The prenatal tests you may receive and what they mean
Today, expectant parents can take advantage of a wealth of prenatal tests to help ease fears and make even a healthy pregnancy less stressful — and sometimes treat serious problems. Here are the ones you’re most likely to undergo.
For many women, prenatal testing begins before conception: Women of certain ethnic groups or who belong to families with a history of birth defects are advised to see their doctors and get genetic counseling before they become pregnant. But for most, prenatal testing begins with that all-important first visit to the obstetrician.
At your first visit, a Pap smear and a blood and urine test will be done. At later visits, several tests will be performed to track your pregnancy and monitor your baby’s health. For example, at each visit, your blood pressure will be measured and a sample of your urine examined. Your weight will be charted, and the doctor may listen to the baby’s heartbeat and take external measurements of your uterus to monitor the baby’s growth. An ultrasound may also be performed to determine your due date or make a diagnosis.
Other tests include the maternal serum alpha-fetoprotein (MSAFP or AFP), which is performed at about 15–20 weeks. This blood test helps screen for such conditions as neural-tube defects and Down’s syndrome. The results of the test are not conclusive, however; abnormal results indicate that more testing is necessary.
If any of your screening tests produce abnormal results, or if you are over the age of 35 or have a personal or family history of birth defects, diagnostic tests may be required to pinpoint the problem — or lack thereof. The two most common are amniocentesis and chorionic villus sampling, or CVS. “These tests are overwhelmingly reassuring,” says Michelle Kling, a spokeswoman for the March of Dimes. “Ninety-five percent of the time, they reveal that the fetus does not have the condition you’re testing for.” In fact, only 4 percent of babies in the United States are born with birth defects.
Amniocentesis can detect all known chromosomal “errors,” such as Down’s syndrome, as well as some genetic disorders. Typically performed between the 14th and 18th weeks, amniocentesis involves inserting a needle through the abdomen into the uterus to draw an amniotic sample. Because it is an invasive test, it poses certain risks, including a 1 in 200–400 risk of miscarriage following the procedure.
CVS is a slightly less accurate test (the accuracy rate is still higher than 99 percent), with a higher risk of miscarriage (1 in 100–200), but it can be performed earlier than amniocentesis — as soon as 10 weeks after conception. With this procedure, a needle is inserted through the abdomen or a catheter is inserted through the vagina to take a sample from the chorion, the tissue that develops into the placental.
If amniocentesis or CVS testing is indicated, you may want to consult a genetic counselor or your religious adviser to discuss the kinds of decisions such testing may require. The March of Dimes offers detailed information about birth defects and prenatal testing on its Web site (www.modimes.org) and operates a toll-free hot line at (888) MODIMES.
|Test||Who needs it?||What’s involved?||What does it reveal?||Potential harm to fetus?||Follow-up: What’s Next?|
Blood type and Rh factor
|All pregnant women; at first visit.||A blood sample is drawn and examined.||Maternal blood type and whether Rh-negative.||None.||If Rh-negative, Rh immune globulin is given at 28 weeks to protect fetus.|
|Pap smear; gonorrhea culture||All; at first visit.||Cervical/vaginal secretions examined.||Pre-existing health problems.||None.||Depends on findings.|
|All; early in pregnancy and at 32–34 weeks.||A blood sample is drawn and examined.||Can indicate anemia.||None.||If anemic, extra iron will be prescribed.|
|Screening for certain illnesses||All; early in pregnancy. Repeated if mother is exposed to an illness.||A blood sample is drawn and examined.||If mother has been exposed to a disease or illness.||None.||Treatment can protect the fetus and prevent transmission of the infection at birth.|
|Blood pressure||All; at every visit.||Blood pressure is measured.||A sudden rise can signal complications.||None.||May indicate need for additional testing.|
|Urinalysis||All; at every visit.||A urine sample is collected and examined.||Protein can indicate pre-eclampsia; glucose can indicate gestational diabetes.||None.||High levels of protein or glucose require further testing (see blood sugar test, below).|
|Alpha-fetoprotein (AFP)||All; at 15–20 weeks.||Blood is drawn and examined for AFP, a substance produced by the fetus.||Abnormal levels of AFP can indicate neural-tube defects or Down’s syndrome; may indicate a multiple pregnancy.||None; abnormal result does not diagnose a defect but calls for further testing.||Ultrasound and/or amniocentesis if abnormal AFP levels are found.|
|Ultrasound||Used to determine due date or gestational age. Also used to perform amniocentesis and CVS.||A device placed on the abdomen or in the vagina creates an image of the fetus.||Age and position of fetus, rate of growth, placement of placenta and visible defects.||None.||Depends on findings; may indicate need for further testing.|
|Amniocentesis||Women with a family history of birth defects or over age 35 (or possibly in the event of abnormal AFP results); at 14–18 weeks.||Needle is inserted through the abdomen and into the uterus; a sample of the amniotic fluid is drawn.||Accuracy rate is 99.4 percent in diagnosing chromosomal abnormalities such as neural-tube defects or Down’s syndrome.||Slightly increased risk of miscarriage (1 in 200–400 pregnancies).||Depends on findings; further genetic counseling may be advised if an abnormality is found.|
|Chorionic villus sampling (CVS)||Women with a family history of certain birth defects or over age 35; at 10–12 weeks.||A needle is inserted through the abdomen and into the uterus or through the vagina and cervix for a sample of the chorionic villi.||Accuracy rate is greater than 99 percent in diagnosing chromosomal abnormalities such as neural-tube defects or Down’s syndrome.||Riskier than amniocentesis, CVS increases the chance of miscarriage (1 in 100–200) and damage to the fetus.||Depends on findings; further genetic counseling may be advised if an abnormality is found.|
|Blood sugar||All women; at 24–28 weeks or when glucose is present in the urine.||A blood sample is drawn after drinking a sugary mixture.||Glucose level in the blood may indicate gestational diabetes.||None.||Gestational diabetes can be controlled with diet or insulin.|
|Group B strep||All women; at 35–37 weeks.||Vagina and perineum are swabbed and sample is examined.||Presence of deadly bacteria that can be passed to baby at birth.||None.||If bacteria are found, antibiotics are given during labor.|
|Nonstress||Women who are post-term or ill; performed close to term.||External monitor is strapped to the abdomen to measure fetal heart rate.||Assesses fetal well-being.||None.||May indicate need for further testing.|