Antenatal testing, also known as prenatal testing, includes any diagnostic procedures performed during a woman’s pregnancy, before the baby’s birth.
Antenatal tests and exams are essential for protecting the health of a pregnant woman and her developing child.
Some tests, such as amniocentesis, carry a small risk of miscarriage or other complications that could harm the mother or baby.
Description of Antenatal testing
Women who become pregnant undergo a wide variety of tests throughout the nine months before delivery.
In the early stages, physicians order blood tests to screen for possible disorders or infections, such as the human immunodeficiency virus (HIV), passed from the mother to the fetus.
Later, the focus shifts to checking on fetal status with various technological tools such as ultrasound scans.
When a woman first learns she is pregnant, her physician runs a series of routine urine and blood tests to determine her blood type and Rh factor,
check for anemia and gestational diabetes, make sure she is immune to rubella (German measles), and check for infectious diseases like HIV, hepatitis, chlamydia, and syphilis.
Physicians also usually conduct a pelvic examination to screen for cervical cancer. They also check the patient’s blood pressure. As the pregnancy progresses, more tests follow.
Ultrasound is a device that records sound waves as they bounce off the developing fetus to create an image, which is projected onto a computer screen.
Physicians order ultrasound scans to listen for the fetal heartbeat, determine a woman’s precise due date, and check for twins, among other uses.
An ultrasound scan also is known as a sonogram. The procedure takes a few minutes, is painless and often noninvasive, and is usually covered by health insurance.
The ultrasound technician asks the pregnant woman to remove her clothes and change into a gown.
The technician may rub some gel on the woman’s stomach, which helps the handheld device pick up sound waves better.
In certain cases, the technician may insert a plastic probe into the woman’s vaginal canal to get a clearer picture of the fetus.
Early in pregnancy, the test may need to be done with a full bladder.
Unlike x-rays, ultrasound technology is safe to use during pregnancy. It does not cause any known side effects that might harm the mother or fetus.
Pregnant women usually have their first ultrasound between 8 and 12 weeks of gestation. In normal cases, the technician identifies the fetal heartbeat, which appears as a flashing light on the screen.
Closer to the due date, physicians use ultrasound to ensure the fetus is in the correct position to exit the birth canal headfirst.
Sometimes an ultrasound shows that a fetus has stopped growing, or a gestational sac has formed without a fetus, and a miscarriage has occurred.
Later in pregnancy, it may also show that the child is in a breech position (oriented feet first), which can cause difficult labor.
Tests for congenital disabilities
Most obstetricians offer parents various ways to discover if the developing fetus might have congenital disabilities such as spina bifida or Down syndrome.
An alpha-fetoprotein screen can be done through a simple blood test in the doctor’s office between weeks 16 and 18.
It determines the odds of the fetus having severe congenital anomalies. The test works by measuring the level of alpha-fetoprotein, a substance produced by a fetus with congenital disabilities.
Low levels of alpha-fetoprotein in the mother’s blood may indicate Down syndrome.
In that case, the next step for most couples is amniocentesis because the alpha-fetoprotein test can give false-positive results.
Amniocentesis is a more accurate test, but it also has higher risks of complications.
A test that measures the level of alpha-fetoprotein, a substance produced by a fetus with congenital disabilities, in the mother’s blood.
An invasive procedure allows physicians to check for congenital disabilities by collecting a sample of fetal cells from inside the amniotic sac.
When a child is oriented, feet first in the mother’s uterus just before delivery.
Group B streptococci (GBS)
Group B streptococci are a type of bacteria that can cause inflammation of the brain, spinal cord, blood, or lungs if passed to a fetus. In some cases, GBS can result in infant death.
A device that records sound waves as they bounce off a developing fetus to create an image projected onto a large computer screen.
This procedure is typically used to diagnose Down syndrome while a developing child is still in the womb, at 15–28 weeks.
During amniocentesis, a doctor inserts a needle through a woman’s abdomen. Using ultrasound as a guide, the doctor pierces the uterus to withdraw a fluid sample from the amniotic sac.
Afterward, tiny cells shed by the fetus can be studied in the laboratory. Scientists can analyze DNA samples to determine if the fetus has Down syndrome or another genetic condition. Amniocentesis can also determine the sex of the fetus (as can ultrasound).
Women who have a history of recurring miscarriages may not want to have this procedure. Amniocentesis is usually performed in a doctor’s office on an outpatient basis. Common side effects include cramping and bleeding.
In about 1 in 1,000 cases, the needle used in amniocentesis punctures the uterine wall, resulting in miscarriage.
Due to the risk of miscarriage with certain invasive tests, such as amniocentesis, there has been a push for non-invasive methods of testing for genetic disorders.
New technologies, as well as the discovery of fetal DNA in maternal blood plasma, allow for prenatal cell-free DNA screening.
This testing can detect many of the same disorders as older testing methods while also being safer for the fetus.
In most cases, couples find out their baby does not have a congenital disability.
If the results come back positive for Down syndrome or another serious condition, the couple must decide to end the pregnancy.
Others use the knowledge to plan and prepare any special care needed for their future child.
Group B strep
Group B strep tests for group B streptococci (GBS) infection. By testing for GBS, physicians can determine if a woman is at risk of passing this infection along to her child.
Women who have had a prior child with GBS or who have a fever or prolonged or premature rupture of the amniotic sac may be at higher risk for this type of infection.
GBS is a type of bacterium commonly found in the vagina and rectum. Unlike a common strep infection (that is, strep throat), GBS can be present in a person’s body without causing any symptoms. Thus many women do not realize they are infected.
To test for the presence of GBS, doctors may take a urine sample. They also may collect samples from the vagina or rectum, which are then analyzed in a lab.
This test is usually performed late in pregnancy, at 35–37 weeks of gestation, and involves a routine urine test or pelvic exam with no side effects.
In many cases, doctors do not find any evidence of this type of infection. However, if a woman has been infected with Group B strep, the physician usually waits to treat this condition until just before labor begins.
At that time, the mother may be treated with antibiotics, so the baby is not born with the infection.
Newborns exposed to Group B strep can suffer inflammation of the brain, spinal cord, blood, or lungs. In some cases, this serious complication can result in infant death.
Berghella, V., ed. Maternal-Fetal Evidence-Based Guidelines. 3rd ed. London/New York: Informa Healthcare/CRC Press, 2016.
Pos, Ondrej, et al. “Recent Trends in Prenatal Genetic Screening and Testing.” F1000Researcg 8 (May 31, 2019): F1000 Faculty Rev-764. https://doi-org.ezproxy.slq.qld.gov.au/10.12688/f1000research.16837.1 (accessed April 15, 2020).
Salmoukas, C., et al. “Background Knowledge and Attitude of Pregnant Women towards Ultrasound Screening at 20–23 Weeks Gestation.” Zeitschrift f⊠r Geburtshilfe und Neonataologie (Journal of Obstetrics and Neonatology) 3 (June 2015).
American Academy of Family Physicians. “Prenatal Care.” AAFP.org. https://www.aafp.org/afp/topicModules/viewTopicModule.htm?topicModuleId=25 (accessed April 15, 2020).