Neural tube defects

The neural tube defects are the commonest of the major congenital abnormalities and include anencephaly, microcephaly, spina bifida with or without myelomeningocele, encephalocele, holoprosencephaly and hydranencephaly ( Fig. 10.1 ). The incidence is approximately 1/200 and the chance of having an affected child after one previous abnormal child is 1/20. Infants with anencephaly or microcephaly do not usually survive. Many die during labour and the remainder within the first week of life. Infants with open neural tube defects often survive, particularly where it is possible to cover the lesion surgically with skin. However, the defect may result in paraplegia and bowel and bladder incontinence. The child often has normal intelligence and becomes aware of the problems posed for the parents. Closed lesions generally do not cause problems and may escape detection until after birth.

Two common abnormalities of the central nervous system. (A) Anencephaly. (B) Spina bifida with open neural tube defect.

There is good evidence that pre- and periconceptual folic acid supplementation (400 µg/day) reduces the incidence of this condition. Once a woman is pregnant, the major effort is directed toward screening techniques that enable recognition of the abnormality and the offer of a termination of the pregnancy where there is a lethal abnormality.

Folic acid dietary supplementation is indicated both before and during pregnancy in those women who have experienced a pregnancy complicated by a neural tube defect.

Congenital cardiac defects

Some of these infants present with intrauterine growth retardation and oligohydramnios, but in many cases the diagnosis is recognised and diagnosed after delivery. With improvements in real-time ultrasound imaging, recognition of many cardiac defects has become possible; however, early recognition is essential if any action is to be taken. A four-chamber view of the fetal heart is shown in Figure 10.2 .

Four-chamber ultrasound view of the fetal heart.

The most common defects are ventricular and atrial septal defects, pulmonary and aortic stenosis, coarctation and transpositions, including the tetralogy of Fallot. These lesions can generally now be recognized on the four-chamber views recorded during detailed 18-week gestation scans.

Defects of the abdominal wall

Defects of the abdominal wall can be diagnosed by ultrasound imaging. They include gastroschisis and exomphalos ( Fig. 10.3 ). In both cases, the bowel extrudes outside the abdominal cavity. The main differences between the two are that a gastroschisis is a defect that is separate from the umbilical cord (usually 2–3 cm below and to the right), does not have a peritoneal covering and is usually an isolated problem. In contrast, an exomphalos is essentially a large hernia of the umbilical cord with a peritoneal covering and an increased risk of an underlying chromosomal abnormality,

Bowel extrusion associated with exomphalos.

Chromosomal abnormalities

A considerable number of chromosomal abnormalities have been identified from the culture and karyotyping of fetal/placental cells in the amniotic fluid or from the chorionic plate. The chromosomal abnormalities include both structural and numerical abnormalities of the karyotype. The commonest abnormality is that associated with trisomy 21 or Down’s syndrome.

Down’s syndrome

Down’s syndrome (DS) is characterized by the typical abnormal facial features ( Fig 10.4 ), mental retardation of varying degrees of severity and congenital heart disease. The karyotype includes an additional chromosome on group 21 (‘trisomy 21’; Fig 10.5 ). The incidence overall is 1.5/1000 births. However, the risk increases with advancing maternal age (see below). The underlying reason is thought to be an increased frequency of non-disjunction at meiosis.

Facial appearance of infant with Down’s syndrome.

Trisomy 21 karyotype.

About 6–8% of affected infants have the disease as a result of a translocation and the extra 21 chromosome carried on to another chromosome, usually in group 13–15. The mother or the father will usually show evidence of being a balanced translocation carrier.

Screening

Screening in this context is the process whereby women with a higher risk of fetal abnormality are identified in the general population. This screening is undertaken using identification of clinical risk factors, ultrasound (US) and biochemical testing of maternal serum. Clinical risk factors can be identified throughout pregnancy though the options for management are different depending on the gestational age. US and biochemical screening is offered to women in the first half of pregnancy. Ideally women should be offered a combined screening test (using US and biochemistry) for DS towards the end of the first trimester and a detailed US scan at about 20 weeks. The early scan also allows gestational age to be confirmed. If a woman presents too late for the first trimester DS screening, then she should be offered a biochemical screening test at about 16 weeks.