What tests should my doctor perform during pregnancy?

When a woman first becomes pregnant, the amount of information given to her about prenatal care may be overwhelming. Throughout this page, we will explain prenatal tests (also known as antenatal tests) that physicians perform to ensure the health and proper development of a newborn baby. The main goal of these tests is to identify high-risk conditions so that harm to the mother and baby can be prevented. 

Prenatal Testing

Prenatal testing has been part of obstetrical practice since the 1970s. Physicians use a number of prenatal assessments to identify fetal risks and complications, allowing them to intervene if necessary. Physicians pay close attention to two abnormal findings in particular: oxygen deprivation (hypoxia) and acidosis (acidity of the baby’s blood due to oxygen deprivation). These findings are the final pathway to infant brain damage, and therefore require careful surveillance.

Prenatal testing is used to detect fetal oxygen deprivation

When a baby starts to show signs of oxygen deprivation, it is known as fetal distress. If a baby is oxygen-deprived, prenatal tests may show the following:

  • Decreased fetal movement
  • Changes in blood flow
  • Slow or abnormal fetal heart rate
  • Decreased amniotic fluid

Below, we discuss prenatal tests that detect the major indications of fetal distress.

Nonstress test (NST)

The nonstress test (NST) can be performed as early as the beginning of the third trimester and as late in the pregnancy as needed. It is frequently performed between weeks 38 and 42 of the pregnancy, but many times started at 28 to 30 weeks depending on risk factors.

How do nonstress tests work?

The nonstress test is a simple electronic fetal monitor test performed to ensure that unborn babies’ are getting enough oxygen. The test looks at the baby’s heart rate to assess whether it is normal and whether the baby is moving appropriately. In babies, an acceleration is defined as an increase in heart rate for at least 15 seconds, with an increase of at least 15 beats/minute. If the baby’s oxygen supply is adequate and the baby is moving, heart rate accelerations will occur, resulting in a reactive test. When these tests are reactive, they show the presence of normal fetal autonomic function, and the absence of acidosis or neurologic depression.

Sometimes when babies are not receiving sufficient oxygen from the placenta, their heart rates will not accelerate in response to an NST, resulting in a nonreactive test, which may require intervention.

When are nonstres tests performed?

Nonstress tests are performed if a pregnancy has extended past its due date, as well as during high-risk pregnancies. Some additional reasons for a nonstress test include the following:

  • Preeclampsia or maternal high blood pressure 
  • Gestational diabetes or diabetes treated with medication
  • Maternal obesity
  • Other maternal medical conditions that might affect the pregnancy
  • Intrauterine growth restriction (the baby is not growing properly)
  • The baby is less active than normal
  • Oligohydramnios (too little amniotic fluid)
  • Polyhydramnios (too much amniotic fluid)
  • The mother has had a procedure such as an external cephalic version (which turns a baby from the breech position to the proper birthing position) or a third-trimester amniocentesis (to determine if the baby’s lungs are mature enough for birth, or to check for a uterine infection such as chorioamnionitis)
  • The mother has previously lost a baby during the second half of pregnancy. When this is the case, NST testing may begin as early as 28 weeks.
  • The baby has been diagnosed with an abnormality or birth defect and requires more frequent monitoring
  • The pregnancy is post-term and post-dates

Contraction stress test (CST)

A contraction stress test (CST) is performed prior to labor and delivery to predict how a baby will tolerate labor. CSTs are typically performed at 34 weeks of gestation or later. A CST is performed to make sure the baby and placenta are healthy and that the baby will receive adequate amounts of oxygen from the placenta during the labor and delivery process and contractions.

How does a CST work?

During a CST, the mother lies on her left side with two devices attached to her abdomen – one monitors the baby’s heartbeat and the other records the mother’s contractions. These are graphed next to each other for the physicians to interpret. If the mother doesn’t have contractions for the first 15 minutes of the test, she may be given Pitocin to induce contractions.

Understanding the results of a CST

The results of a CST are based on the fetus’ response to oxygen-limiting conditions present during contractions. During a contraction, oxygen stops flowing to the placenta and baby. When the placenta is healthy, it has extra stores of oxygen-rich blood ready for the baby during or after a contraction. If the placenta isn’t functioning properly, the baby may not receive enough oxygen, and their heartbeat will slow down after a contraction in what is referred to as a late deceleration.

The CST is similar to the NST, except the fetal heart rate is evaluated in response to contractions as well as accelerations. The CST is not used very often, but may be used if an NST or a biophysical profile come back abnormal.

Amniotic fluid volume (AFV)

Amniotic fluid is the protective, sterile fluid that surrounds the fetus in the womb. Amniotic fluid volume (AFV) can be dangerous for the baby if it is too low (oligohydramnios) or too high (polyhydramnios).

Pregnancy complications associated with AFV

Abnormalities in AFV are associated with many different pregnancy complications, including:

How is the amniotic fluid measured?

An ultrasound can record a measurement known as the amniotic fluid index, or AFI. AFI is calculated by measuring the depth of the amniotic fluid in four sections of the womb and adding them together. Near term, an AFI of 9-18 centimeters is considered normal, 5-8 is considered borderline, and 5 or below is considered abnormal. A sudden decrease in amniotic fluid or a significant decrease over a short period of time is considered abnormal even if the AFI is above 5.

At 20-35 weeks of gestation, the AFI in a healthy pregnancy is approximately 14 cm. At weeks 34-36, the amniotic fluid starts to decrease in preparation for birth.

