Prenatal Testing

From WikiMD

Prenatal diagnosis is the diagnosis of 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.

There are both invasive and non-invasive methods of diagnosing a problem before birth. Examination of the mother's womb through ultrasonography are routinely done to check for any abnormality in the pregnancy.

If an abnormality is suspected by a non-invasive procedure, a more invasive technique may need to be offered to get more information. Invasive techniques include 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 is slightly more risky to the unborn child.

Reasons for prenatal diagnosis

Knowing a fetus is abnormal before birth allows parents to plan for any health needs of their baby in advance of it being born. It can also prepare them for the birth, enabling them to receive counselling before birth, reducing the shock and other reactions when the baby is born. As well as this, it gives the option of abortion of the fetus.

Healthcare staff can also prepare for the delivery, and have suitable treatment ready for when the baby is born.

The type of prenatal diagnosis done depends on the situation of the parents. In an older mother (for instance over 35), or a parent with an inheritable genetic condition, a more invasive technique may need to be done. This can detect chromosome abnormalities (such as Down's syndrome) which are more common as a woman's age increases, or a specific genetic problem that might run in the family. Genetic counseling is often offered to help parents decide what type of testing is right for their situation.

Risk Factors Qualifying a Woman for Prenatal Testing

  • Pregnant 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 whose partners have ethnic backgrounds prone to genetic disorders

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.
  • 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.
  • Knowing about certain birth defects, such as spina bifida, 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.
  • Are mentally or physically different children less valuable in our 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.

Some say the argument not to have prenatal diagnosis is a Right-to-life argument. Pro-life activists believe that “because in utero treatment is in its infancy, prenatal diagnosis today is used to seek out and destroy ‘defective’ infants. However, this view assumes that the goal of prenatal diagnosis is simply to abort abnormal pregnancies. Some believe that knowing about a baby's health problems before birth can prepare the family, allow them to make informed decisions about treatment or surgery options, and alert the medical professionals who would be caring for the abnormal baby. Also, many women decide not to have prenatal diagnosis because of the risks of complications associated with the invasive procedures. It is the decision of the pregnant woman whether to have testing and what to do with the information if the results show a problem.

Methods of prenatal diagnosis

Non-invasive methods: (to the baby)

  • Examination of the mother's uterus from outside the body. (i.e. Feeling the mother's 'stomach'.)
  • Ultrasound detection - Commonly dating 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.
  • AFP screening can check levels of alpha fetoprotein, β-hCG, and estriol in the mother's serum.
  • 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.

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.

External Links

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