**1. Introduction**

Nowadays, 2D ultrasound is the most important diagnostic technique in obstetrics, especially in the diagnosis of congenital malformations.

The first diagnostic ultrasound screening usually takes place at around 11–13 weeks of pregnancy, when the thickness of the nuchal translucency is measured and the presence of the nasal bone is confirmed. Nuchal translucency (NT) is an excess fluid under the nuchal skin of the fetus in the first trimester. In 1866, Langdon Down described this phenomenon as trisomy 21 patients' having "too large skin" [1]. In the 1990s it was recognized that the excessive fluid

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is responsible for this thickening and that it was possible to measure the thickness of this fluid at the back of the neck around the third month of pregnancy [2, 3]. A thicker nuchal translucency might suggest chromosomal abnormality, but it can be present in cardiovascular malformations and genetic syndromes as well.

Malformations are usually easier to diagnose later in pregnancy as the organ develops and grows (i.e., heart malformations). Also, it is easier to detect anomalies when the defect grows with the

Congenital Fetal Anomalies and the Role of Prenatal Ultrasound

http://dx.doi.org/10.5772/intechopen.71907

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Brain anomalies are one of the most common group of congenital malformations. To be examined: the cerebellum, choroid plexuses, cisterna magna, lateral ventricles, cavum septi pellu-

**3.** cerebellar view: cerebellum, cisterna magna (Between the 15–22 weeks of gestation, the diameter of the cerebellum in mms is usually equal to the gestational weeks of the pregnancy.) [3, 4]

Any abnormality in these views may suggest a brain malformation, such as: neural tube defects, ventriculomegaly, holoprosencephaly, hydranencephaly, Dandy-Walker malformation, agenesis of the corpus callosum, porencephaly, intracranial tumor. The sensitivity of the ultrasound

Neural tube defects (NTDs) are the second most common types of malformations. The incidence of NTDs is around 1:1000 in the USA and 8:1000 in the UK. Anencephaly, encephalo-

Anencephaly is the absence of a major portion of the skull and the brain hemispheres caused by an abnormal closure in the cranial part of the brain. Exencephaly is an early stage of anencephaly, when the brain is still present but is located outside the skull. Anencephaly develops as the neural tissue degenerates. Exencephaly can be detected by first trimester ultrasound as floating brain tissue outside of the skull. CRL is usually lower than normal. In the second and third trimester, polyhydramnios is usually also present. Anencephaly is a fatal condition and

Encephalocele and meningocele are defects of the skull. In the former, brain tissue and meninges protrude through the defect of the skull, while in the latter only the meninges are affected. The defect of the skull can be detected with ultrasound. In 75% of the cases the defect is occipital.

Spina bifida is present when the spine does not close properly. There are three types based on

**b.** myelomeningocele: contains meninges, cerebrospinal fluid and neural structures

**3.** rachischisis/myeloschisis: the neural tube is completely open, no meninges or skin

gestational age (i.e., pyelectasis).

**2. Central nervous system**

what structures are affected:

**1.** occulta: no protrusion, covered with skin

**2.** cystica: protrusion, covered or not covered (**Figure 1**)

**a.** meningocele: contains meninges and cerebrospinal fluid

cidi. There are three major scan planes for the fetal brain:

is high in the diagnosis of these malformations [5, 6].

**2.** ventricular view: lateral ventricles, choroid plexuses, arteria

cele/meningocele and spina bifida are the most common NTDs [7–9].

most affected pregnancies are terminated after the diagnosis [7, 8].

**1.** thalamic view: BPD, HC measurement, thalamus, cavum septi pellucidi

NT is usually measured around 11–13 weeks of gestation. The thickness of the nuchal oedema is proportional to the crown-rump length (CRL) of the fetus so it is measured when the CRL is 45–84 mm. The higher the NT is at a specific CRL, the higher the risk for chromosomal abnormalities is [2]. For example, in Turner-syndrome, the NT value is around 8 mm higher than the median [2].

In Down-syndrome, nasal bone is absent in 60–70% of the affected fetuses. Langdon Down observed that those with 21-trisomy had small noses, caused by the hypoplasia of the nasal bone. The hypoplasia of the nasal bone can also be detected during pregnancy. According to a meta-analysis, the nasal bone was absent in 1.4% of healthy fetuses, while in 69% of fetuses with trisomy 21. Furthermore, the maxilla was shorter in 25% of the affected fetuses, and ductus venosus flow was abnormal in 80% [1, 2].

In Edwards-syndrome (trisomy 18), early growth retardation and bradycardia can be detected at 11–13 weeks. Also, the nasal bone is absent in 55% of the cases and 75% has a singular artery in the umbilical cord.

In Patau syndrome (trisomy 13), 70% of the fetuses have tachycardia. Early growth retardation, megacystis and holoprosencephaly are also frequently present [2].

The second ultrasound screening is usually done around 18–20 weeks of gestation. The goal of this screening is to diagnose congenital malformations and to detect other signs of chromosome anomalies and other syndromes. Therefore, this ultrasound examination is called "genetic screening."

Structures to be examined:


Malformations are usually easier to diagnose later in pregnancy as the organ develops and grows (i.e., heart malformations). Also, it is easier to detect anomalies when the defect grows with the gestational age (i.e., pyelectasis).
