Antenatal screening

Screening differs from diagnosis

Antenatal screening is the process of identifying those at high risk of a disorder. Prenatal diagnosis establishes whether or not the disorder is definitely present. Screening is used to select a high risk group so that they can be offered prenatal diagnosis. Selection is needed, since for most disorders diagnosis is only possible by an invasive procedure and this carries a slight risk of miscarriage. The main procedures are amniocentesis and chorionic villus sampling (CVS). Screening does not replace diagnosis; it aims to provide information which can help decision-making.

Decision-making

Those considering screening need to make a number of decisions. Initially there are three: whether to be screened at all, for which specific disorders, and which test. For those screened and identified as high risk, there is the decision whether to undergo prenatal diagnosis. Following this, if the pregnancy is found to be affected a further decision will need to be made: whether to have a termination of pregnancy.

Disorders

Screening tests available today are designed principally to screen for Down's syndrome, fragile X syndrome (FXS), Spinal Muscular Atrophy (SMA) and Cystic Fibrosis (CF). In addition the Down's syndrome screening tests can also detect other disorders. These are Edwards' syndrome, and if testing is after 15 weeks gestation, Neural Tube Defects (NTDs) and Abdominal Wall Defects (AWDs). The table shows that most of the additional disorders are either rare, incompatible with survival, or surgically correctable:

† in the 1970's: current rates unknown but thought to be lower because of dietary changes.

Some Terminology

The results of a screening test are either 'screen positive' or 'screen negative'. A screen positive result means that the risk is high enough to consider having prenatal diagnosis. It does not mean that the baby is definitely affected. Most of those with screen positive results go on to have normal babies. Similarly, a screen negative result means that there is not a high risk. It does not mean that an affected pregnancy has been completely excluded.

The quality of a screening test is determined by the 'detection rate' and the 'false-positive rate'. The detection rate is the percentage of affected pregnancies identified as high risk, and the false-positive rate is the percentage of unaffected pregnancies identified as high risk.

Procedures and gestations

The gestational age at which the various screening and diagnostic tests are carried out and the type of procedure involved are:

*These tests are not generally available on the NHS but can be obtained privately

Amniocentesis and CVS are used to obtain fetal cells which can be analysed for chromosome number and genetic mutations. Amniocentesis can also be used in the diagnosis of spina bifida and AWDs by measuring the concentration of alpha-fetoprotein (AFP) which leaks from an open fetal lesion into the fluid surrounding the baby (amniotic fluid).

Amniocentesis involves inserting a fine needle through the woman's abdomen into the womb. A sample of the amniotic fluid is taken and from this fetal cells are extracted. CVS can be done either by the insertion of a fine needle through the abdomen or through the vagina. A sample is taken of the placenta which comprises fetal cells only.

Both procedures take about 10 minutes and are performed on an out-patient basis. Some women may experience slight discomfort. The advantage of CVS is that it can be done earlier in pregnancy, from about 10 weeks; amniocentesis is usually performed around 16 weeks, although practice varies. Both procedures carry an element of risk and about one woman in every 100-200 may miscarry, over and above those who would have miscarried anyway. In some cases other mild complications occur such as bleeding or leakage of amniotic fluid.

For chromosomal anomalies, results are usually available within 2-3 weeks. With CVS, a provisional result may be given within 1-2 days; however, it may take 2-3 weeks to confirm. Occasionally, a conclusive result is not obtained and the procedure needs to be repeated. For FXS, SMA and CF results are available within days.

In most countries a specialist ultrasound ('anomaly') scan is carried out routinely at 18-20 weeks gestation to identify structural abnormalities in the fetus.  Other signs seen on the scan, known as 'soft' markers can also be used to modify the results of Down's syndrome screening but the scan cannot be used to exclude this disorder or Edwards' syndrome.

