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Prenatal Tests: Third Trimester

Throughout your pregnancy, you'll want to know how your baby is growing. You'll also want to know if how you're feeling is normal. Prenatal tests can offer you valuable information about your health and the health of your growing child.

If your doctor recommends a screening or test, be sure to learn about the risks and benefits. Most parents agree that prenatal tests offer them peace of mind while helping to prepare them for their baby's arrival. But it's your choice to accept or decline a test.

Read on to learn about tests that may be available to you during your third trimester.

What's a Screening Test

OB Visit Screenings & Tests

Your first visit to the obstetrician should have included a full physical, urine test, and blood test to check for things like:

  • your blood type and Rh factor. If your blood is Rh negative and your partner's is Rh positive, you may develop antibodies that prove dangerous to your fetus. This can be prevented through a course of injections given to you.
  • anemia, a low red blood cell count
  • hepatitis B, syphilis, and HIV
  • immunity to German measles (rubella) and chickenpox (varicella)
  • cystic fibrosis — health care providers now routinely offer this screening even when there's no family history of the disorder

After the first visit, you can expect to get your urine tested and your weight and blood pressure checked at every (or almost every) visit until you deliver. The reason for this is to identify conditions such as gestational diabetes and preeclampsia.

Between your 35th and 37th weeks of pregnancy, the doctor will check you for group B streptococcus (GBS) infection. GBS bacteria are found naturally in the vaginas of many women but can cause serious infections in newborns. This test involves swabbing the vagina and rectum. A woman whose test comes back positive must go to the hospital as soon as labor begins so that intravenous (IV) antibiotics can be started to reduce the chance of the baby being infected with this illness.

Throughout your third trimester, you'll be offered more tests depending on your age, health, family medical history, and other factors.

Glucose Screening

Why Is This Test Performed?

Glucose screening checks for gestational diabetes, a short-term form of diabetes that develops in some women during pregnancy. Gestational diabetes can cause health problems for the baby, especially if it is not diagnosed or treated.

Should I Have This Test?

Most women have this test, and if they have gestational diabetes, are treated to reduce the risk to the baby.

When Should I Have This Test?

Screening for gestational diabetes usually is done at 24 to 28 weeks. Testing may be done earlier for women who are at higher risk for it, such as those who:

  • have previously had a baby that weighed more than 9 pounds (4.1 kilograms)
  • have a family history of diabetes
  • are obese
  • are older than age 25
  • have sugar in the urine on routine testing
  • have high blood pressure
  • have polycystic ovary syndrome (POS)

How Is the Test Performed?

This test involves drinking a sugary liquid and then having your blood drawn after an hour. If the sugar level in the blood is high, you'll have a glucose-tolerance test, which means you'll drink a glucose solution on an empty stomach and have your blood drawn once every hour for 3 hours.

When Are the Results Available?

The results are usually available within 1 to 2 days. Ask if your health care provider will call you with the results if they are normal or only if the reading is high and you need to come in for another test.

Nonstress Test

Why Is This Test Performed?

A nonstress test (NST) can determine if the baby is responding normally to a stimulus. It is used mostly in high-risk pregnancies or when a health care provider is uncertain of a baby's movements, and usually can be performed after 26 weeks. This is when a baby's heart rate can appropriately respond by speeding up or slowing down.

This test may also be done if you've gone beyond your due date. The NST can help a doctor make sure that the baby is receiving enough oxygen and is responding to stimulation. However, an unresponsive baby isn't necessarily in danger, though further testing might be needed.

Sometimes, a biophysical profile (BPP) is done, which includes an ultrasound and an NST. A BPP examines a baby's breathing, movement, amount of amniotic fluid, and tone, in addition to the heart rate response.

Should I Have This Test?

Your health care provider may recommend this if you have a high-risk pregnancy, if there are concerns during your pregnancy, or if you have a low-risk pregnancy but are past your due date.

When Should I Have This Test?

An NST may be recommended any time after 26 to 28 weeks, depending on why it is needed.

How Is the Test Performed?

The health care provider will measure the response of the fetus' heart rate to each movement the fetus makes as reported by the mother or observed by the doctor on an ultrasound screen. If the fetus doesn't move during the test, he or she may be asleep and the health care provider may use a buzzer to wake the baby. You also may be asked to drink or eat to try to stimulate the baby more.

When Are the Results Available?

Immediately.

Ultrasound

Why Is This Test Performed?

Ultrasounds were once used only in high-risk pregnancies but have become so common that they're often part of routine prenatal care.

During an ultrasound, sound waves are bounced off the baby's bones and tissues to construct an image showing the baby's shape and position in the uterus. Also called a sonogram, sonograph, echogram, or ultrasonogram, ultrasounds are used to:

  • verify the expected date of delivery
  • detect pregnancies outside the uterus
  • see whether there might be more than one fetus
  • determine whether the fetus is growing at a normal rate
  • record fetal heartbeat or breathing movements
  • check the amount of amniotic fluid in the uterus
  • indicate the position of the placenta in late pregnancy (which can sometimes block the baby's way out of the uterus)
  • guide doctors during other tests, like amniocentesis
  • find structural defects that may indicate Down syndrome, spina bifida, or anencephaly
  • detect other problems such as congenital heart defects, cleft lip or palate, and gastrointestinal or kidney malformations

Should I Have This Test?

This test is considered safe, but it's up to you to decide whether you want to undergo it. Talk to your health care provider to find out why this test is recommended for you.

When Should I Have This Test?

Ask your doctor when you will need to have an ultrasound. Women with high-risk pregnancies may need to have multiple ultrasounds throughout their third trimester to make sure a baby is developing normally.

Sometimes, when a doctor is uncertain of a baby's movements, a biophysical profile (BPP) is done, which can include an ultrasound and a nonstress test (NST). A BPP examines a baby's breathing, movement, amount of amniotic fluid, and tone, in addition to the heart rate response.

When are results available?

Although the technician can see the images immediately, a full evaluation by a doctor may take up to 1 week if one is not on site during the exam.

Depending on where you have the ultrasound done, the technician may be able to tell you that day whether everything looks OK. However, most radiology centers or health care providers prefer that technicians not comment until a specialist has taken a look — even when everything is OK.

Contraction Stress Test

Why Is This Test Performed?

This test stimulates the uterus with pitocin, a synthetic form of oxytocin (a hormone secreted during childbirth), to determine the effect of contractions on fetal heart rate. It may be recommended when a nonstress test or biophysical profile indicates a problem and can determine whether the baby's heart rate remains stable during contractions.

Should I Have This Test?

This test may be ordered if the nonstress test or biophysical profile indicates a problem. However, it can induce labor.

When Should I Have This Test?

Your doctor may schedule it if he or she is concerned about how the baby will respond to contractions or feels that it is the appropriate test to determine the fetal heart rate response to a stimulus.

How Is the Test Performed?

Mild contractions are brought on either by injections of pitocin or by squeezing the mother's nipples (which causes oxytocin to be secreted). The fetus' heart rate is then monitored.

When Are the Results Available?

Immediately.

Reviewed by: Elana Pearl Ben-Joseph, MD
Date reviewed: January 2014