Congenital abnormalities and assessment of fetal wellbeing

Neural tube defects

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.

Commonly cases are identified by ultrasound (US) screening in the early second trimester. Where there is an open neural tube, maternal serum alpha-fetoprotein (MSAFP) levels are raised. US assessment for other abnormalities should be undertaken and karyotyping offered if they are found.

Multidisciplinary counselling and care are recommended. 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 after birth. However, the defect may result in paraplegia and bowel and bladder incontinence and the need for repeated further surgery. The child often has normal intelligence and has insight, being particularly aware of the problems posed for the parents. Closed lesions generally do not cause problems and may escape detection until after birth.

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. In these cases, a higher dose of folic acid prophylaxis (5 mg/day) is recommended.

Congenital cardiac defects

The likelihood of a pregnant woman having a fetus with a congenital cardiac defect is about 0.6%. Some of these infants present with intrauterine growth restriction and oligohydramnios, but in many cases the condition is only recognized and diagnosed after birth. With improvements in real-time US imaging, recognition of many cardiac defects has become possible. However, early recognition is essential if any action is to be taken. Screening is performed by US. Most centres would undertake nuchal translucency scanning in the first trimester (around 11 weeks) and a four-chamber view of the fetal heart in the second trimester (at 18–20 weeks) ( Fig. 10.2 ). In addition, some centres are looking at blood flow in the ductus venosus and across the tricuspid valve at 11 weeks and the left and right outflow tracts at 18–20 weeks to try and identify more abnormalities. Despite this screening, even in the best centres less than 50% of congenital heart defects are currently identified prenatally.

The most common defects are ventricular and atrial septal defects, pulmonary and aortic stenosis, coarctation and transpositions of the great vessels, including the tetralogy of Fallot. These lesions can generally now be recognized on the four-chamber views recorded during detailed 18- to 20-week-gestation scans.

Once an abnormality is identified, management comprises further diagnostic US examination to establish the extent of the defect and whether there are any other fetal abnormalities (this may include karyotyping) and multidisciplinary counselling and care. In some cases where the prognosis is poor, the parents may opt for a termination of pregnancy. In other cases, delivery should occur in a centre with full neonatal cardiological services, including cardiac surgery.

Defects of the abdominal wall

Defects of the abdominal wall can be diagnosed by US imaging. US screening in the second trimester (18–20 weeks) in the best centres detects over 95% of cases. They include gastroschisis ( Fig. 10.3A ) and exomphalos ( Fig. 10.3B ). 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 2–3cm 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, especially trisomy 18.

If a gastroschisis is diagnosed, the parents can be told the prognosis is very good. Multidisciplinary care is needed. Delivery can be vaginal and should take place in a hospital with neonatal surgical facilities. All babies will require neonatal surgery to correct the defect; however, over 90% will survive.

In contrast, exomphalos has a very poor prognosis. Apart from the association with chromosomal abnormality, there is an increased risk (over 60%) of co-existent structural defects, especially cardiac. Further careful detailed US examination of the fetus should be undertaken once the diagnosis is made and karyotyping offered. Multidisciplinary care is advisable. If the parents opt to continue with the pregnancy rather than have a termination of pregnancy, this should take place in a unit with comprehensive neonatal facilities, including surgery.

Chromosomal abnormalities

Chromosomal abnormalities are common, with an estimated incidence of at least 7.5% of all conceptions. However, many of these result in a miscarriage, and the liveborn incidence is much less than that, at about 0.6%. Chromosomal abnormalities can be identified from culturing and karyotyping 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 (DS). In this condition, in at least 92% of cases the chromosomal abnormality is that each cell has three rather than two number 21 chromosomes (about 8% of cases are translocations – see later). This condition is discussed further later. The next most common are abnormalities of the sex chromosomes (Klinefelter’s syndrome with one extra sex chromosome in the form of two X-chromosomes and one Y-chromosome; Triple-X syndrome with an extra sex chromosome in the form of three X-chromosomes; and Turner’s syndrome with one only one sex chromosome, an X-chromosome), followed by trisomies 13 and 18 (Patau and Edwards syndromes, respectively).

Down’s syndrome

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

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 be a carrier of a balanced translocation.

Assessing fetal normality