Prenatal diagnosis of congenital heart defects

Introduction: prenatal diagnosis—routine and specialised examinations

Today, ultrasound examinations of the unborn child are an inherent part of routine prenatal care almost everywhere. Most European countries have an official country-wide policy for routine ultrasound anomaly scanning, which includes between one and three ultrasound examinations at different developmental stages of the unborn child (see Table 1 below). So, for example, in Germany, where the standard of prenatal care is relatively high, foetal ultrasound examinations are usually done at three different stages of pregnancy - around week 9 - 12 (mostly transvaginal), week 19 - 22 and week 29 - 32. They are an inherent part of routine prenatal care almost everywhere. Ultrasound is a non-invasive method and has hardly any risks for both the mother and the unborn child. It is used to monitor the development of the foetus and check for potential delays or malformations. For parents, the routine ultrasound provides the opportunity to “see” their child during pregnancy and to establish a strong emotional bond to their offspring.

In most cases, these routine examinations can help to ease parents’ fears and worries about their baby’s health by showing healthy development. Sometimes, however, the sonography shows abnormal findings, suggesting that there might be some disorder, developmental retardation or malformation. At this point, the examiner will probably recommend further, specialised examinations to investigate the suspected underlying problem in more detail to establish a firm diagnosis.
On the basis of individual risk factors of the expectant mother and on the suspected diagnosis, different methods are available for a close examination of the foetus.

Established risk factors for congenital heart defects

In addition to abnormal findings in a routine ultrasound examination, several factors are known to increase a mother’s risk to give birth to a child with a congenital heart defect. If one or more of these risk factors are present, the attending physician will probably recommend more specialised tests to look more closely at the baby’s cardiovascular system and search for potential malformations or dysfunctions. However, studies have found that a high proportion of affected children are born to parents without any established risk factors.

Risk factors for a congenital heart defect:

• Familial history of congenital heart disease—if a congenital heart defect has already occurred in either the father’s or mother’s side of the family, the unborn child has an increased risk of being affected. This risk is approximately 2–4%, depending on the degree of the relationship with the respective family member and on the type of heart defect (the risk in the general population for congenital heart disease is about 1%).
• Maternal metabolic diseases—for example, diabetes mellitus, epilepsy, phenylketonuria.
• Infections during early pregnancy.
• Exposure of the expectant mother to teratogenics—ie, various external influences that are known to cause malformations and developmental defects in the embryo or foetus. These influences include:
  • substances (eg, alcohol, nicotine, cocaine, benzodiazepines [or “benzos”], lithium, thalidomide [better known as Contergan©], specific solvents, dioxins);
  • pathogens (eg, rubella virus); and
  • radiation (eg, X-ray, radioactive radiation).
• Multiple pregnancy
• In-vitro fertilisation

Should I have prenatal diagnostic tests?

Aims, benefits and drawbacks of specialised prenatal diagnostic examinations

When it comes to deciding whether or not to have a specific test, remember that you alone should be the one to choose whatever seems best for you. No one can make this decision for you, just as no one can dictate what you must do. Specialised tests are not obligatory, even though they might be advisable in many cases. Whatever the results, prenatal tests offer no guarantee concerning your baby’s state of health. Besides, studies suggest that detection rates can differ considerably from region to region and depending on the examiner’s skills. So take your time to get all the information and advice you need before reaching a decision.

If you have opted to have specialised prenatal tests, you should prepare yourself for the possibility that the test might detect some abnormalities. Without doubt it is a great and moving experience to see the unborn child and thus be able to build up a strong connection to it, but you should keep in mind that ultrasound examinations are primarily performed to look for anomalies.

Being told that there is something wrong with one’s unborn baby is always a shock. If you have opted to have specialised prenatal tests, you should prepare yourself for the possibility that the test might detect some abnormalities. The knowledge of your baby’s illness might burden you and your family during pregnancy, causing fear and anxiety instead of eager and happy anticipation of birth and everything that is to come. Tests that do not yield definite results but only provide a risk assessment (eg, nuchal-fold scan) could add to troubled thoughts and even increase the feeling of uncertainty rather than provide reassurance.

However, learning about the baby’s condition during pregnancy can lead to the best possible preparation, for both the parents and the attending medical staff. For expectant parents, knowing about their baby’s disease at an early stage allows them to gather comprehensive information and to receive both medical advice and psychological counselling. Furthermore, they have enough time to discuss everything with friends and relatives. This confrontation of the problem will help them come to terms with the situation, allowing them to make informed decisions at all stages.

This decision might also include a termination of pregnancy, especially when the condition is severe and poses a threat to the child’s or the mother’s life. It is important to emphasise that prenatal diagnostic tests are not used to facilitate some sort of preselection to sort out “defective” children. However, termination of the pregnancy might seem the best possible solution in some circumstances, and no one else but the prospective parents should make the decision.

