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Testing: Trimester by Trimester

Over the course of a 40-week pregnancy, you're likely to undergo more tests than ever before in your life. Some monitor your health; others, the baby's. Many are routine, while some are optional. A few are cutting-edge; most have been around for years. But one thing's for sure: The number and variety of them -- not to mention all the acronyms you're supposed to remember -- can make you feel overwhelmed.

Keep in mind that there are two types of genetic tests: A screening test assesses your risk of having a baby with a certain birth defect. A diagnostic test gives you a near-definitive answer. Here, to help, is a trimester-by-trimester guide to the tests you're likely to face, the risks involved, and what to do if there's a problem.

The First Trimester

ROUTINE TESTS

Initial Blood Tests

A blood sample will be screened for a number of conditions. A complete blood count (CBC) will determine whether you have enough hemoglobin in your red blood cells; too little can be a sign of anemia, which can make you tired and weak. Another test will determine your blood type and whether your Rh factor, a protein found on red blood cells, is positive or negative. In addition, you'll be screened for hepatitis B, which can put your baby at risk of future liver disease if it's not treated. Finally, you'll be screened for antibodies to rubella (German measles) and chicken pox; if you aren't immune, exposure during pregnancy can cause birth defects.

When performed: At your first prenatal visit (usually six to eight weeks into your pregnancy)

Results and follow-up: If you're low on iron, your doctor will likely have you take a daily supplement along with your prenatal vitamin. If you're part of the 10 to 15 percent of the population who lack the Rh protein (you're Rh-negative) and your baby is Rh-positive, injections of Rh immune globulin (Rhlg) can prevent your body from producing antibodies that may attack the fetus. If you're infected with hepatitis B, doctors will give your newborn medication and the hepatitis B vaccine soon after birth. It's 95 percent effective in preventing the disease in infants. Finally, many women in this country are immune to rubella and chicken pox from childhood immunizations, but if you don't think you've been vaccinated, speak with your doctor.

Sexually Transmitted Disease (STD) Tests

A blood test can check for HIV, the virus that causes AIDS, which can cross the placenta before or during delivery and infect the fetus. The cervix is also swabbed and tested for gonorrhea and chlamydia (untreated, both can cause preterm delivery or eye infections in the baby), while a blood test screens for syphilis (unchecked, it can cause miscarriage or stillbirth). If you have symptoms of herpes, a culture can confirm the condition.

When performed: At your first prenatal visit

Results and follow-up: If you're a carrier of HIV, you'll be given medication during pregnancy and delivery that decreases the risk of mother-infant transmission from 25 percent to about 7. If you have gonorrhea, chlamydia, or syphilis, you'll be treated with antibiotics that are safe for your fetus. If you have herpes, an antiviral medication can control breakouts. If a herpes infection is active at the time of delivery, your doctor may recommend a c-section.

Pap Smear

Cells are swabbed from your cervix to detect cervical cancer.

When performed: At your first prenatal visit

Results and follow-up: If the test reveals precancerous or cancerous growth, which affects less than 1 percent of women, treatment will begin during or after pregnancy, depending on how advanced the cancer is.

Blood Pressure

The standard armband test reveals any sudden changes.

When performed: At each prenatal visit

Results and follow-up: A spike, along with protein in your urine, may indicate preeclampsia (high blood pressure during pregnancy). The condition, which affects between 5 and 8 percent of pregnant women, can keep your baby from gaining weight properly, cause the placenta to separate from the wall of the uterus, and put you at risk for a stroke, liver dysfunction, and other problems. Mild cases can be treated with bed rest; severe ones may require antihypertensive drugs or an early delivery.

Urine Tests

These check for protein (a possible sign of preeclampsia; see above), sugar (too much may signal gestational diabetes), and, in some cases, for blood and bacteria (which could indicate a urinary-tract infection).

When performed: At each prenatal visit

Results and follow-up: Gestational diabetes, which can make your baby grow too quickly, rarely develops until the second or third trimester. Milder cases may be controlled with diet, and in more severe cases, insulin. A urinary-tract infection can be cured with baby-safe antibiotics (untreated, it can lead to a kidney infection and preterm labor).

Ultrasound

A handheld instrument is moved across your belly or placed inside your vagina, and the sound waves it emits produce images of your fetus. An early ultrasound, at around nine weeks, is used to determine the age, rate of growth, position, and heart rate of your baby, and whether you're carrying multiples. It's standard to have a second one at 18 to 20 weeks, when you can learn the baby's gender. An ultrasound also looks at the placement of the placenta and the amount of amniotic fluid in the uterus. It can reveal some birth defects, such as a heart defect, cleft palate, or spina bifida (an opening in the spine). Most doctors use two-dimensional technology, though 3-D may be available in some larger cities.

When performed: Most women will be given two, the first at 9 weeks, and then at 18 to 20 weeks.

Results and follow-up: Follow-up depends on the reason for the ultrasound. If your ob was trying to determine the fetus's age, your due date may be adjusted. If she suspects a condition such as spina bifida, you have the option of more tests (see "Additional Tests," below) to verify the diagnosis.