Down's Syndrome

This is the most common cause of intellectual disability. All children with Down's syndrome have a degree of intellectual disability and although they have special educational needs, many attend mainstream schools. The ability of adults with Down's syndrome varies considerably. This is reflected in the degree of independence and level of employment.

Certain medical conditions are more common in people with Down's syndrome; these include dry skin, slow feeding, poor tongue control and a tendency to develop chest and sinus infections. Some 40% of affected babies have a heart defect, ranging from a slight murmur to a severe abnormality requiring surgery. Hearing, vision and thyroid problems may also occur. However, many people with Down's syndrome enjoy a healthy life, and a life span of 40-60+ years is not uncommon.

The Cause

Down's syndrome is the result of a chromosomal anomaly. It can be inherited, though this rarely happens, and most cases occur in couples with no family history. Every human cell contains chromosomes, which incorporate the genes that influence our individual characteristics. In a normal human cell there are 46 chromosomes. In individuals with Down's syndrome there are 47 chromosomes. The extra one is a copy of chromosome number 21, hence the disorder is sometimes called 'trisomy 21'. Most cases are known to result from an error in cell division during the early stages of egg production.

Marker screening for Down's Syndrome

In the past, advanced maternal age or a previous Down's syndrome pregnancy were the only ways of identifying a high risk group. The risk of having a baby with Down's syndrome increases with the mother's age. For example the risk is about 1 in 910 at age 30, and 1 in 28 at age 45. A previous affected pregnancy increases the risk further, to about 1 in 200 at age 30 and 1 in 25 at age 45.

However, with this approach a large number of normal pregnancies and relatively few affected pregnancies were identified as high risk. Most babies with Down's syndrome are born to young women - about half are born to women under 30 - since most pregnancies are in this age group. A very small proportion of affected births occur in couples with a family history.

Now, a simple blood test or a special ultrasound examination can be used to screen more effectively. This involves measuring 'markers' which are either chemicals in the mother's blood or structures seen on ultrasound.

Markers

The level of each marker is typically either increased or reduced on average in a Down's syndrome pregnancy.  The table shows a typical profile for the most important markers found so far:

† '+' and '-' signs indicate increase or decrease in a typical affected pregnancy

Marker levels, except NB, change with gestation. To quantify the extent of increase or decrease in level, they are expressed as multiples of the normal median (MoMs) for the gestation.  For example, 2.0 MoM means that the level is double the average for the gestational age of the pregnancy.

Interpretation

Although, on average, a Down's syndrome pregnancy follows a typical profile there is a lot of variability and many are atypical. Equally, some unaffected pregnancies have a profile similar to Down's syndrome. When someone is screened we use a computer program to calculate how close their profile is to that of an affected pregnancy.

Taking the maternal age, family history and profile together our program calculates the risk of the pregnancy ending in the birth of a baby with Down's syndrome. If the risk exceeds a specified cut-off risk result is regarded as screen positive, otherwise it is screen negative. Most centres also report the actual risk too.  Risks can be expressed as either the chance that the fetus has Down's syndrome or as the chance of a birth with Down's syndrome.  Because a large proportion of affected pregnancies miscarry spontaneously these probabilities can be very different.  The cut-off risk varies in different countries.  Most use 1 in 250 at birth.

Choice of tests

Various combinations of markers can be used in a screening test. The most common today is the 'Combined' test using PAPP-A, free beta-hCG (or hCG) and NT carried out at 9-13 weeks.  For women who present too late for this test the most widely offered is the 'Quadruple' test using free beta-hCG (or hCG), AFP, uE3 and inhibin at 14-20 weeks.  The Combined test has a much higher detection rate than the Quadruple test.  In some centres NB, NF, NBL and PT are available and their use as additional markers will increase detection further.  Some centres are also able to combine a 9-13 week test with a 14-20 week test.  This gives the highest detection rate of all.  The best performance is achieved by a 'Contingent' test where a Combined test is followed by a Quadruple test in about 10% of women with Combined test risks between about 1 in 50 and 1 in 1500 at birth.  The borderline risk is adjusted according to the Quadruple test results.

