Screening Tests
Various screening tests are recommended during the first and second trimesters; these normally include ultrasound examinations and lab/blood work. These tests, taken together, can evaluate a fetus’ risk for one or more birth defects. If risk is considered high, diagnostic tests (amniocentesis, chorionic villus sampling, cell-free DNA) are then recommended to confirm diagnosis.
The purpose of screening is to allow for earlier education of the condition, earlier potential treatment, and better management after delivery. However, these screening tests are optional.
There are also screening tests for the mother’s health, which are strongly recommended for the continued health of the pregnancy (infection, blood sugar, thyroid and other hormone levels, antibodies, anemia, and risk of depression).
Pregnant women should engage in thorough risks and benefits discussions regarding any screening and diagnostic tests with their HCP, as well as a genetic counselor, if necessary.
Background
Pregnant women are offered numerous screening tests and/or diagnostic tests during pregnancy. It is their choice which tests they want completed. Screenings allow for potential diagnoses to be made earlier, which also allows for better management and treatment before, during, and after delivery.
Screenings assess for risks of possible birth defects; this begins with a discussion of family medical history with an HCP or genetic counselor. This information is combined with the results of other tests to further assess and evaluate risk.
Screening tests include ultrasounds and blood work which are used to determine risk, but they do not confirm a diagnosis. If a screening test reveals a higher risk, it is considered positive, and then diagnostic tests are recommended.
Screening tests are done first as they have better safety profiles than diagnostic tests, which do carry some risks by themselves.
Diagnostic tests are used to confirm whether a baby has a certain birth defect that were flagged as a possibility during screening tests. Examples of diagnostic tests include amniocentesis, chorionic villus sampling, and cell-free DNA.
With any type of testing, there is a possibility of false-positive/negative results. Women should receive pretest counseling with a genetic counselor regarding the risks and benefits of available options, as well as thorough post-test counseling, with individualized interpretation of results.
First Trimester
First and second trimester screenings involve counseling, ultrasound exams, and lab/blood work.
First-trimester screening is usually done between 10 and 13 weeks of pregnancy:
Carrier screening is a blood or saliva test that can be done prior to, or during pregnancy and determines if a woman or her partner carry a gene for a certain disorder that can affect a baby. If one parent carries a specific gene, it is possible for that condition to be passed to their baby even if that parent does not actually have the condition – as they are still a carrier. If both parents are carriers, the risk for the baby increases further.
Ultrasound is used to conduct nuchal translucency screening to examine the area at the back of the fetal neck for increased fluid or thickening, which is a common finding in Down Syndrome (false positive rate is about 5%). A nuchal translucency of greater than 3 mm is significantly associated with both chromosomal abnormalities and structural malformations.
It recommended that pregnant women with a positive nuchal translucency screening undergo a more thorough ultrasound and fetal echocardiogram (evaluates the heart) to assess for the presence of potential complications.
Ultrasound is also used to assess all other physical structures of the fetus, to include the nasal bone, which is not observed in some fetuses with chromosomal abnormalities.
If any results of these first trimester screening tests are abnormal, genetic counseling is recommended, if not done prior, as well as chorionic villus sampling, amniocentesis, cell-free fetal DNA, or additional ultrasounds.
Blood tests are also completed early in pregnancy. When used together with other first trimester screening tests, nuchal translucency screening and blood tests have a greater ability to determine if the fetus might have a birth defect. These blood tests look for the following:
Hormones: progesterone, human chorionic gonadotropin (HCG), and estriol
Complete blood count (CBC): red and white blood cells and platelets; can help identify anemia early in pregnancy as well as any blood clotting concerns (read Blood Volume).
Blood type (especially RH factor): RH factor is a protein found on the surface of red blood cells; most people have this factor, but some individuals do not. If the mother is negative, and the baby is positive, the woman’s body will make antibodies against an RH factor, that will harm a future pregnancy.
Rubella antibodies: if antibodies are low, a booster vaccine is recommended in the postpartum period, or before pregnancy if this was found during a preconception appointment; contracting rubella during pregnancy can cause birth defects (very, very rare in the United States; part of the MMR series).
