos and early fetuses within 12 weeks of gestation are usually examined by transvaginal ultrasonography, whereas those beyond 12 weeks of gestation are examined by transabdominal ultrasonography. Ultrasonography is used for examining embryos and fetuses for several reasons, one of them is to determine the gestational age and estimate the fetal weight. A formula for estimating the latter was first suggested in the late 1970s [34]. Since then, a number of formulae have been proposed and accepted [35–39], while new formulae for the estimation have also been frequently promoted [40, 41]. Another purpose of ultrasonography is to detect (and occasionally, to assess) congenital fetal anomalies. Ultrasonography was first applied to evaluate anencephaly [42], but now it is able to detect a wide range of anomalies. The definition of optimal fetal anatomy survey has been published as guidelines by the International Society of Ultrasound in Obstetrics

scheduled when the child is 1 or 2 years old.

32 Congenital Anomalies - From the Embryo to the Neonate

*3.3.3. Sirenomelia (meromelia)*

the risk of sirenomelia [31].

**4.1. Imaging modalities**

*4.1.1. Ultrasound*

**4. Diagnostic strategies for human embryos**

Magnetic resonance (MR) microscopy refers to MR imaging for screening small samples. It is significantly useful for the 3D measurement of chemically fixed human embryos, due to the large amount of mobile or NMR responsive protons existing in the preservation fluid (formalin) [48]. Being a non-invasive and non-destructive imaging process, it has been applied to a number of animal models for understanding developmental embryology [49–52]. MR imaging provides highly beneficial features [50, 53, 54], reaching a resolution of 40 μm/pixel or higher when scanning a sample for an extended amount of time. Superconducting magnets with field strength of 1.0–9.4 T [52, 54, 55] have been used for describing human embryo using MR imaging. **Figure 5C**–**D** and **E–F** is obtained with MR microscopes equipped with 7.0 and 2.34 T magnets, respectively.
