What is Prenatal Screening and What Should You Know
Perhaps you are pregnant or have a friend or family member who is. Many of us fear that the baby may be unhealthy, or carry certain genetic conditions, especially if you are considered high risk. To find out, many countries offer pregnancy screening, sometimes called genetic testing, that can help determine if the baby has certain genetic conditions.
What does genetic testing do?
Genetic testing helps to find out if a person’s chromosomes or genes may be linked to certain genetic conditions. Genetic testing is available for some conditions, such as Down syndrome, but not all health conditions. It is most often possible if the gene change that leads to the illness or condition is already known. This is not the case for most health problems.
Prenatal screening is done before baby is born and identifies pregnancies with increased risk of being affected by a chromosomal abnormality. It estimates risk, and cannot confirm if the baby has a condition. It is a screening test, not a diagnostic test. For example a women may find out that her baby has a 1:10,000 risk of having Down syndrome. In this case, 9999 kids in 10,000 will not have Down syndrome. But you do not know if your baby is among the 1 or 9999 unless more invasive diagnostic testing is done.
Prenatal screening identifies pregnancies with increased risk of being affected by a chromosomal abnormality. It estimates risk, and cannot confirm if the baby has a condition.
Who should consider prenatal screening?
Some people want to know everything they can, some want minimal information. You may want to consider genetic testing if:
- You or your partner have a chromosomal condition, or have another child with a condition, such as Down syndrome.
- You or your partner have or are a carrier for a genetic condition that can be passed on through the genes, such as cystic fibrosis.
- You are a woman over 35 years of age, and therefore more at risk of having a child with a genetic condition such as Down syndrome.
When is prenatal screening done?
There are several types of screening tests available. Some common ones include; integrated prenatal screening (IPS), first trimester screening (FTS) and ultrasound testing.
First trimester screening
The first trimester screening includes two parts. A blood test measures the levels of pregnancy- associated plasma protein-A (PAPP-A) and human chorionic gonadotropin (HCG). Abnormal levels of these may indicate a problem with the baby.
Secondly, an ultrasound measures certain areas of the baby including the neck (nuchal translucency screening). When this is too large, it may indicate chromosomal abnormalities such as Down syndrome.
A women’s age, blood and ultrasound results will be used to calculate her risk of having a baby with Down syndrome and other genetic abnormalities. Both parts are completed in the first trimester.
Integrated screening test
The integrated screening test is completed in two parts in the first and second trimesters of pregnancy. The results of the two parts are used to estimate the risk to the baby. The results are similar to that of the first trimester screening, though with a lower false-positive rate, meaning fewer women are falsely identified as carrying babies with Down syndrome.
The first trimester test involves a blood test for PAPP-A and an ultrasound, as above. In the second trimester, a quad screen (meternal serum screen or MSS) measures blood levels of alpha fetoprotein (AFP), HCG, inhibin A and estriol, four pregnancy associated substances. Results from these two sets of testing determine the risk of genetic conditions and neural tube defects such as spina bifida.
Newer techniques: Cell-free fetal DNA analysis
Cell-free fetal DNA testing checks for fetal DNA (genetic material) that is circulating in mom’s blood. At my local hospital, it is offered, free of charge to mothers who are older than 35, as they have a higher risk of chromosomal abnormalities such as Down syndrome. In addition, if one of the tests above demonstrates a high risk of Down syndrome or another genetic abnormality, it can be offered to confirm. Cell-free fetal DNA analysis can be done after 10 weeks gestation.
Cell-free fetal DNA analysis is more specific than other screening tests, though it will not confirm a case. It is a screening test, albeit more accurate than the ones above.
Diagnostic tests following prenatal screening
If you are at high risk of having a baby with Down syndrome, or have screening tests that are worrisome, you may consider having a diagnostic test to confirm the diagnosis.
Amniocentesis – in this test a sample of amniotic fluid, which surrounds the baby, is drawn through a long needle inserted in mother’s stomach. The sample is used to analyze the chromosomes of the fetus, called a chromosomal karyotype. An amniocentesis can be performed after 15 weeks of gestation. There is a small, but real, risk of miscarriage (1/100-1/1000 depending on the institution and level of expertise with the test).
Chorionic villus sampling (CVS) – in CVS, cells are taken from the placenta to analyze the chromosomes. This test can be done sooner than the amniocentesis, between 10-15 weeks. Though parents will have an answer sooner than with amniocentesis, there is a higher risk of miscarriage.
Preimplantation genetic testing – This test is available to couples undergoing in vitro fertilization and there is an increased risk of passing along certain genetic conditions, such as cystic fibrosis. In this case the embryo is tested for genetic abnormalities before it is implanted into the uterus.
If you are getting testing, or have a screen positive or a confirmed diagnosis, please seek support and advice from your doctor, friends and family. There are many support groups available and people that have gone through the same situation.
Not everyone has the same feelings about testing and the results. Some will feel relived to know the results, others anxious, and some may appreciate knowing results to make a plan during or after pregnancy. When in doubt, please speak to your doctor or genetic counselor to discuss the impact this testing will have on you, your baby, and your family.
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