Prenatal Tests

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What are prenatal tests?

Prenatal tests are set of specific medical tests that are done during pregnancy with the purpose of screening or diagnosing potential health problems for the expectant mother or her unborn baby.

Why are prenatal tests done?

The main purpose of doing prenatal tests is to assess some key parameters that can affect the mother and the baby’s health.

A healthy pregnant woman may carry certain diseases without any symptoms. There may be serious consequences if such conditions are left untreated.

Some of the identified diseases can be treated at an early stage, so these tests must be carried out as per the recommended schedule.

Different types of tests may be advised before pregnancy and during the first, second and third trimesters of pregnancy. Concerning the mother’s health, these tests can help in determining:

• The blood type of the mother.
• Health conditions like anaemia, gestational or pregnancy-related diabetes, etc
• Certain tests are done to determine the presence of antibodies against certain infections with viruses like rubella, syphilis, hepatitis, human immunodeficiency virus (HIV), etc

Fetal monitoring tests in the developing child can help in determining:

• Treatable health conditions that can affect the baby.
• Observe the physical characteristics of the baby within the uterus like position, size, and growth, etc.
• Determine the presence or probability of any birth or genetic defects.

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What are the types of prenatal tests?

Broadly, prenatal tests may be of two types, namely screening and diagnostic tests,

• Screening tests are not very definitive, they just reveal the possibility of the existence of a health condition.
• Diagnostic tests are definitive. They can accurately detect the presence of a medical condition in the mother or the baby has a specific problem.
• A diagnostic test may be followed by a screening test.

Who should have prenatal tests?

Although there are a large number of prenatal tests, doctors usually do not recommend all tests to all pregnant women. Some tests may be recommended only when a woman is at a high risk for certain diseases. Some tests, on the other hand, may be recommended for all women.

The appropriate prenatal tests are advised as follows:

• Routine prenatal tests: Recommended for all pregnant
• Non-routine tests: Recommended only for women with high-risk pregnancies, multiple pregnancies, pregnancies after fertility treatment. For example:
  • Advanced maternal age, 35 years or older.
  • History of or presence of conditions like diabetes, high blood pressure, lupus, heart disease, kidney disorders, cancer, asthma, etc.

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What are the prenatal tests done to plan a healthy pregnancy?

A woman who is considering having a child should consult with her gynaecologist who may advise certain tests based on the medical and family history. Some of the commonly advised tests to help ensure a pregnancy as healthy as possible include:

• Blood tests: The blood group whether type A, B, AB or O, and Rh- or Rh+ factor in the blood can be determined by blood tests. A complete blood count helps in evaluating the counts of different types of cells in the blood and the haemoglobin for anaemia.
• Urine tests: Urinalysis is usually done to check the appearance, concentration, and content of urine for a range of disorders, such as bladder and urinary tract infections, kidney disease, and diabetes. Sometimes an infection may be present without having any symptoms. A urine culture is done to identify the type of microorganism if the infection is present.
• Infections: Blood tests are also done for screening of infections like rubella, hepatitis, HIV, syphilis, etc as and when indicated. Appropriate vaccines may be recommended if the woman is not protected against rubella or chickenpox. Attempts to conceive should be avoided for at least one month in such cases.
• Pap smear – It is a screening test for the risk of cervical cancer.
• Genetic tests & counselling: Couples with a family history of genetic or birth defects, intellectual disabilities are generally recommended genetic counselling.

Many types of disorders can be detected in the lab, but the most common genetic tests are:

• Cystic fibrosis.
• Fragile X syndrome.
• Sickle cell disease.
• Spinal muscular atrophy.
• Tay-Sachs disease.

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What are the prenatal tests done in the first trimester (week 1 to 12 of pregnancy)?

Prenatal care should ideally be initiated by 10 weeks of gestation of the first trimester. Some prenatal screening and diagnostic tests can be performed at this gestational age.

A gynaecologist would advise most of the tests described below in the first prenatal visit during the first trimester of pregnancy.

The purpose of these tests is to check for some medical conditions and infections that can likely affect the mother’s health or harm the unborn baby during the pregnancy.

Ultrasound: A fetal sonogram or ultrasound is a safe imaging technique that uses sound waves to produce images of a fetus in the uterus. This test is performed during all stages of pregnancy albeit with different techniques and purposes. It may be of two types:

• Transabdominal ultrasound: A transducer which is a small handheld device is rolled over the abdomen after it is lubricated with jelly and the images are observed on a monitor.
• Transvaginal ultrasound: A wand or probe like transducer is placed in the vagina to transmit images on the screen. It may be recommended if the transabdominal ultrasound doesn’t provide enough information.

