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There are many different types of prenatal tests, testing both the mother and fetus for potential problems. Different tests are appropriate at different times in your pregnancy. Some tests are done routinely; others are completed only when there is an important reason to do so, such as a family history of a condition.
Some prenatal tests are screening tests, while others are diagnostic. A screening test gives information about the possibility that a problem exists. For example, an alpha-fetoprotein (AFP) screening test can show that there is a 1% risk of spina bifida . A diagnostic test can tell that a specific problem is present.
The different types of prenatal testing include:
At your first prenatal care visit, your healthcare provider will conduct blood and urine tests.
In a blood test, blood is drawn from your arm using a syringe and sent to a laboratory for evaluation. Blood tests check for:
Blood Type and Rh Factor
There are four major blood types: A, B, AB, and O. Each of these blood types is also classified by the types of antigens present—proteins on the surface of blood cells, one of which is called the Rh factor. If you carry this protein, you are Rh positive. If you do not carry this protein, you are Rh negative. About 85% of people are Rh positive.
When the mother is Rh negative and the father is Rh positive, the fetus has at least a 50% chance of being Rh positive (by inheriting the Rh factor from the father). Potential Rh incompatibility results, and the mother may develop antibodies to the Rh antigens in the baby’s blood if a small amount of the baby’s blood mixes with the mother’s blood. This rarely occurs in a woman’s first pregnancy, as fetal blood cells usually only gain access to the mother’s circulation very late in the pregnancy or during childbirth. The woman can also become sensitized to the RH factor during a miscarriage or ectopic pregnancy .
Symptoms and complications of Rh incompatibility only affect the fetus or newborn; the mother’s health is not affected. If the mother’s antibodies cross the placenta and attack the fetus’s blood, the fetus’s red blood cells will be broken down and fetal anemia will result. This can cause swelling of the baby’s body, brain damage, heart failure, respiratory problems, or even death of the fetus or newborn.
Rh incompatibility can be prevented in most cases by maternal injections of Rh immunoglobulin (RhIg), which prevents sensitization of an Rh negative mother’s blood. The first injection is given at week 28 of the pregnancy. The second injection is given within 72 hours of delivery (or miscarriage or abortion) to desensitize the mother’s blood for future pregnancies.
Once a Rh negative woman develops antibodies to an Rh positive baby, RhIg treatment does not help. Therefore, prenatal testing and preventative measures are extremely important. The fetus will be closely monitored during the pregnancy, and the baby may need a blood transfusion after delivery. In severe cases, the fetus may be given a blood transfusion while in the uterus.
Iron-deficiency Anemia
If you are anemic , you have low levels of iron in your blood. Anemia can lead to extreme tiredness and increase your risk of preterm delivery. You may be prescribed iron tablets or injections.
Sexually Transmitted Diseases (STDs)
Many STDs can harm your developing baby. Healthcare providers routinely test for chlamydia and gonorrhea , which are done by culture, as well as the following blood tests: hepatitis B , syphilis , and human immunodeficiency virus (HIV) .
Immunity to Certain Diseases
Chicken pox and German measles (rubella) can cause birth defects if a mother is infected for the first time during pregnancy.
CF is a genetic disease. Both parents must be CF gene carriers for the baby to incur the condition, of which there is no cure. This test is usually offered to all expecting women, even if there is no family history. CF is most likely to occur if the parents are of Eastern European Jewish descent.
In a urine test, you collect a sample of your urine in a sterile container. The urine is sent to a laboratory for evaluation. Urine tests are done at each prenatal visit and check for:
Many different types of tests can be done to assess the health and development of your baby.
An ultrasound uses sound waves to examine internal structures, including the developing fetus and position of the placenta. Ultrasound is a noninvasive exam that poses no risk to the mother or fetus. Depending on which trimester you are in, an ultrasound can be used for different reasons.
First Trimester :
Second trimester :
Identify possible fetal abnormalities
Third trimester :
Early in the pregnancy, your healthcare provider may use a transvaginal ultrasound . During this procedure, a slender probe is inserted into your vagina and images of the uterus and ovaries are generated on a computer screen and evaluated by a trained professional. You will need to empty your bladder prior to the test.