Hepatitis B, Hepatitis C, Sexually Transmitted Infections, and Human Immunodeficiency Virus: these viruses/infections can be passed to the fetus
Tuberculosis
Urine tests are also completed in the first trimester (some practices may do one at every appointment) to screen for glucose, proteins, and bacteria (potential infection).
Second Trimester
Second trimester screening is usually conducted between 16 and 22 weeks of pregnancy and includes additional blood work and a lengthier ultrasound exam.
Maternal serum screen, also called quad screen, triple test, triple screen, multiple marker screen or AFP, has been used since the 1980s and measures the levels of three or four different substances in the blood to screen for Down syndrome, trisomy 18, and neural tube defects. These substances include alpha-fetoprotein (fetal liver), HCG, inhibin A, and estriol (form of estrogen). The triple test does not include inhibin A.
An ultrasound exam, known as the anomaly scan, occurs between 18 and 22 weeks when more anatomical details are present (usually a woman’s second scan); the main purpose of this scan is to screen for potential birth defects by observing, assessing, and measuring anatomical details to include the placenta, umbilical cord, and amniotic fluid.
The results of these screening tests are most accurate when they are all used and then combined in various ways.
A glucose tolerance test is also conducted in the second trimester (24 to 28 weeks) to screen for gestational diabetes.
Third Trimester
Group B streptococcus (Group B strep) is a bacteria commonly present in the intestines and lower genital tract. Group B strep is usually harmless in adults but can cause serious illness in newborns. Pregnant women are screened for this bacteria (via swab) between 35 and 37 weeks of pregnancy.
The non-stress test was introduced in 1975 and is used to assess fetal health by comparing fetal movement to fetal heart rate. Although the reliability of the test needs improvement, it still remains very popular for use near term.
Diagnostic Tests
Because fetal urine and cells enter the amniotic fluid, studies of fetal enzymes, amino acids, hormones, and other substances can be conducted through the removal and testing of amniotic fluid.
An amniocentesis is a procedure used to obtain a small sample of the amniotic fluid to diagnosis chromosomal disorders and neural tube defects in pregnancies considered at risk for these defects. The procedure involves inserting a long, thin needle, guided by ultrasound, through the woman’s abdomen into the amniotic sac to remove a small amount of fluid for analysis. Complications are considered rare, with pregnancy loss attributed to amniocentesis estimated at 1 in 900.
Chorionic villus sampling (CVS) is a diagnostic test performed in the first trimester that involves taking a sample of placental tissue which carries the same genetic material as the fetus. It does not provide information on neural tube defects.
The procedure can vary but involves inserting a small tube (catheter), guided by ultrasound, through a woman's vagina and into her cervix. The catheter contains a syringe on one end to remove tissue from the placenta. The procedure can also be performed abdominally, similarly to amniocentesis. The risk of miscarriage is less than 1 in 100. It is possible results are inconclusive, or an inadequate amount of tissue was taken, and a follow up amniocentesis may be needed.
Cell-free DNA is a blood test introduced in 2011 to identify DNA fragments that are derived primarily from placental cells that enter the mother’s blood supply. The test can be performed any time after 9 to 10 weeks of pregnancy. This DNA can be screened for Down syndrome, trisomy 13, trisomy 18, and problems with the number of sex chromosomes.
Cell-free DNA can identify fetal sex with excellent accuracy in the first trimester, as well fetal Rh status. It is also possible – in the future – that cell-free DNA can be used to determine a cause for sporadic and recurrent miscarriage.
Action
Women who have any questions or concerns regarding the risks and benefits of the various screening tests available need to talk with their HCP, who will likely recommend a genetic counselor. Women should not consent to any test unless they are comfortable with their knowledge of the test, its purpose, and its safety.
Women who are afraid of needles should read this page and talk to their HCP about their concerns.
Resources
Carrier Screening (American College of Obstetricians and Gynecologists)
Screening for Fetal Aneuploidy (American College of Obstetricians and Gynecologists)
Routine Tests during Pregnancy (American College of Obstetricians and Gynecologists)
Common Tests during Pregnancy (Stanford Children's Health)
Screening Tests in Pregnancy (U.K. National Health Service)