 A fetal ultrasound in the first trimester helps to:

• Confirm the pregnancy and its location i.e pregnancy outside of the uterus or ectopic pregnancy, or to diagnose any potential miscarriage.
• An early ultrasound examination acts as a key baseline against which later examinations are compared for the evaluation of the growth and development of the fetus. It helps to determine the baby’s gestational age, determine the due date and track milestones throughout pregnancy.
• Confirm the number of fetuses or babies.
• Evaluate the growth and monitor baby’s movement, breathing and heart rate.
• To look for any kind of growth in the fetus that is not normal.
• Determine the risk of certain genetic defects by assessing the fetal nasal bone and Fetal Nuchal Translucency (NT): The nasal bone or the bone of the nose is not visible, or hypoplastic at 11-13 weeks’ pregnancy in fetuses with chromosomal abnormalities and genetic defects like trisomy 21 or Down’s syndrome. Assessment of the nasal bone at 11-13 weeks is used as a screening for trisomy 21 by maternal age.
• Fetal nuchal translucency (NT) and serum biochemistry: An ultrasound in the 12^th week is used for the measurement of the thickness of the nuchal translucency. Nuchal translucency is the fluid-filled region at the neck of the fetus. The thickness of more than 2.5 mm may be suggestive of chromosomal abnormalities.

Fetal ultrasound is recommended by gynaecologists for medical reasons. It is not advisable to use it as a tool to determine a baby’s sex (as per the law of  Republic of India) or producing keepsake videos or pictures.

Blood work: Blood tests may be advised for the screening of certain genetic abnormalities and as a standard panel for assessing the health status, to determine the blood group if not done earlier and the presence of any infections in the body. The panel may be augmented by additional tests in women at risk for specific conditions.

Double marker test or biochemical markers in the blood, human chorionic gonadotropin (fbhCG) and pregnancy-associated plasma protein-A (PAPP-A) for Down syndrome: HCG which is a hormone produced by the placenta is detected in blood around 11 days after conception and reaches its peak around 8 to 11 weeks after conception. The test can be useful in:

• Confirmation of pregnancy.
• Determining the approximate age of the fetus.
• Potential miscarriage.
• Screening for Down’s syndrome.

Maternal serum screening test & blood antibody tests: Screening for blood antibodies is recommended to be done in all women even if a test for blood antibodies was done earlier. This is so because antibody concentrations change over time.

Blood should be assessed for the presence of antibodies for infectious conditions like Rubella, Hepatitis B, Tuberculosis (TB), Human immunodeficiency virus (HIV) & other sexually transmitted diseases (STDs) as this could adversely affect the health of the expectant mother and harm the baby as the pregnancy progresses.

Genetic testing: Genetic testing gives information about whether their fetus has certain genetic disorders,

• Non-invasive prenatal testing (NIPT) or non-invasive prenatal screening (NIPS)/Cell free DNA prenatal screening test: The test can be done as early as 9 weeks of pregnancy and like all prenatal tests, it is optional. This test measures the freely circulating DNA from the baby’s placenta in a sample of the mother’s blood to determine the risk of a genetic disorder. Since it is a screening test, it measures only the likelihood of having that condition. A positive result is called the positive predictive value (PPV) of the test whereas the negative result is called the negative predictive value (NPV) of the test.

Even though it is a screening test, it has a 97-99% accuracy for three of the most common genetic conditions i.e.

• Trisomy 21 (Down syndrome)
• Trisomy 18 (Edwards syndrome)
• Trisomy 13 (Patau syndrome)

The results of a NIPT screening serve as a guide whether to have a diagnostic test like chorionic villus sampling (CVS) or amniocentesis (“amnio”).

NIPT is considered non-invasive since it can be performed on the mother’s blood and does not pose any risk to the fetus.

The test may detect a genetic condition in the mother too as it analyzes both fetal and maternal cfDNA.

Chorionic villus sampling (CVS): The growing baby receives nutrition and oxygen from the placenta. It also removes waste products from the baby’s blood.

Chorionic villi are wisp-like projections of placental tissue that also contain information about the baby’s genetic makeup.

In CVS, a test that can be done as early as 10 weeks of pregnancy, a sample of chorionic villi is taken through the cervix (transcervical) or the abdominal wall (transabdominal) route from the placenta for testing.