An alternative to the AFI is to determine the maximum pool, which is when the single deepest vertical pocket of amniotic fluid is identified by ultrasound and measured. This is part of the biophysical profile (BPP), which we’ll discuss in the next section.

Does the amount of amniotic fluid fluctuate during pregnancy?

Amniotic fluid normally increases steadily to about one liter by 34-36 weeks, and then decreases thereafter. Most studies report a decrease of about 25% per week. The rate of decline may be as high as 150 milliliters per week at 38-43 weeks. In some cases of oligohydramnios, the volume may be reduced to only a few mL.

Polyhydramnios typically is defined as having around 2,000 mL of amniotic fluid, > 8cm maximum pool, or an AFI greater than or equal to 24 cm. It can be caused by the baby producing too much urine, decreased fetal swallowing, and increased water transfer across the placenta to the baby.

If AFV is abnormal, the baby may need to be delivered early, especially if the cause is unknown. Factors used to determine if a baby should be delivered early include whether other factors indicate the baby is in distress, such as an abnormal or nonreassuring heart rate, or whether the baby’s lungs are mature.

Biophysical profile (BPP)

The biophysical profile (BPP) predicts whether a baby is healthy and the presence or absence of fetal asphyxia (severe hypoxia) as well as the risk of death during the antenatal period (the short period after birth). The BPP can be performed as early as the beginning of the third trimester and is done using an ultrasound over 30 minutes. When BPP results identify a compromised baby, the physician must take steps before progressive acidosis causes death or permanent brain damage in the baby. This may potentially include delivery by emergency C-section.

Which tests are included in a BPP?
  • A nonstress test (NST)
  • Measurement of the amniotic fluid index/volume (AFI/AFV) using ultrasound
  • Observation of fetal breathing movements
  • Observation of gross body movements
  • Observation of tone (reflex and extension movements)

During the BPP, each test parameter has points assigned to it, which when added together, create a score to assess indicators of ongoing or sudden onset hypoxia. Sudden onset (acute) hypoxia is reflected in the NST, breathing, and body movement portions of the BPP, while chronic (ongoing) hypoxia is reflected in the AFI/AFV portion.

The modified BPP (mBPP) consists of a nonstress test (a measure of acute oxygenation) and an AFI/AFV (as a measure of longer-term oxygenation).

Doppler velocimetry

Doppler velocimetry is a prenatal test that gives physicians information about uteroplacental blood flow and the baby’s responses to physiological challenges. It can be performed at the beginning of the third trimester. When blood vessels in the placenta are developing abnormally (as in preeclampsia, hypertension, or sickle cell anemia), there are progressive changes in:

  • Placental blood flow
  • Fetal blood flow
  • Fetal blood pressure
  • Fetal heart rate
Types of prenatal Doppler tests

There are several kinds of prenatal Doppler tests. Each provides different information about the growing baby:

  • Umbilical Artery Doppler: Alerts physicians about possible uteroplacental insufficiency that could lead to intrauterine growth restriction (IUGR).
  • Middle Cerebral Artery Doppler: Used to monitor and report on fetal anemia; can also provide information about IUGR.
  • Venous Doppler: Can assess compromised blood supply in the liver, blood flow through the cardiac cycle, and the presence or absence of cardiac instability.

Doppler measurements are very specific and show blood flow in different vessels. They can even pinpoint blood flow in the maternal component of the placenta. Any abnormal Doppler finding needs very close monitoring and immediate consideration regarding when to deliver.

Medical malpractice and prenatal testing errors

All medical personnel must follow standards of care in prenatal testing. Regular tests are required, and if the pregnancy is high-risk or the mother has signs of a pregnancy complication, appropriate prenatal testing must take place. When prenatal testing reveals health problems with the mother or baby, medical professionals must intervene.

Medical professionals must constantly monitor for fetal distress. When fetal distress is present, medical personnel must act promptly and treat the condition causing it, since fetal distress almost always is an indication that the baby is not receiving enough oxygen. Often, delivery by emergency C-section is the best intervention when a baby is in distress. It is negligence if physicians fail to perform appropriate prenatal tests or dismiss signs of fetal distress. If this negligence leads to harm, it constitutes medical malpractice.

Trusted Legal Help for Birth Injuries

If your child was permanently harmed as the result of prenatal testing errors, you could be eligible for compensation from a medical malpractice case. The attorneys at ABC Law Centers: Birth Injury Lawyers (Reiter & Walsh, P.C.) have helped families of children with birth injuries to secure the resources necessary to pay for long-term care and support.

Contact our award-winning attorneys today for a free consultation. We will review the medical records and determine if the necessary prenatal tests were not performed, your baby’s fetal distress was mismanaged, or any other act of negligence occurred. If your child was injured due to malpractice, we will fight to obtain the compensation they deserve for medical care, therapy, and a secure future. Our firm has numerous multimillion dollar verdicts and settlements that attest to our success, and you pay nothing unless we win your case.

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Dealing with a birth injury diagnosis can be difficult, but our attorneys can help. The ABC Law Centers: Birth Injury Lawyers team focuses exclusively on birth injury and are dedicated to earning justice for families like yours.

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Related Resources

Video: What to Expect During Prenatal Care Appointments

In this video, nurse Andrea Shea discusses what to expect during a prenatal appointment, as well as the proper management of a high-risk pregnancy.