Later in the pregnancy, your healthcare provider will do a transabdominal ultrasound . During this procedure, your abdomen is coated with a gel and a technician passes a wand-like tool called a transducer across your abdomen. Images of the fetus are generated on a computer screen and evaluated by a trained professional. When this procedure is conducted during the first trimester, a full bladder is required.
If your pregnancy is considered high-risk, you may have an advanced ultrasound , including an assessment of the fetal anatomy and assessment of the fetal heart. This procedure is similar to the transabdominal ultrasound, but targets a specific problem and uses more sophisticated equipment. You may also have a 3D ultrasound, which uses specially designed probes to create a 3D image of the fetus.
Additionally, you may have a Doppler ultrasound , which can detect moving blood cells and measure their direction and speed.
The triple screen is a maternal blood test that measures the levels of three substances normally produced by the fetus or placenta. The test is used to identify pregnancies that have a higher-than-average risk of certain birth defects, such as Down syndrome and neural tube defects.
The triple screen can be done between the 15th and the 22nd week of pregnancy. However, it is most accurate when done between the 15th and 17th week. The screening is done through a normal blood draw and poses no risk to the mother or fetus.
The triple screen is offered to all pregnant women, but is recommended for women who:
The three substances the triple test measures are:
Levels are expressed as high, low, or normal. The following table presents the possible evaluation based on the substance level:
| AFP | Estriol | hCG | |
| Neural tube defects | High | Normal | Normal |
| Down syndrome (trisomy 21) | Decreased | Decreased | Increased |
| Trisomy 18 | Decreased | Decreased | Decreased |
The triple screen is a screening test, not a diagnostic test. This means the test can only assess risk, it cannot diagnose a condition. A woman’s individual risk of birth defects is calculated using the levels of AFP, estriol, and hCG combined with the mother’s age, weight, race, and whether she requires insulin due to diabetes.
The test results are usually available in 3-5 days, but may take up to two weeks. The results are expressed as odds. For example, “the odds this baby has Down syndrome are now greater than, less than, or equal to” what was expected based on the mother’s age and medical history.
If the triple screen shows increased risk, additional testing is done, as well as a referral to a genetic specialist. It is important to keep in mind that the test only indicates the likelihood of an outcome—it is not diagnostic. In addition, many women with a positive screening test do not have a child with a birth defect. This is considered a “false positive” result.
AFP is sometimes tested alone, without hCG and estriol. Also, an additional marker can be added to the test, making the triple screen a quadruple screen. The substance is inhibin A, which is a glycoprotein. Studies have found that a greater number of markers increases the accuracy of the screening test.
An amniocentesis is a diagnostic procedure during which a sample of amniotic fluid is removed and evaluated. Amniotic fluid is the fluid surrounding a developing fetus and contains cells normally shed from the fetus.
When done early in pregnancy (15th to 18th week of pregnancy), it is used to detect:
Amniocentesis is recommended for women who:
An amniocentesis can also be performed near the time of delivery to assess the baby’s lung maturity or to check whether there is an infection around the baby.
During an amniocentesis, a physician inserts a needle through the abdominal wall and into the uterus to remove a small amount of amniotic fluid (1-2 tablespoons). An ultrasound is used to guide the physician when inserting the needle. A local anesthetic may be used. You may experience some cramping, pain, or pressure. The procedure will take about 45 minutes, but withdrawing the fluid takes less than five minutes.
After the procedure, the doctor will check the fetus’ heartbeat to make sure it is normal. You should rest for several hours following the procedure and refrain from intercourse and strenuous activity for the remainder of the day. Call your healthcare provider if you experience fever, ongoing bleeding, severe cramping lasting several hours, or leakage of amniotic fluid.
The fetal cells in the amniotic fluid are grown in a cell culture, and the DNA is analyzed to detect chromosomal abnormalities. Results can take up to one month, but timing varies. Results from an amniocentesis are highly accurate (99%).