The test can confirm the presence of genetic defects such as Down syndrome Tay-Sachs and cystic fibrosis. The test is usually recommended if:

• A prenatal screening test was positive.
• History of a chromosomal condition in a previous pregnancy.
• The pregnant woman is 35 years or older.
• There is a family history of genetic disorders.

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What are the prenatal tests done in the second trimester (week 13 to 27 of pregnancy)?

Some of the tests recommended during the second trimester for regular monitoring and depending on any health risks observed during the earlier prenatal visits include:

Ultrasound: Also known as second-trimester anatomy scan/ fetal anomaly scan or TIFFA (targeted imaging for fetal anomalies), the second-trimester ultrasound is done between 18 and 20 weeks.

A confirmation of or change in the menstrual date and stage of pregnancy within 10 to 14 days can be measured with an ultrasound in the second trimester.

A detailed anatomical evaluation of the fetus from head to toe is the primary objective of TIFFA for the detection of any anomalies.  A thorough and detailed study helps to,

• To assess structural normalcy of the baby as expected.
• To detect the presence of any abnormalities.
• To screen if the case requires any further testing or serial scans.

One must keep in mind that diagnosis by fetal ultrasound has its limitations. While it is easier to see some problems or defects more easily, others may not be diagnosed before birth.

Triple screen or Multiple-marker blood tests: Though all women should have it, the triple screen test is highly recommended for certain groups of women during the second trimester. These include women with:

•  A family history of birth defects.
• 35 years or older.
• History of any harmful medications or drugs during pregnancy.
• Diabetes on insulin.
• History of viral infection during pregnancy.
• History of high level of radiation.

Performed between the 15th and 20th weeks of pregnancy, the test is also known as “multiple marker screening” and “AFP plus.” The test is done by drawing a sample of the mother’s blood and measuring:

• AFP: alpha-fetoprotein, a protein that is produced by the fetus.
• HCG: human chorionic gonadotropin, a hormone produced within the placenta.
• Estriol: estriol, an estrogen produced by both the fetus and the placenta.

Fetal abnormalities like Trisomy 21 (Down Syndrome), trisomy 18 (Edwards Syndrome), and spina bifida can be screened with this test. Further testing may be recommended by high definition ultrasound or amniocentesis in case of positive triple screen test results.

Amniocentesis: Usually performed between 14 and 20 weeks, this test can provide a definite diagnosis after a positive triple screen test. During this procedure, a sample of the amniotic fluid is taken by inserting a small needle through the skin of the belly into the amniotic sac. The amniotic fluid is then checked for chromosomal and genetic abnormalities like Down’s syndrome. Amniocentesis may also be occasionally done late in pregnancy to assess the maturity of the baby’s lungs and determining paternity if required. Even though amniocentesis is a safe procedure, being invasive, it carries some risks, miscarriage being one of them. Usually, the rate of risk ranges from 1 in 400 to 1 in 200.

Glucose Challenge Screening and Glucose tolerance test: A condition is known as gestational diabetes or pregnancy-induced diabetes can pose a risk to both mother and baby. Potential gestational diabetes can be screened with a preliminary screening test performed between 26-28 weeks, known as Glucose Challenge Screening. If the Glucose Challenge Screening is positive, the second test called the Glucose Tolerance Test (GTT) to diagnose whether diabetes exists or not.

The American Congress of Obstetrics and Gynecologists (ACOG) recommends that all pregnant women should be screened for gestational diabetes.

For this test, the pregnant woman is made to drink a solution containing around 50 grams of sugar. A blood test is performed after an hour to check the sugar level. No preparation or fasting is needed.

If the test is positive for gestational diabetes dietary and exercise advice is given in addition to medication if required. Gestational diabetes normally goes away after you have your baby.

 Cordocentesis or percutaneous umbilical blood sampling: Usually done after week 18 of pregnancy, cardiocentesis is a diagnostic prenatal test to detect certain genetic disorders, blood conditions, and infections. A sample of the baby’s blood is taken from the umbilical cord for testing. The procedure can also be used to deliver medications and blood to the baby if required through the umbilical cord.

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What are the prenatal tests done in the third trimester (week 28 to 40 of pregnancy)?