If parents-to-be decide to continue the pregnancy after receiving the prenatal diagnosis of a congenital heart defect, they still have enough time to prepare themselves mentally for what is to come and to learn everything that they need to know about the condition of their child. By getting in touch with self-help groups and other professional organisations, they can get information about the consequences of their child’s heart defect and the effect it will have on all their lives, and find out everything about the course of disease and possible treatments. This knowledge will make coping with the situation after their baby is born much easier.

Furthermore, the attending medical staff, ideally a multidisciplinary team of specialists, can provide the best possible management of both pregnancy and delivery, and make all necessary arrangements to ensure that the child receives the optimum treatment. Studies suggest that foetal echo leads to a decrease in neonatal morbidity and mortality due to CHD. Dependent on the type of heart defect, the expectant mother might be advised to give birth in a specialised centre or at a hospital with a paediatric intensive-care unit. Some newborn babies might need certain drugs to be administered immediately or an intervention to be done as soon as possible. Sometimes, even intrauterine treatment might be an option. In either case, the careful and individual preparation of all necessary steps will enhance the child’s chances of survival significantly and lay the foundation for a life as normal as possible for the entire family.

Prenatal diagnostic methods to detect a congenital heart defect

Foetal echocardiography

Foetal echocardiography is the most common diagnostic method for the prenatal detection of CHD. A foetal echocardiogram (echo) is a special type of ultrasound examination that enables a very detailed assessment of the structure and function of a baby’s heart before birth. With use of refined ultrasound techniques such as colour Doppler echocardiography, the examiner is able to visualise in detail the heart chambers, valves and vessels and the direction and speed of the blood flow. Thus, an exact assessment of the heart function, including heart rate size and rhythm, is possible and malformations or other abnormalities can be detected at an early stage.

Pregnant women who have a high risk of having a child with a congenital heart defect (eg, if they have already given birth to a child with this disease or if chromosomal or extracardiac abnormal changes have already been confirmed), can have an early foetal echo, mostly transvaginal, around the 15th week of gestation. Although details of the heart structure can already be seen at that stage, the results from this early examination are sometimes unclear and the chances that something is missed or misinterpreted are fairly high. Therefore the early foetal echo can be complemented with a second scan that, according to guidelines of the International Society of Ultrasound in Obstetrics and Gynaecology, should take place between week 18 and 22. This period is considered to be the ideal time to detect a cardiac malformation, since the baby’s heart and organs have developed enough by then to be seen clearly.

The value of a foetal echocardiogram greatly depends on the examiner’s skills. Therefore, an expert who is specially trained in looking after the cardiovascular system of unborn babies and in the management of pregnancies involving a congenital heart defect should do the foetal echocardiography. This person can be a cardiologist or an obstetrician, who is specially trained in echocardiography.

However, some abnormalities are not detectable before birth, even with a detailed expert examination. These conditions are mainly minor defects such as small holes between the chambers or mild valve abnormalities. Additionally, some congenital heart malformations do not become evident until a few days after birth because of changes in the heart and lung physiology: the transformation from foetal to normal circulation.

Despite these drawbacks, foetal echocardiography is still a reliable and common diagnostic method for the detection of congenital heart defects. This examination is non-invasive and uses sound waves and no ionising radiation. It is painless and has no risks for either mother or unborn child. Therefore it can be safely recommended, especially in pregnancies with an increased risk of a congenital heart defect.

Should foetal echocardiography detect the presence of a congenital heart malformation, it could indicate that this is not just an isolated heart defect but part of an underlying syndrome or genetic disorder (eg, Down’s syndrome or Marfan syndrome). There is also the possibility of multiple malformations. According to a survey by Eurocat (European Surveillance of Congenital Anomalies), 18% of cases diagnosed with a CHD between 2000 and 2005 were associated with other major anomalies. As the presence of further anomalies can have a considerable impact on the child’s prognosis, a complete and differential assessment is recommendable to get a clear picture. Further examinations such as amniocentesis can be used to clarify the situation.

Nuchal-fold scan

This scan is another type of specialised ultrasound examination that is done between week 11 and 14 of pregnancy if considered necessary, for example in cases of suspected abnormalities or a known maternal predisposition. It can be easily combined with the routine pregnancy scan.

In the early stage of development, every foetus has an accumulation of fluid in the soft parts at the back of its neck, called the nuchal fold. During the examination this area is measured, since evidence suggests that an increased nuchal-fold thickness can indicate the presence of some abnormalities. Although this test is mainly used to assess the baby’s risk of having Down’s syndrome or other chromosomal abnormalities, several studies have reported that an increased nuchal-fold thickness can also be associated with a greater risk of a congenital heart defect.

