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Prenatal testing is testing for diseases or conditions in a fetus or embryo before it is born. The aim is to detect birth defects such as neural tube defects, Down syndrome, chromosome abnormalities, genetic diseases and other conditions. It can also be used to determine its sex.
Diagnostic prenatal testing can be by invasive or non-invasive methods. An invasive method involves probes or needles being 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 fetus. 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 woman'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.
Most often this is performed to test for birth defects. Common procedures include amniocentesis, sonograms, nuchal translucency testing, or genetic screening. The tests can be used to check for conditions such as Down syndrome, spina bifida, cleft palate, Tay Sachs disease, sickle cell anemia, thalassemia, cystic fibrosis, and fragile x syndrome. In some cases, the tests are administered to determine if the fetus will be aborted.
Fetal screening has also been done to determine characteristics generally not considered birth defects. In some parts of the world, if a fetus is determined to be female, it is sometimes aborted. The rise of designer babies and parental selection for specific traits raises a host of bioethical and legal issues that will dominate reproductive rights debates in the 21st century.
Fetal versus maternal
Some screening tests performed on the woman are intended to detect traits or characteristics of the fetus. Others detect conditions in the woman that may have an adverse effect on the fetus, or that threaten the pregnancy.
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 and parents for the delivery of a child with a health problem.
Many expectant parents would like to know the sex of their baby before birth. Methods include amniocentesis with karyotyping, and prenatal ultrasound. In some countries, health care providers are expected to withhold this information from parents, while in other countries they are expected to give this information.[How to reference and link to summary or text]
Methods of prenatal screening and diagnosis
- Examination of the woman's uterus from outside the body.
- Ultrasound detection - Commonly dating scans (sometimes known as booking scans) from 7 weeks to confirm pregnancy dates and look for twins. The specialised nuchal scan at 11-13 weeks may be used to identify higher risks of Downs syndrome. Later morphology scans from 18 weeks may check for any abnormal development.
- Listening to the fetal heartbeat (see stethoscope)
- External fetal monitoring, often known as a non-stress test
Less invasive methods
- Second trimester maternal serum screening (AFP screening, triple screen, quad screen, or penta screen) can check levels of alpha fetoprotein, β-hCG, inhibin-A, estriol, and h-hCG (hyperglycosolated hCG) in the woman's serum.
- First trimester maternal serum screening can check levels of free β-hCG, PAPP-A, intact or beta hCG, inhibin-A, or h-hCG in the woman'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 fetal DNA using blood cells from the fetus that have made their way into the woman's bloodstream. Tests such as Baby Gender Mentor allegedly use this method to determine the sex of a fetus as early as six weeks into a pregnancy.
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 may have a higher risk of miscarriage.
- Amniocentesis - This can be done once enough amniotic fluid has developed to sample. Cells from the fetus 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 embryo or fetus.
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)
- Women who have previously had miscarriages
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 fetus, a more accurate but also riskier invasive technique may be used.
Ethical and practical issues
Ethical issues of prenatal testing
- The option to continuing or aborting a pregnancy is the primary choice after most prenatal testing. Rarely, fetal intervention corrective procedures are 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 assure that the appropriate treatment and/or surgery be provided immediately after birth.
- Questions of the value of mentally or 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 fetus. 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. This does give parents the option to consider abortion.
If abortion isn't an option for a particular couple (because of their own beliefs or the laws of their country), invasive prenatal diagnosis of such a condition is unhelpful as the test puts the fetus at risk, and knowing the result doesn't help the fetus. Genetic counseling can help families make informed decisions regarding results of prenatal diagnosis.
False positives and false negatives
Ultrasound of a fetus, which is considered a screening test, 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 as an Echogenic intracardiac focus or a 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 on 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.
A higher maternal serum AFP level indicates a greater risk for anencephaly and open spina bifida. This screening is 80% and 90% sensitive for spina bifida and anencephaly, respectively.[How to reference and link to summary or text]
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 the patient's partner may also influence the woman's decision.
- Congenital disorders
- Genetic counseling
- Genetic disorders
- Guthrie test
- Newborn screening
- Pre- and perinatal psychology
- Prenatal care
- Prenatal development
- Reproductive technology
- Special non-invasive advances in fetal and neonatal evaluation network (SAFE)
- SAFE Network non-invasive prenatal diagnosis FAQs page
- Our Bodies Ourselves chapter on Prenatal Testing and Disability Rights
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