In the UK, the NHS generally provides at best only a standard Combined test.  For a higher detection rate it may be necessary to obtain a more comprehensive test privately, using more and better markers.

Edwards' syndrome

Like Down's syndrome, this is caused by an extra chromosome, in this case number 18, hence it is also known as 'trisomy 18'. Although most infants die in the first weeks of life there are long-term survivors with profound physical and intellectual disability. The typical profile for some of the markers is different from Down's syndrome. Again results are interpreted by calculating the risk of this disorder. The estimated detection rate is high and the false-positive rate is very low.

Other chromosomal anomalies such as Turner's syndrome and triploidy are not specifically screened for but are detected as a result of a Down's syndrome screening test.  Structural abnormalities such as heart defects may be found when measuring NT.

NTDs and AWDs

These are caused by failures in the early development of the embryo. With NTDs the neural tube has failed to close fully, leaving a hole. If this results in absence of the brain it is anencephaly, if it leads to damage in the spinal chord it is spina bifida. Anencephaly is incompatible with life and only about half of those with spina bifida survive infancy. The extent of handicap due to spina bifida varies considerably but many have paralysis of the lower limbs and incontinence. Generally, there is no intellectual disability. With AWDs it is the abdominal wall that has failed to close fully. As a result some of the abdominal organs, although still attached, are displaced outside the body.

AFP is produced in the fetal liver, and during normal pregnancy a very small amount reaches the maternal blood. In NTDs and AWDs this level is increased as more AFP leaks from the fetus through the open lesion. After 15 weeks' gestation the level is sufficient for AFP to be a strong marker. If the AFP level exceeds 2.5 MoM the result will be interpreted as screen positive. More than three-quarters of NTDs and two-thirds of AWDs have raised AFP levels. The false-positive rate is 2-3%.

Non-marker screening for Down's syndrome

In recent years a new screening method has been developed that does not use markers.  Instead cell-free (cf)DNA is tested in a maternal blood sample.  This form of DNA is predominantly produced by the mother but about 15%, on average, is produced by the fetus/placenta.  The test measures the proportion of DNA fragments from chromosome 21 in the maternal blood and if the fetus has Down’s syndrome there will be a small increase.  As with marker screening most affected pregnancies have a positive test result but some unaffected pregnancies are also positive.  However, the test performs much better than marker screening, detecting almost all Down’s syndrome cases and yielding relatively few false-positives.  The test will also detect Edwards’ syndrome, the less common Patau’s syndrome and the more common Turner’s syndrome.  Some versions of the test will also detect ‘micro-deletion/duplication’ syndromes. These are either rare or have variable severity.  Testing for outcomes other than Down’s syndrome will increase the false-positive rate.

Failure of cfDNA testing

In about 2% of tests, the test fails because the amount of fetal cfDNA in the blood sample is too small and a repeat test will be required.  This is more likely to occur in women who are over-weight or when the blood sample is taken early in pregnancy.

Choice of cfDNA tests

There are various different test methods but they yield similar results for Down’s syndrome.  All of them are more expensive than marker screening.  The NHS plans to introduce a limited amount of cfDNA testing.  It will be offered only to women when are at high risk based largely on marker screening results.

Fragile X syndrome

FXS is the most common inherited cause of severe learning disability although most cases occur in families with no history of the syndrome.  In general, FXS is the second most common cause of learning disabilities, after Down's syndrome.  Boys have learning difficulties ranging from mild to severe intellectual impairment.  Girls can have normal intelligence but a large proportion have learning difficulties comparable with affected boys.

Common behavioural features in boys with FXS include short attention span, distractability, impulsiveness, and overactivity. Girls with FXS, including those without learning difficulties, may have concentration problems and social, emotional and communication difficulties related to extreme shyness and anxiety in social situations.  Many affected individuals exhibit autistic-like features: a dislike of eye contact, difficulty in relating to other people, anxiety in social situations, insistence on familiar routines and hand flapping or biting.