ADDITIONAL TESTS

Cystic Fibrosis (CF)

This blood or saliva test determines whether one or both parents are carriers of CF, a genetic disorder that causes lifelong problems with digestion and breathing. If both parents are carriers, there is a one in four chance that their baby will have CF.

When performed: During the first or second trimester, ideally before 20 weeks

Results and follow-up: CVS (see below) or amniocentesis can confirm a diagnosis.

Chorionic Villus Sampling (CVS)

This procedure tests for such genetic disorders as Down syndrome, sickle-cell anemia, and cystic fibrosis. A small cluster of cells is withdrawn from the chorionic villi, which are tiny parts of the placenta, either through a vaginal catheter or through a needle inserted into the abdomen. CVS cannot detect an open neural tube defect (when the spinal cord is exposed). It's an option for women over age 35 (the risk of chromosomal abnormalities increases with age) or those with a family history of chromosome disorders. There are risks involved: Up to 1 in 100 women will have a miscarriage as a result of the procedure.

When performed: At 10 to 13 weeks

Results and follow-up: The results, which are more than 99 percent accurate, are generally available within ten days. A faster test, known as "FISH" (fluorescent in situ hybridization), can quickly check for five major chromosomal abnormalities, one of which points to Down syndrome; this test gives results in a day or two. The main advantage of CVS is that it can be done early in the pregnancy.

Nuchal Translucency Screening

This relatively new test, which is a combination of two tests, screens for Down syndrome earlier than standard second-trimester testing, and may be helpful in detecting other disorders in the first trimester as well. A specialized ultrasound measures the fluid accumulated in the nape of the fetus's neck (the nuchal measurement); too much can be a sign of Down syndrome. A blood test measures hormone levels associated with increased Down syndrome risk. The test doesn't incur the miscarriage risks of CVS or amniocentesis, but it isn't widely available yet, so check with your doctor about finding a hospital that offers it.

When performed: At 11 to 13 weeks

Results and follow-up: Researchers believe that the accuracy of nuchal translucency screening may be as high as 90 percent. If your results show a risk of an abnormality, you'll be offered CVS or amniocentesis to confirm the diagnosis.

Second Trimester

ROUTINE TESTS

Urine and blood pressure (see The First Trimester)

ADDITIONAL TESTS

Multiple Marker Screening

There are two types of this blood test that screens for Down syndrome and neural tube defects such as spina bifida and anencephaly, in which the skull does not form properly. The first is known as the triple screen: It measures levels of the pregnancy hormones hCG, estriol, and alpha-fetoprotein (AFP). The quadruple screen checks for all of these, plus inhibin-A, which can increase the accuracy of screening. All women should be offered one of these tests.

When performed: Between 16 and 18 weeks

Results and follow-up: Results are available within one to two weeks. If your levels of AFP and estriol are low and the level of hCG is high, your baby has an increased risk of Down syndrome. The triple-screen test detects about 60 percent of those affected; the quadruple screen, 75 percent. The tests also reveal about 80 percent of neural tube defects. If your results are abnormal, you'll be offered an amniocentesis (see below). These tests have a high false-positive rate: 50 out of every 1,000 women will be told that their fetus is at risk; of these, only one or two babies will actually have Down syndrome.

Amniocentesis

A sample of amniotic fluid is withdrawn through a needle inserted into your abdomen and tested for Down syndrome, open neural tube defects, and other genetic disorders. It's offered to women who are 35 and over, or at high risk of chromosomal or genetic disorders. Between 1 in 400 and 1 in 200 women will have a miscarriage as a result of this procedure.

When performed: At 15 to 18 weeks

Results and follow-up: The test is 99 percent accurate; results are available in two to three weeks. The FISH test can also check for abnormalities at this time.

Third Trimester

ROUTINE TESTS

Blood Glucose Test

This test screens for gestational diabetes (see above) by checking blood sugar levels after you consume a sugary drink. If levels are high, another test can more accurately diagnose the condition.

When performed: 24 to 28 weeks

Results and follow-up: Elevated blood-sugar levels can be controlled with diet and, in some cases, insulin.

Group B Streptococcus (GBS) Test

In this test, a culture of your vagina and rectum are checked for the bacteria, which are carried by 10 to 30 percent of all pregnant women and can be passed to the baby during delivery and, without treatment, cause inflammation of the lungs, brain, and spinal cord or even death.

When performed: At 35 to 37 weeks

Results and follow-up: If you test positive, you'll receive antibiotics during delivery to help prevent perinatal transmission. If the condition is diagnosed early enough in your baby, GBS can be successfully treated with antibiotics.

ADDITIONAL TESTS

Nonstress Test

This test measures fetal heart rate patterns in women who have high-risk pregnancies, including those who have preeclampsia or diabetes.

When performed: Anytime during the last trimester to check the well-being of the fetus

Results and follow-up: The fetal heart rate usually quickens in response to stimuli, such as noise. If the fetus's heart rate doesn't change, you may be given the test again or take further tests, such as a biophysical profile (see below).

Biophysical Profile

A nonstress test combined with a detailed ultrasound to look at such factors as heart-rate activity, body movements, and the volume of amniotic fluid.

When performed: During the third trimester

Results and follow-up: If a problem, such as low amniotic fluid, is found, the baby may need to be delivered early.

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