The primary risk with amniocentesis is miscarriage. The risk of miscarriage ranges from 1 in 400 to 1 in 200. The risk of miscarriage increases to about 3 in 100 cases when the procedure is done as early as 11 weeks. If the procedure is preformed before 13 weeks, there is a slight increase in the risk of clubfoot . Infection can also occur after amniocentesis, but it is rare. A slight amount of amniotic fluid may leak, but it usually stops on its own. It is rare that leakage would be ongoing. If it is, the pregnancy is monitored closely.
If amniocentesis results indicate that there may be a problem, counseling is recommended.
Chorionic villus sampling (CVS) is a diagnostic procedure during which a sample of chorionic villi cells from the placenta is removed and tested for chromosomal abnormalities and genetic birth defects. CVS is done at 10-12 weeks gestation and can test for diseases such as Down syndrome, Tay Sachs , and cystic fibrosis. However, the procedure does not test for neural tube defects. CVS can also be used for paternity testing.
The test is recommended for women who:
CVS is not recommended for women who:
There are two ways a physician can remove chorionic villi cells:
After the procedure, you should plan to rest and refrain from intercourse and strenuous activity for the remainder of the day. You may experience cramping or pain and light spotting of blood. Call your healthcare provider if you experience fever, chills, or leaking amniotic fluid.
If you are carrying more than one fetus, the physician will take a sample from each placenta. This can be done using a combination of TA CVS and TC CVS. However, CVS is technically more difficult in multiple gestation.
Preliminary results may be available within two to four days, but may not be accurate. Final results are available about seven to 10 days after the procedure. The procedure has high accuracy rates—98%-99%.
The primary risk with CVS is miscarriage, occurring in 1 out of 100. An infection as a result of CVS is rare. When the test is performed before 9 weeks, an increased risk of fetal limb abnormalities has been reported. CVS can cause a small amount of fetal blood to leak into your circulation. If you are Rh negative, this can create a problem; you should receive RH immunoglobulin after the procedure.
If CVS results indicate that there may be a problem, additional testing and counseling is recommended.
Percutaneous umbilical blood sampling (PUBS) is a diagnostic procedure that examines blood from the fetus. The blood is drawn from the umbilical cord where it inserts into the placenta. Performed at 18 weeks or later, PUBS is used to detect:
PUBS can also be used to give certain medications to the fetus.
This test is recommended for women who:
During PUBS, a physician will guide a thin needle through the abdomen wall and uterine wall to the umbilical cord. Ultrasound is used to help guide the physician and decrease risk to the fetus. A local anesthetic may be used. The physician inserts the needle where the umbilical cord enters into the placenta and withdraws a small sample of fetal blood.
After the procedure, you should plan to rest and refrain from intercourse and strenuous activity for the remainder of the day. You may experience cramping or pain. Call your healthcare provider is you experience fever, chills, or leakage of amniotic fluid.
The blood is sent to a laboratory for analysis, which usually takes about three days. Results from PUBS are highly accurate.
PUBS is considerably more invasive than other diagnostic studies, because the needle enters the fetal circulation. It has a slightly higher risk for miscarriage than amniocentesis at 1.2% to 4.9%. Other risks include blood loss from the puncture site, infections, and preterm labor. In addition, about 3% to 12% are complicated by drop in fetal heart rate. In some cases, this can be severe and may require cesarean delivery , depending on gestational age and fetal condition.
Group B streptococcus (GBS) is a bacterium that can cause life-threatening infections in newborns. Many people carry GBS but do not become ill. One of every four to five pregnant women carry GBS in the rectum or vagina. During birth, a newborn may come in contact with this bacterium.
One out of every 100 to 200 babies born to a mother who carries GBS will develop signs and symptoms of the disease. A majority (75%) of cases occurs within the first week of life and most are apparent a few hours after birth. GBS infection can lead to severe infection, pneumonia , and meningitis . Once diagnosed, GBS infections can be treated through intravenous antibiotics.
A woman can be checked for GBS during pregnancy. The test should be done late in pregnancy, during weeks 35 to 37. The physician takes a swab of both the vagina and rectum, which is sent to a laboratory and grown in culture.