Speech and language is usually delayed, with continuing speech difficulties.  About 20% of children develop epilepsy but this is usually well controlled by medication.

The cause

At a particular point on the X chromosome there is normally a block of between 11 and 54 copies of a small DNA sequence.  FXS results from a fault at this point where the block is abnormally extended to over 200 copies.  This is called a full mutation (FM).  The block is so long that it interferes with the production of an important protein.

Since boys only have one X chromosome, those with an FM are severely affected.  Girls have two X chromosomes and if one of produce sufficient protein.  Nonetheless about half the girls with an FM have FXS.

Inheritance

An FM in a child always comes from the mother (never the father), who has a larger than normal block of DNA.  Usually the mother has a moderately extended block with 55-199 copies called a pre-mutation (PM). This is harmless to the mother, being insufficient to alter her protein production, but it becomes unstable at the time of conception when it might expand to an FM in the fetus.

The chance of a maternal PM being passed to the fetus and expanded into an FM varies according to the number of copies.  For example, a PM with 65 copies has a 1 in 40 chance whilst a PM with 90 copies has a 1 in 3 chance.

Occasionally, the mother of a child with an FM is not a PM carrier, and is one of those females who has an FM but is an unaffected carrier of the disorder.  An FM mother will pass the FM to the child in half her pregnancies.

About one in 150 pregnant women is a PM carrier and 1 in 8000 women is an unaffected FM carrier.  These proportions do not vary according to maternal age or ethnic origin.

Most babies with FXS are born to women who do not have a family history of the disorder.  Therefore, screening is needed to determine any woman's PM or FM carrier status.  Screening can be carried out on a woman's sample antenatally or pre-pregnancy.

Interpretation

This is much simpler than with Down's syndrome screening.  DNA is extracted from a blood sample and the number of copies is measured.  If a PM or FM is detected, the result is classified as screen positive, otherwise screen negative.  From the number of copies in the PM, the chance of expansion to an FM fetus is calculated.

The carrier detection rate is over 99% so women with a screen negative result have a close to zero chance of a child with FXS.

Unlike the other screening tests the diagnostic tests available to those with screen positive results is not necessarily definitive in all cases. The diagnostic test is to see if the fetus has inherited the mutated X chromosome and the block has expanded to an FM.  If there is a male FM fetus, he will definitely have FXS.  However, there is no way of distinguishing those female FM fetuses who have FXS with its associated intellectual and behavioural difficulties from those who will not go on to exhibit these traits.

This does not seem to be a major problem for most couples as studies show that termination of pregnancy is the chosen option for nearly all those with FM fetuses.

SMA & CF screening

All genes, apart from those on the X and Y chromosomes in males, come in pairs.  SMA and CF have a similar pattern of inheritance, each occurring when both genes in a specific pair are faulty. This leads to loss of production of an important chemical.  The parents are carriers with only one faulty gene and so are healthy.

Children receive one gene of a pair from each parent.  So a child is only affected if (1) both parents are carriers of the same disorder and (2) both pass on their faulty gene, which happens in 25% of pregnancies.

Screening

Most babies with these disorders are born to couples, neither of whom has a family history of the disorder.  Therefore, screening is needed to determine any couple's carrier status.

Screening can be carried out either antenatally or pre-pregnancy in couples or single individuals.  The best method is a 'paired sequential' approach.  This is the process whereby both partners provide blood samples and the woman's sample is tested to see whether she is a carrier.  The man's sample is tested only if the woman is found to be a carrier.

Interpretation

If a carrier couple is detected the result is regarded as screen positive, otherwise it is screen negative.  Thus, a screen negative result can arise in two ways.  Either the woman is not a detectable carrier, or she is a carrier but the man is not.

