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. Common maternal conditions that warrant antenatal testing include diabetes, hypertension, autoimmune disorders and intrahepatic cholestasis while common fetal indications include fetal growth restriction, fetal anomalies, an adverse pregnancy history, certain 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 frequency of antenatal testing ordered by your primary obstetrician is variable and will depend on the risks associated with your pregnancy. The higher the risk, the more frequent the antenatal testing. The most common frequency of antenatal testing is once weekly amniotic fluid evaluations and twice weekly non-stress tests.
Which pregnancies should undergo antenatal testing?
Pregnancies complicated by:
- Autoimmune disease
- Intrahepatic cholestasis
- Fetal growth restriction
- Adverse pregnancy history
- Certain multiple gestations
- Decreased fetal movement
- Post dates
A Perinatologist's Perspective:
The nuchal translucency (NT) exam at 11-14 weeks and the detailed is an absolutely critical exam. It not only provides early information about the health of the pregnancy, but also accurately dates the pregnancy - and while many people are sure of their due date before the NT exam, statistics show that about 60% of women do not get their conception date correct (and thus their due date). It may not seem like an important matter at 12 weeks, but having a due date off by 5-7 days 28 weeks into the future could be critical to the timing of your delivery and health of your baby.