However, this test is a screening tool that can only indicate the statistical probability of the baby having a defect or disease. It cannot confirm or rule out the presence of an abnormality with certainty. In the same way, an increased nuchal-fold thickness is not specific for any congenital heart defect.

Currently, the risk that an increased nuchal-fold thickness is accompanied by a congenital heart defect is estimated to be approximately 4–7%. However, the relationship between specific types of heart malformation and nuchal-fold thickness is still very uncertain, and the issue is controversial among health professionals.
Therefore, the nuchal-fold scan should be regarded as a screening examination. A positive result of the nuchal-fold measurement can be an important indication for further tests such as complete foetal echocardiography or an amniocentesis.

Amniocentesis

Amniocentesis is the most common invasive prenatal examination. It is mainly used to detect or rule out chromosomal and specific genetic abnormalities. It cannot identify congenital heart defects, but it can be performed to establish whether a suspected or confirmed heart malformation is part of an underlying syndrome that is genetically detectable. Several genetic syndromes are known to frequently involve congenital heart defects. For some of them such as Down’s syndrome, Marfan syndrome, Turner’s syndrome, genetic tests are available. For these diseases, amniocentesis can be an important method for the prenatal diagnosis of congenital heart defects.

During the examination, a thin needle is inserted into the uterus of the woman and a small sample (2–4 teaspoons) of amniotic fluid, which surrounds and protects the baby, is withdrawn in a syringe. To protect the foetus from any potential damage from the needle, ultrasound is used simultaneously to identify the baby’s exact location and to observe its movements. Thus the risk of injury can be kept to a minimum.

The amniotic fluid contains foetal cells, which are tested for anomalies. By a chromosomal analysis, several or structural chromosomal abnormalities like Down’s or Turner’s syndrome can be detected. Specific DNA analysis can discover some genetic disorders such as William’s syndrome. The first results are already available after a few days, whereas the whole assessment is usually completed after 2 weeks. Amniocentesis can detect specific genetic disorders such as Down’s syndrome accurately and with certainty. However, this test cannot identify all birth defects, including heart defects, and the fact that no abnormal change is detected does not guarantee that the baby will be born healthy.

Amniocentesis for the detection of genetic disorders is usually done between the 15th and 20th week of gestation (second trimester), when the risks involved are fairly low. However, a slight risk of complications does remain, even if the examination went well and no abnormality was detected. These complications include:

• Injury to mother or foetus from the needle, although this risk is reduced by using ultrasound to guide the needle. If an injury does occur, for example puncture of the placenta, it usually heals without any further problems.
• Maternal infection due to the intrusion of bacteria into the amniotic sac; however, this complication is very rare.
• Exposure of the mother to the baby’s blood, although this is a problem only if the mother’s blood is rhesus-negative and the baby’s blood is rhesus-positive. In this case, complications can be avoided by injecting the mother with appropriate antibodies. However, the risk of this complication occurring is low.

Very rarely, complications due to amniocentesis can lead to miscarriage. Data reported in published work suggest that miscarriage can occur for between 1 in 100–400 cases.

If considering amniocentesis, you should always weigh up the risks and the benefits of the procedure and the use of the results in your particular case. You should discuss the issue with your attending doctors to help you make an informed and appropriate decision.

Other prenatal diagnostic tests

In addition to the diagnostic tests described in this section, several other examinations can be undertaken prenatally, such as umbilical cord blood sampling or chorionic villus sampling. These are other procedures by which to obtain foetal cells for genetic testing. Because these procedures inherit a greater risk for both the foetus and the mother, they are rarely used to make a prenatal diagnosis of congenital heart defects and are not described here in detail.

Concluding remarks

Several prenatal diagnostic methods can help in both the management of pregnancy and the preparation of delivery. These methods are particularly helpful for cases in which a certain predisposition or other indication for the presence of a congenital heart defect is known.

Although many conditions can be identified by one or more of these tests, these methods do have limitations. Not all abnormalities are detected by prenatal tests. If the results of an examination are negative, there is still no guarantee that the child will be born healthy.

According to published work, only 20–30% of all congenital heart defects are detected in routine prenatal care. In specialised centres, however, this figure rises to around 60–95%. Generally, studies suggest that the rate of detection of malformations has improved in line with advances in technology and examining skills. Furthermore, the gestational age at the time of diagnosis has been lowered, which means malformations are detected earlier. Studies have also found out that the detection rates of isolated CHDs are lower than those of CHDs associated with a syndrome, a chromosomal anomaly or other malformations.

Again, it is up to you to decide whether specialised prenatal diagnostic tests are an appropriate option for you.

National policy/ recommendations for routine prenatal ultrasound scans in place in 2004 in 18 European countries

*Not official policy but usually performed.

Source: Boyd et al. 2008