**1. Introduction**

According to the World Health Organization (WHO), the term congenital anomaly includes any morphological, functional, biochemical or molecular defects that may develop in the embryo and fetus that is present at birth, whether detected at that time or not [1].

Major congenital anomalies impair function or greatly interfere with cosmetic value.

They may be life threatening and therefore need immediate management. Major anomalies could have a negative impact on a child's well-being and development, if early corrective surgery is not done. Minor anomalies, on the other hand, may cause little or no functional effects. They do not cause distress in the newborn and hence there is no urgency for their correction in the neonatal period [2].

Congenital anomalies occur in 2–3% of all births and are an important cause of perinatal morbidity and mortality accounting for 20–30% of perinatal deaths.

In a study conducted by Nayab Alia in Madina Teaching Hospital, Faisalabad on gray scale ultrasound, antenatal prevalence of congenital anomalies was 29.75 per 1000 and 2.97% [3]. A similar study in Saudi Arabia showed the antenatal prevalence of congenital anomalies to be 27.96 per 1000. The median maternal age at diagnosis was 27.5 years and the median gestational age at diagnosis was 31 weeks [4].

However, the actual numbers of these anomalies vary among different countries with prevalence of anomalies reported to be 2% in England and 1.49% in South Africa [5]. The reason for the regional difference of congenital anomalies might be attributed to variable factors, such as: maternal risk factors, environmental exposures, ecological, economical and ethnic factors [6, 7].

#### **1.1 Pattern of congenital anomalies**

The patterns of congenital anomalies may be different between regions and the actual numbers may vary significantly between countries [8]. In different countries, people have varied cultural and religious practices including exposure during antenatal period to various environmental factors. This may contribute to varied patterns of congenital anomalies. In some instances a common exposure to teratogenic factors or a hereditary condition with variable penetrance, may lead to high numbers of some anomalies, where severely affected individuals were not observed because of fetal/infant mortality [9]. Certain population groups are also regarded as "high risk groups" for congenital anomalies such as those living in heavily polluted industrial zones [10].

According to one study, frequency of congenital anomalies was more in males than females, with CNS anomalies being the most common. The anomalies were more common in gestation age of 29–32 weeks [11]. In a retrospective study of 200 cases of congenital anomalies carried out in Jos, Nigeria, the highest incidence was reported in the gastrointestinal system 61 cases. No association was found between the occurrences of the various congenital anomalies [12].

In yet another study conducted among South African live born neonates at Kalafong Hospital, Pretoria, in which the incidence of congenital anomalies was 11.87 per 1,000 live births, the most commonly affected system was the central nervous system (2.30 per 1,000 live births) [13].

#### **1.2 Risk factors for congenital anomalies**

According to WHO, approximately 50% of all congenital anomalies cannot be associated with a specific cause. However, there are some known risk factors which include socioeconomic factors with an estimated 94% of severe birth defects occurring in middle and low income countries. This is because mothers are more susceptible to macronutrient and micronutrient malnutrition and may also have increased exposure to agents that cause or increase the incidence of abnormal prenatal development, especially infection and alcohol. Other known factors are genetic and environmental factors [14, 15]. This is reaffirmed by a study that was conducted in Tanzania, that showed significant association between congenital anomalies and lack of periconceptional use of folic acid, maternal age above 35 years, exposure to pollutants and high birth order above [16].

Women with uterine anomalies have also been found to be at risk for particular CAs. In one study, the risk for some specific defects such as nasal hypoplasia, omphalocele, limb deficiencies, teratomas, and anencephaly was four times higher among offspring of mothers with a bicornuate uterus [17].

#### **1.3 Ultrasound imaging IN congenital anomalies**

Many congenital anomalies are identified prenatally on usual work up which includes detailed ultrasound and amniocentesis.

The diagnostic ability of ultrasound is well established by several studies with detection rate dependent on a number of factors which include the type of

#### *Antenatal Diagnosis of Congenital Anomalies on Ultrasound Screening DOI: http://dx.doi.org/10.5772/intechopen.98586*

abnormality, sophistication of equipment and experience of operator [18]**.** In a study conducted by Shama Munim at Aga Khan University Hospital (AKUH), Karachi on the accuracy of ultrasound in the diagnosis of congenital abnormalities, antenatal ultrasound successfully diagnosed fetal abnormalities in 48.8% of cases and more than 90% Central Nervous system defects and renal abnormalities [18]**.**

However, a 1997 Report of the Royal College of Obstetricians and Gynecologists Working Party on Ultrasound Screening for Fetal Abnormalities identified that one of the problems with screening scans was the variable way in which they were conducted because there are no clear guidelines about what should, or should not, be examined. In a multicenter study, comparison was made of the precision of sonographic examinations done before 24 weeks gestational age at tertiary ultrasound laboratories contrasted with nontertiary ones. All the institutions were amply furnished with up-to-date equipment and supported with didactic and practical inservice training as required. The study found global sensitivity for sonographically demonstrable fetal abnormalities was 35% in tertiary centers, significantly higher compared to 13% in community hospitals. This further emphasizes that operator experience, competence, and training are vital determinants [19].

Ultrasound imaging is now routinely used in most countries for the purpose of screening pregnancies for fetal malformations but modalities, reliability and value of such screening is controversial [14, 19].

Regarding the time in gestation at which sonographic screening should be done, it is worthy noting that majority of the structural abnormalities are increasingly detected with advancing gestation. In early pregnancy, it is possible to diagnose with confidence certain categories of fetal anomalies, such as anencephaly, which can be reliably demonstrated at 10–14 weeks gestational age [20]. In certain instances, omphalocele and extremity malformations are also detectable using sonography in the first trimester, whereas other structural defects, like urinary system anomalies, are demonstrable later in pregnancy. However, a routine anomaly scan should be performed between 18 and 22 weeks of gestation. This period compromises between dating the pregnancy and the timely detection of major congenital anomalies [21].

Ultrasound examination at 10–14 weeks includes measurement of nuchal translucency, which is the maximum thickness of the subcutaneous translucency between the skin and the soft tissue overlying the cervical spine of the fetus. An increased nuchal translucency is associated with aneuploidy and cardiac malformations [22].

Ultrasound at around 20–21 weeks is done to screen fetuses for morphological anomalies. The utility of second trimester sonographic scan for detection of chromosomal anomalies was first recommended in 1985 [23]. Chromosomal aberrations were increasingly found to be associated with certain ultrasound findings, including biometric parameters (e.g., shortened femur and humerus, pyelectasis, thickened nuchal fold, dilated ventricles, fetal growth retardation) and morphologic features.

Ultrasound is the main diagnostic tool in the prenatal detection of congenital anomalies. It allows examination of the external and internal anatomy of the fetus. Even though a number of women are at increased risk of fetal malformations, either as a result of family history or owing to exposure to teratogens like infection and some drugs, the great majority of fetal abnormaitlies arise in the low risk category. As a result, sonographic evaluation ought to be offered routinely to all pregnant mothers. This is typically performed at 18–23 weeks of gestation, and should be done to a high level of precision. The scan should comprise systematic evaluation of the fetus for the detection of any defects.
