What is antenatal testing?
Antenatal testing encompasses a group of specialized tests that include the non-stress test, biophysical profile and amniotic fluid evaluation. These tests, used to determine the health of your unborn baby, are usually employed in pregnancies where a maternal or fetal condition has placed the pregnancy at risk for an adverse outcome or increased risk of stillbirth. The purpose of antental testing is to identify a pregnancy where the health of an unborn baby is deteriorating and allow for an early delivery that could mean the difference between a live birth and a stillbirth. Although not often discussed, the backgrount rate of stillbirths in the general population is 2-4 per 1,000. While studies with antenatal testing have not demonstrated an ability to change that number in a low risk population, there is evidence that antenatal tesing reduces stillbirths and adverse pregnancy outcomes in higher risk populations. Because of this benefit, we recommend antenatal testing when the risk of stillbirth for any patient exceeds twice the background stillbirth death rate (4-8/1,000). Common maternal conditions/characteristics that warrant antenatal testing include diabetes, hypertension, autoimmune disorders and intrahepatic cholestasis, advanced maternal age, obesity, adverse pregnancy history, African American race, and IVF pregnancy while common fetal indications include fetal growth restriction, fetal anomalies, multiple gestations, decreased fetal movement and post dates pregnancies
What is a non-stress test, an amniotic fluid index and a biophysical profile?
Non-stress Test (NST): Scientifically known as cardiotocography, a non-stress test is a non-invasive recording of the fetal heart rate (with the fetus under “no stress”) over a specified period of time – usually 20-30 minutes. During that time, spontaneous fetal movement results in accelerations of the fetal heart rate, which are recorded and interpreted by our perinatologists. A healthy fetus will normally respond to movement by increasing his/her heart rate – much the same way a normal adult heart rate increases when climbing a flight of stairs. This normal response indicates the fetus has adequate oxygen reserves for activity and therefore, a normal functioning placenta. When the placenta is not functioning properly and the fetus does not have adequate oxygen reserves, the heart rate will not increase (no accelerations) due to lack of fetal activity.
Amniotic Fluid Index (AFI): The amniotic fluid index is a measurement of the amount of amniotic fluid surrounding the fetus. The final index number is the addition of 4 measurements taken in each quadrant of the uterus. A low AFI indicates a fetus that is poorly hydrated (since amniotic fluid is fetal urine) and may represent a failing or compromised placenta while a normal AFI indicates a well hydrated fetus and a normally functioning placenta.
Biophysical Profile (BPP): A specialized ultrasound evaluation used to determine fetal well being by observing the presence or absence of fetal breathing, fetal tone, fetal movement and a normal or abnormal amount of amniotic fluid. Similar to a non-stress test, a fetus that has inadequate oxygen reserves (and therefore a failing or compromised placenta) will breath less, move less, have poor tone and produce less amniotic fluid while a fetus with adequate oxygen reserves will breath regularly, move, have good tone (flex and extend the spine or open and close a hand) and produce a normal amount of amniotic fluid. Since a non-stress test is often unable to differentiate between a fetus in a normal sleep cycle (which occurs regularly throughout the day and is the reason pregnant women do not feel fetal movement all the time) and an inactive fetus due to a failing placenta – a biophysical profile is often used to differentiate the two.
How often is antenatal testing performed?
The most common frequency of antenatal testing is once weekly amniotic fluid evaluations and twice weekly non-stress tests beginning at 32 weeks and continuing until delivery.
Which pregnancies should undergo antenatal testing?
Pregnancies complicated by:
- Autoimmune disease
- Intrahepatic cholestasis
- Fetal growth restriction
- Adverse pregnancy history
- Mulitple Gestations
- Decreased fetal movement
- Post dates
- IVF pregnancies
- Advanced Maternal Age
- African American race
A Perinatologist's Perspective:
Antenatal testing is without a doubt - inconvenient. Twice weekly visits to undergo antenatal testing between 32 weeks and the delivery of your baby is a real time committment and disruptive to work, family and social calendars. As care providers, we empathize. But the benefits of antenatal testing far outweigh the horror of a stillbirth that may have been preventable.