Prenatal diagnosis

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

Overview

Prenatal diagnosis is the diagnosis of a disease or condition in a fetus or embryo before it is born. The aim is to detect birth defects such as neural tube defects, chromosome abnormalities, genetic diseases and other conditions. It can also be used to determine the sex of the unborn baby.

Diagnostic prenatal testing can be by invasive methods or non-invasive methods. An invasive method is when probes or needles are inserted into the placenta, e.g. amniocentesis, which can be done from about 14 weeks gestation, and usually up to about 20 weeks and chorionic villus sampling, which can be done earlier (between 9.5 and 12.5 weeks gestation) but which is slightly more risky to the unborn child. Non-invasive methods, called "screens", can only evaluate risk of a condition and cannot determine 100% if the fetus has a condition. Non-invasive techniques include examinations of the mother's womb through ultrasonography and maternal serum screens (i.e. Alpha-fetoprotein) If an abnormality is indicated by a non-invasive procedure, a more invasive technique may be employed to gather more information.

Reasons for prenatal diagnosis

There are three purposes of prenatal diagnosis: (1) to enable timely medical or surgical treatment of a condition before or after birth, (2) to give the parents the chance to abort a fetus with the diagnosed condition, and (3) to give parents the chance to "prepare" for a baby with a health problem or disability, or for the likelihood of a stillbirth.

Having this information in advance of the birth means that healthcare staff can better prepare themselves (have suitable treatment ready ) and parents (providing counselling) for the delivery of a child with a health problem.

An estimated 50-70% of expectant parents would like to know the sex of their baby before birth , which can also be determined through prenatal testing.

Methods of prenatal screening and diagnosis

Non-invasive methods

• Examination of the mother's uterus from outside the body. (i.e. Feeling the mother's 'stomach'.)
• Ultrasound detection - Commonly dating scans (sometimes known as booking scans) from 7 weeks to confirm pregnancy dates and look for twins. Later morphology scans from 18 weeks may check for the baby's sex and any abnormal development. The specialised nuchal scan at 11-13 weeks may be used to identify higher risks of Downs syndrome.
• Listening to the fetal heartbeat using a fetoscope or Doppler ultrasound fetal heart monitor
• External fetal monitoring, often known as a non-stress test

Less invasive methods

• Second trimester maternal serum screening (AFP screening, triple screen, or quad screen) can check levels of alpha fetoprotein, β-hCG, inhibin-A and estriol in the mother's serum.
• First trimester maternal serum screening can check levels of free β-hCG and PAPP-A in the mother's serum, and combine these with the measurement of nuchal translucency (NT). Some institutions also look for the presence of a fetal nasalbone on the ultrasound.
• Integrated, Sequential, and Contingent screening tests use serum samples from both first and second trimester, as well as the nuchal translucency measurement to calculate risks. With Integrated screening, a report is only produced after both samples have been analyzed. With Sequential screening, a first report is produced after the first trimester sample has been submitted, and a final report after the second sample. With Contingent screening, patients at very high or very low risks will get reports after the first trimester sample has been submitted. Only patients with moderate risk will be asked to submit a second trimester sample, after which they will receive a report combining information from both serum samples and the NT measurement.
• Detection of fetal blood cells in maternal blood. With this technique it is technically possible to obtain a sample of the baby's DNA using blood cells from the fetus, that have made their way into the mother's bloodstream. Tests such as Baby Gender Mentor allegedly use this method to determine the sex of a baby as early as six weeks into a pregancy.

More invasive methods

• Chorionic villus sampling - Involves getting a sample of the chorionic villus and testing it. This can be done earlier than amniocentesis, but is a more risky procedure.
• Amniocentesis - This can be done once enough amniotic fluid has developed to sample. Cells from the baby will be floating in this fluid, and can be separated and tested.
• Embroscopy and fetoscopy - These involve putting a probe into a women's uterus to observe (with a video camera), or to sample blood or tissue from the baby.

Risk factors qualifying a pregnant woman for prenatal testing

• Women over the age of 35
• Women who have previously had premature babies or babies with a birth defect, especially heart or genetic problems
• Women who have high blood pressure, lupus, diabetes, asthma, or epilepsy
• Women who have family histories or ethnic backgrounds prone to genetic disorders, or whose partners have these
• Women who are pregnant with multiples (twins or more)

The type of prenatal diagnosis done depends on the situation of the parents. If the woman is classified as being at high risk for a defect in the baby, a more accurate but also riskier invasive technique may be used.

Ethical and practical issues

Ethical issues of prenatal testing

• The option to continue a pregnancy or abortion is the main choice after most prenatal testing, with just occasionally fetal intervention corrective procedures possible.
• Are the risks of prenatal diagnosis, such as amniocentesis worth the potential benefit?
• Some fear that this may lead to being able to pick and choose what children parents would like to have. This could lead to choice in sex, physical characteristics, and personality in children. Some feel this type of eugenic abortion is already underway (sex-selective, etc.)
• Knowing about certain birth defects such as spina bifida and teratoma before birth may give the option of fetal surgery during pregnancy, or to assure that the appropriate treatment and/or surgery be provided immediately after birth.
• Questions of the value of mentally/physically disabled people in society?
• How to ensure that information about testing options is given in a non-directive and supportive way.
• That parents are well informed if they have to consider abortion vs. continuing a pregnancy. See wrongful abortion.

Will the result of the test affect treatment of the fetus?

In some genetic conditions, for instance cystic fibrosis, an abnormality can only be detected if DNA is obtained from the baby. Usually an invasive method is needed to do this.

If a genetic disease is detected, there is often no treatment that can help the fetus until it is born. It does give parents the option to consider abortion of the baby.

If abortion isn't an option for a particular couple (because of their own beliefs), invasive prenatal diagnosis of such a condition is unhelpful as the test puts the child at risk, and knowing the result doesn't help the child. Genetic counseling can help families make informed decisions regarding results of prenatal diagnosis.

False positives and false negatives

Ultrasound, which is considered a screening test, of an unborn baby can sometimes miss subtle abnormalities. For example studies show that a detailed ultrasound, also called a level 2 ultrasound, can detect about 80% of spina bifida. Ultrasound results may also show "soft signs," such an Echogenic intracardiac focus or Choroid plexus cyst, which are usually normal, but can be associated with an increased risk for chromosome abnormalities.

Other screening tests, such as the AFP triple test, can have false positives and false negatives. Even when the AFP triple test results are positive, usually the pregnancy is normal, but additional diagnostic tests may be offered. Both false positives and false negatives will have a large impact to a couple when they are told the result, or when the child is born. Diagnostic tests, such as amniocentesis, are considered to be very accurate for the defects they check for.

No prenatal test can detect all forms of birth defects and abnormalities.

Societal Pressures on Prenatal Testing Decisions Amniocentesis has become the standard of care for prenatal care visits for women who are “at risk” or over a certain age. All obstetricians offer patients the AFP triple test, HIV test and ultrasounds routinely. However, almost all women meet with a genetic counselor before deciding whether to have prenatal diagnosis. It is the role of the genetic counselor to accurately inform women of the risks and benefits of prenatal diagnosis. Genetic counselors are trained to be non-directive and to support the patient's decision. Some doctors do advise women to have certain prenatal tests and patient's partner may also influence the woman's decision.

See also

• Genetic counseling

References

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