Many different faults in a gene can lead to the same disorder.  DNA tests are designed to test for the most common faults but inevitably will not detect 100% of carriers.

Spinal Muscular Atrophy (SMA)

This is a debilitating disorder that destroys the nerve cells for walking, head and neck control, swallowing and breathing.

It causes the motor neurons in an area of the spinal cord called the anterior horn to deteriorate. Motor neurons are nerve cells in the spinal cord that send impulses to the muscles, telling them to expand or contract. The deterioration of the motor neurons gradually breaks the link between the brain and the muscles that this part of the spinal cord controls. As the link is broken, the muscles are used less and less and so become weaker, or shrink (atrophy).

It primarily affects children and is the main genetic cause of death in infancy.

SMA is caused by a fault in a particular gene on chromosome 5, called SMN1.  The normal function of the gene is not known precisely and the protein it produces has not yet been characterised.  In infants with SMA there is a faulty SMN1 gene on both chromosomes 5 so that only an abnormal form of the protein is produced.

The carrier frequency is 1 in 60, hence for one couple in about 3600, both partners are carriers.

About 95% of carriers are accounted for by a specific deletion within the SMN1 gene.  Current carrier tests determine whether there is a single normal gene but 5% of carriers have an extra normal gene as well as the faulty gene. Hence DNA testing has a carrier detection rate of about 90%.

Cystic Fibrosis (CF)

This is a serious inherited condition.  People are born with it; they cannot acquire it.  CF is normally diagnosed in the first year of life due to the presence of numerous symptoms; however, occasionally diagnosis is not made until later in childhood.  Both males and females can have the condition.  It does not affect intelligence. CF causes glands to produce excessive amounts of thick mucus in the lungs and digestive tract.  In order to avoid chest infections, and to aid digestion, affected individuals have daily treatment with inhalers, regular physiotherapy and enzyme tablets.  The quality of life for CF sufferers has greatly improved due to the continuous efforts of research.  However, life expectancy is still only 30-40 years and some people suffer serious health problems.

CF is caused by a mutation in a particular gene on chromosome 7, called CFTR.  Normally, the CFTR gene has instructions for a  protein which forms a channel, allowing salt and water to pass in and out of the cell. Certain tissues in the body, such as the airways in the lungs, rely on this mechanism to keep them moist.   CFTR mutations lead to the production of an abnormal form of the protein.

In people affected by CF there is a mutated CFTR gene on both chromosomes 7 so that only the abnormal protein is produced.  This prevents passage of salt and water and results in accumulation of thick mucus in the lungs and digestive tract.

It is not uncommon to be a CF carrier.  In UK Caucasians approximately one person in 25 is a carrier and for one couple in about 625 both partners are carriers.  Among those of Asian or Afro-Caribbean origin the CF carrier frequency is less than half that in Caucasians.

There are about 1000 CF-causing mutations in the CFTR gene.  In the UK one mutation called delta-F508 is much more common than others.  Among Caucasians over 80% of carriers are accounted for by a few of the most common mutations.  This is not so for Asians and Afro-Caribbeans; only one-third of their CFTR mutations have been identified to date.  In Ashkenazi Jews there are fewer CFTR mutations and nearly all carriers can be detected.

Ashkenazi Jewish disorders

Most Ashkenazi Jews know that they are at increased risk of having a child with Tay-Sachs disease, but few realise that their chance of carrying another genetic disorder is much higher.

In Israel and the USA, it is routine to offer a DNA test that will establish carrier status for several such disorders simultaneously in a single blood sample.

In the UK it is possible to have a private (non-NHS) DNA test for seven Jewish disorders plus CF.  They are Tay-Sachs disease, Canavan's disease, Neimann-Pick disease A & B, Bloom's syndrome, Familial dysautonomia, Fanconi's anaemia C and Mucolipidosis IV.

This article was first published on the site in 2002. It was updated by the author in 2010 and again in October 2018.