In the third trimester, apart from routine testing of the urine and blood, other recommended tests include:

Ultrasound & fetal monitoring: Third-trimester ultrasound examination is done to assess fetal growth, assessment of amniotic fluid and position of the placenta. Some of the uses of the ultrasound in the third trimester are,

• Growth scanning to monitor the growth and position of the fetus and assess if it is breech, transverse, cephalic, or optimal.
• Identify any congenital abnormalities or birth defects.
• Detect any structural abnormalities or problems of placental blood flow and determine the adequacy of oxygen supply to the fetus.
• Monitor amniotic fluid level.
• Identify issues of the ovaries or uterus, like pregnancy tumours.
• Measure cervical length.
• Confirmation of intrauterine death when suspected.

Nonstress Test (NST): It is a simple, non-invasive test performed over 28 weeks of pregnancy. It is called “non-stress” as it places no stress on the fetus during the test. A fetal monitor is placed on the belly of the pregnant woman for 20 to 30 minutes to measure the heart rate of the fetus in response to its movements.

An NST is indicated when:

• Pregnant women report that the baby is not moving as frequently as usual.
• Pregnancy is overdue.
• Suspected inadequacy of placental function.
• High-risk pregnancies.

The test is also useful in assessing if the baby is not receiving enough oxygen due to any placental or umbilical cord problems; or any other reason for fetal distress.

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How to prepare for prenatal tests?

Women who are considering pregnancy are recommended to consult their gynaecologist and undergo screening tests before conception. This helps in planning a healthy pregnancy as the doctor can initiate treatment for any potential disorders or prescribe any vaccinations if required. While some tests are generally recommended for all women, others may be reserved for women who have a high risk of certain diseases or conditions during pregnancy. Knowing about the tests beforehand can also help in preparations for the test:

 Ultrasound: While preparing for USG Scan, the mother can keep in mind the following points,

• Wear clothes that are comfortable and suitable for an ultrasound. Loose two-piece clothes are ideal.
• During an ultrasound in the initial period of pregnancy, one may need to be on a full bladder so that the doctor can get a clear image of the fetus and the reproductive organs. Two to three glasses of water one hour before the scheduled ultrasound helps.
• However, in the third trimester, this may not be needed due to the presence of the amniotic fluid around the baby.

Tests that are done from a urine sample: Urine sample-based tests require the person to urinate into a small jar. This can be done at home at times and carry the sample to the hospital. Alternately the sample can be taken at the hospital. The hospital staff will provide information on exactly what to do and send the sample for testing.

Tests that are done from a vaginal swab: Tests like those for certain infections like Group B streptococcal (GBS) or Pap smear for cervical cancer are done using a vaginal swab. A special stick called a swab may be rubbed gently over the surface of the cervix inside the vagina to collect a sample. The stick is then sent for testing.

Blood tests: Many tests are performed on a sample of blood that may be taken from one of the veins in the arm, through a procedure called a venepuncture. Apart from some bruising at the site of the injected needle, there is very little risk associated with a blood pregnancy test. If certain tests require a person to be fasting before the test, the hospital staff will provide necessary instructions.

In general screening tests pose no threat to the baby or the pregnant woman. Diagnostic tests on the other hand like CVS and Amniocentesis may pose some risk due to their invasive nature. However, studies suggest that when performed at volume centres by expert gynaecologists, the risk of injury to the baby due to the amniocentesis needle becomes negligible.

Most of the recommended tests are optional, but it is recommended that parents discuss the risks and benefits of testing thoroughly with their gynaecologist, who can help in evaluating if the benefits from the results could outweigh any risks from the procedure. Undertaking such tests can certainly help the parents to be in making health decisions about their baby and know what to expect when the baby comes.

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References:

• Mayo Clinic. Prenatal testing: Quick guide to common tests. Available at. https://www.mayoclinic.org/healthy-lifestyle/pregnancy-week-by-week/in-depth/prenatal-testing/art-20045232. Accessed on February 29, 2020
• Mayo Clinic. Cordocentesis. Available at. https://www.mayoclinic.org/tests-procedures/percutaneous-umbilical-blood-sampling/about/pac-20393638. Accessed on February 29, 2020
• The American College of Obstetricians and Gynecologists. Routine Tests During Pregnancy. Available at: https://www.acog.org/patient-resources/faqs/pregnancy/routine-tests-during-pregnancy. Accessed on February 29, 2020
• Screening tests in pregnancy. Available at. https://www.nhs.uk/conditions/pregnancy-and-baby/screening-tests-in-pregnancy/. Accessed on February 29, 2020