Fetal surveillance is a standard part of prenatal care, especially when complications arise or a pregnancy is categorized as high-risk. Monitoring fetal well-being becomes increasingly important during the third trimester, which carries the highest risk for adverse outcomes. Healthcare providers use various non-invasive methods to gather information about fetal health, helping to guide management decisions and ensure appropriate timing for delivery.
Defining the Biophysical Profile
The Biophysical Profile (BPP) is a non-invasive prenatal test designed to assess fetal health by observing biological responses to potential stress within the uterine environment. This diagnostic tool is typically employed after the 28th week of pregnancy, though it is more commonly used after 32 weeks. The test integrates two distinct methods of evaluation to create a composite score that reflects the fetus’s current condition.
The fundamental purpose of the BPP is to predict the presence or absence of fetal asphyxia, which is a state of insufficient oxygen supply. By evaluating several physical parameters, the BPP provides an indication of whether the fetus is adequately oxygenated and thriving. When the BPP identifies a potential compromise, it allows medical professionals to intervene before progressive metabolic acidosis could lead to fetal death.
The profile combines dynamic real-time ultrasonography with cardiotocography, also known as a Non-Stress Test (NST). The ultrasound component observes four separate biophysical variables, while the NST assesses the fetal heart rate pattern. Together, these five parameters offer a detailed picture of the overall fetal condition.
The Five Components of the Test
The complete BPP evaluates five specific variables: four assessed via ultrasound and one through external monitoring. The presence or absence of these variables indicates the fetus’s neurological and physical response to its environment. The observation period for the ultrasound components typically lasts up to 30 minutes to capture fetal activity.
Fetal Tone
One ultrasound component is the assessment of Fetal Tone, which evaluates the fetus’s muscle control. A normal observation requires at least one episode of active extension of a limb or the trunk, followed by a quick return to flexion. The opening and closing of a hand is also considered normal tone. An abnormal finding is a slow extension with an incomplete return to flexion or no movement.
Fetal Movement
Fetal Movement, or Gross Body Movement, is another parameter observed during the ultrasound. To receive a normal score, the fetus must exhibit at least three discrete movements of the body or limbs within the 30-minute observation period. Observing fewer than three movements suggests a potential issue, as these movements reflect central nervous system function.
Fetal Breathing Movements
The BPP also assesses Fetal Breathing Movements, which are observable diaphragm motions even though the fetus is submerged in fluid. A satisfactory finding requires observing at least one episode of continuous breathing movements lasting 30 seconds or longer during the testing period. The absence of such an episode or one lasting less than 30 seconds is considered abnormal.
Amniotic Fluid Volume (AFV)
Amniotic Fluid Volume (AFV) is the final ultrasound variable and is considered a measure of long-term fetal health. The assessment involves measuring the single deepest vertical pocket of fluid free of the umbilical cord or fetal parts. A normal volume is indicated by a pocket measuring at least two centimeters in the vertical axis. A low fluid volume, known as oligohydramnios, may suggest chronic compromise.
Non-Stress Test (NST)
The fifth component is the Non-Stress Test (NST), which monitors the fetal heart rate. The test is considered reactive when the heart rate shows at least two accelerations over a 20-minute period. An acceleration is defined as a temporary increase of 15 beats per minute above the baseline, lasting for 15 seconds. This heart rate variability in response to movement indicates a well-oxygenated fetus.
Interpreting the BPP Score
Each of the five components of the biophysical profile is assigned a score of two points for a normal finding or zero points for an abnormal finding. The highest possible score on the complete BPP is ten out of ten. The scores are then added together, and the total result dictates the subsequent management plan for the pregnancy.
A total score of eight out of ten or ten out of ten is considered reassuring, provided the amniotic fluid volume is adequate. In these cases, antepartum surveillance is typically continued, and the test may be repeated on a schedule determined by the underlying high-risk condition. This high score suggests the fetus is not currently experiencing a lack of oxygen.
A score of six out of ten is considered equivocal or uncertain, indicating a need for further evaluation. Depending on the gestational age and fluid volume, a result of six may lead to repeating the BPP within 12 to 24 hours. If a score of six is combined with a low amniotic fluid volume, the possibility of intervention or delivery is often raised.
When the score is four out of ten or less, the result is considered non-reassuring and prompts immediate medical action. A low score suggests a significant possibility of fetal compromise, which may require expedited delivery. The medical team may recommend continuous monitoring or the induction of labor to prevent further risk to the fetus.
How and When the Test is Performed
The biophysical profile is a safe, non-invasive procedure performed in an outpatient setting. The total time required for the BPP, including the ultrasound observations and the Non-Stress Test, typically ranges from 30 to 70 minutes. During the test, the mother reclines while a technician places an external transducer for the ultrasound and monitors for the NST on the abdomen.
The BPP is not a routine screening test for all pregnancies but is reserved for those deemed high-risk. Common medical indications for ordering a BPP include:
- Maternal conditions like pre-existing diabetes or hypertension, including pre-eclampsia.
- Post-term pregnancy.
- Decreased fetal movement noticed by the mother.
- Suspicion of Intrauterine Growth Restriction (IUGR).
Testing is usually initiated around 32 weeks of gestation for most at-risk patients, though it may begin earlier in cases with severe conditions. The timing is based on the point at which delivery would be considered beneficial for the fetus. If the initial BPP score is satisfactory, the test may be scheduled to repeat once or twice a week for the remainder of the pregnancy.