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The fetal BPP is a frequently utilized method of antepartum fetal surveillance. The BPP score is a composite of four acute or short‐term variables (fetal tone, movement, breathing, and nonstress test) and one chronic or long‐term variable (amniotic fluid index). All four short‐term variables of the BPP are regulated by the fetal central nervous system (CNS). The fetal CNS is highly sensitive to decreases in the level of oxygenation and these biophysical variables are directly influenced by changes in the state of oxygenation of the fetus.

In the presence of progressive hypoxemia, clinical studies have confirmed that reactivity is the first biophysical variable to disappear. This is followed by the loss of fetal breathing and subsequently the loss of fetal movement. Fetal tone is the last variable to be lost in the presence of ongoing in utero hypoxemia.

Fetal urine production is the predominant source of amniotic fluid volume and is directly dependent upon renal perfusion. In response to sustained fetal hypoxemia, there is a long‐term adaptive response mediated by chemoreceptors located in the aortic arch and carotid arteries. This results in chemoreceptor‐mediated centralization of fetal blood flow by differential channeling of blood to vital organs in the fetus (brain, heart, adrenals), at the expense of nonessential organs (lung, kidney) by means of peripheral vasoconstriction. In cases of prolonged or repetitive episodes of fetal hypoxemia, there is a persistent decrease in blood flow to the lungs and kidneys resulting in a reduction in the amniotic fluid production leading to oligohydramnios. Amniotic fluid volume, therefore, is a reflection of chronic fetal condition. On average, it takes approximately 13 days for a fetus to progress from a normal to an abnormal amniotic fluid volume.

The NST is first performed followed by the sonographic evaluation of fetal biophysical activities including fetal tone, movement, and breathing. Amniotic fluid volume is measured by holding the transducer perpendicular to the floor. The largest vertical pocket is selected in each quadrant. The composite of all four quadrants’ deepest vertical pockets is the amniotic fluid index (AFI). A total of 30 minutes is assigned for obtaining ultrasound variables. A normal variable is assigned a score of 2 and an abnormal variable a score of zero (see Table 8.1).

Table 8.1 Fetal biophysical profile

Source: Based on Manning et al. (1980). Reproduced with permission of Elsevier.

Biophysical variable	Normal (score = 2)	Abnormal (score = 0)
Nonstress test	Reactive: More than two accelerations of greater than 15 bpm for more than 15 seconds in 20 minutes	Nonreactive: Less than two accelerations of greater than 15 bpm for more than 15 seconds in 20 minutes
Fetal breathing movements	More than one episode of more than 30 seconds in 30 minutes	Absence or less than 30 seconds in 30 minutes
Gross body movements	More than three discrete body/limb movements in 30 minutes	Fewer than two discrete body/limb movements in 30 minutes
Fetal tone	More than one active extension/flexion of limb, trunk, or hand	Slow or absent fetal extension/flexion
Amniotic fluid volume	More than one pocket of fluid greater than 2 cm in two perpendicular planes	No pocket greater than 1 cm in two perpendicular planes

A composite score of 8 or 10 is considered normal and correlates with the absence of fetal acidemia. A score of 6 is equivocal, and the test should be repeated in 24 hours, except in cases of oligohydramnios with intact membranes. In this particular instance, either delivery or close fetal surveillance is indicated depending on the gestational age. BPP scores of 4, 2 or 0 indicate fetal compromise and delivery should be strongly considered. The BPP score correlates linearly with fetal pH. A normal BPP score virtually rules out the possibility of fetal acidemia being present at the time of testing. A normal BPP result is highly reassuring, with a stillbirth (false‐negative) rate of 0.8 per 1000 within one week of the test. The positive predictive value of an abnormal BPP for evidence of fetal compromise (concerning fetal heart rate tracing in labor, acidemia, etc.) is approximately 50%, and a negative predictive value of 99.9% with a normal BPP. A BPP score of 6 has a positive predictive value of 25% while a score of 0 correlates with a compromised fetus in close to 100% of cases. Vibroacoustic stimulation (VAS) can be used as an adjunct in the assessment of BPP score without changing the predictive value of the test. Further, this may reduce unnecessary obstetric interventions.

The BPP can either be used as a primary test for fetal well‐being in high‐risk conditions or, more commonly, as a back‐up test for a nonreactive NST.