If the result of the test is positive, then you are considered a carrier of GBS. If you test positive for GBS or are at high risk, your doctor will recommend giving you intravenous antibiotics during labor and delivery. Penicillin or ampicillin is usually used. If you have an allergy to those drugs, clindamycin or erythromycin may be used.
It is generally not recommended that women take antibiotics before labor to prevent GBS (unless the condition is identified in the urine), as it does not sufficiently suppress the infection for long enough to prevent the infection in the newborn.
References:
Amniocentesis. American Pregnancy Association website. Available at: http://www.americanpregnancy.org/prenataltesting/amniocentesis.html . Accessed August 24, 2005.
Blood test. American Pregnancy Association website. Available at: http://www.americanpregnancy.org/prenataltesting/bloodtest.html . Accessed August 24, 2005.
Chorionic villus sampling: CVS. American Pregnancy Association website. Available at: http://www.americanpregnancy.org/prenataltesting/cvs.html . Accessed August 24, 2005.
Graves JC, Miller KE, Selers AD. Maternal serum triple analyte screening in pregnancy. American Family Physician. 2002;65:915-920.
Group B streptococcal disease (GBS). Centers for Disease Control and Prevention website. Available at: http://www.cdc.gov/ncidod/dbmd/diseaseinfo/groupbstrep_g.htm . Accessed September 7, 2005.
Mutliple gestation: complicated twin, triplet, and high-order multiple pregnancies [practice bulletin]. American College of Obstetricians and Gynecologists. 2004 Oct; 56.
Multiple marker screening: also known as AFP plus and triple test. American Pregnancy Association website. Available at: http://www.americanpregnancy.org/prenataltesting/multiplemarkerscreen.html . Accessed August 24, 2005.
Prenatal diagnosis of fetal chromosome abnormalities [practice bulletin]. American College of Obstetricians and Gynecologists. 2001 May; 27.
Prenatal genetic testing. Genetics & Public Policy Center website. Available at: http://www.dnapolicy.org/genetics/prenatal.jhtml . Accessed August 24, 2005.
Reference: first-trimester screening for fetal aneuploidy [committee opinion]. American College of Obstetricians and Gynecologists. 2004 July; 296.
Rh factor. American Pregnancy Association website. Available at: http://www.americanpregnancy.org/prenataltesting/rhfactor.html . Accessed August 24, 2005.
Routine tests during pregnancy. American College of Obstetricians and Gynecologists. Medem Medical Library website. Available at: http://www.medem.com/MedLB/article_detaillb.cfm?article_ID=ZZZ84JKXODC&sub_cat=2005 . Accessed August 24, 2005.
Ultrasound. March of Dimes website. Available at: http://www.marchofdimes.com/printableArticles/159_523.asp . Accessed August 24, 2005.
Ultrasound: sonogram. American Pregnancy Association website. Available at: http://www.americanpregnancy.org/prenataltesting/ultrasound.html . Accessed August 24, 2005.
Umbilical cord blood tests. American Pregnancy Association website. Available at: http://www.americanpregnancy.org/prenataltesting/cordocentesis.html . Accessed August 24, 2005.
Urine test: urinalysis. 2003. American Pregnancy Association website. Available at: http://www.americanpregnancy.org/prenataltesting/urinetest.html . Accessed August 24, 2005.
Your first tests. March of Dimes website. Available at: http://www.marchofdimes.com/pnhec/159_519.asp . Accessed August 24, 2005.
Last reviewed July 2007 by Ganson Purcell Jr., MD, FACOG, FACPE
Please be aware that this information is provided to supplement the care provided by your physician. It is neither intended nor implied to be a substitute for professional medical advice. CALL YOUR HEALTHCARE PROVIDER IMMEDIATELY IF YOU THINK YOU MAY HAVE A MEDICAL EMERGENCY. Always seek the advice of your physician or other qualified health provider prior to starting any new treatment or with any questions you may have regarding a medical condition.
Copyright © 2007 EBSCO Publishing All rights reserved.