The BPP combines data from two sources, ie, ultrasound imaging and fetal heart rate (FHR) monitoring. Dynamic realtime B-mode ultrasound is used to measure the amniotic fluid volume (AFV) and to observe several types of fetal movement. The FHR is obtained using a pulsed Doppler transducer integrated with a high-speed microprocessor, which provides a continuously updated reading.

Originally described by Manning and colleagues, the BPP has become a standard tool for providing antepartum fetal surveillance. The BPP integrates 5 parameters to yield a biophysical profile score (BPS) and includes (1) the nonstress test (NST), (2) ultrasound measurement of the AFV, (3) observation of the presence or absence of fetal breathing movements, (4) gross body movements, and (5) tone. Table 1 describes specific criteria for the BPS.

The BPP allows 2 points for each parameter that is present, yielding a maximum score of 10; however, if all the ultrasound variable findings are normal, the FHR variable may be excluded because no change is made in the predicative accuracy of the BPP by including the FHR. If one or more ultrasound variable findings are abnormal, the NST should be performed.

A basic principle of antepartum testing is that a more accurate prediction of fetal wellness is achieved in direct proportion to the number of variables considered. The BPP is a clinical tool that integrates levels of dynamic biophysical activities into a useable standard. The BPP allows 2 points for each parameter that is present, yielding a maximum score of 10; however, if all ultrasound variables are normal, the FHR variable may be excluded because no change is made in the predictive accuracy of the BPP by including the FHR. If one or more ultrasound variable is abnormal, the NST should be performed.

Table 1. Criteria for Coding Prenatal Ultrasound Variables as Normal or Abnormal

Biophysical Variable	Normal (Score = 2)	Abnormal (Score = 0)
Fetal breathing movements	1 or more episodes of >20s within 30 min	Absent or no episode of >20s within 30 min
Gross body movements	2 or more discrete body/limb movements within 30 min (episodes of active continuous movement considered as a single movement)	<2 episodes of body/limb movements within 30 min
Fetal tone	1 or more episodes of active extension with return to flexion of fetal limb(s) or trunk (opening and closing of hand considered normal tone)	Slow extension with return to partial flexion, movement of limb in full extension, absent fetal movement, or partially open fetal hand
Reactive FHR	2 or more episodes of acceleration of >15 bmp* and of >15 s associated with fetal movement within 20 min	1 or more episodes of acceleration of fetal heart rate or acceleration of <15 bmp within 20 min
Qualitative AFV	1 or more pockets of fluid measuring >2 cm in vertical axis	Either no pockets or largest pocket <2 cm in vertical axis

*Beats per minute

The nonstress test
The NST is a noninvasive method used to evaluate fetal well-being. The NST is derived from observations that a fetus that is not acidotic and has an intact normally functioning autonomic nervous system will have periodic accelerations of the FHR. Acceleration is defined as a rise in the FHR-baseline rate that peaks at least 15 bmp above the baseline and lasts for at least 15 seconds from the beginning of the rise until the return to the FHR baseline.

Accelerations almost always occur with fetal movement. Partial umbilical cord compression with transient occlusion of the umbilical vein also can cause accelerations. This occurs with normal autonomic function, which acts to preserve cardiac output by increasing heart rate in response to decreased blood return to the fetal heart.

NSTs are described as either reactive or nonreactive. An NST is considered reactive if at least 2 accelerations are present in a 20-minute period. Occasionally, the NST may require 40 minutes or more of FHR recording to account for variations of the fetal sleep-wake cycle. An NST is considered nonreactive if sufficient accelerations are absent within a 40-minute period.

Decelerations of the FHR may be seen in as many as 50% of NSTs. If decelerations are nonrepetitive and less than 30 seconds in duration, obstetric intervention is not needed; however, repetitive decelerations or decelerations that last longer than 60 seconds are associated with an increased risk of fetal demise and cesarean delivery for the diagnosis of nonreassuring FHR pattern.

The pathophysiologic basis of the biophysical profile
Hypoxemia and acidemia have been shown to interfere with measures of central nervous system (CNS) performance, such as FHR patterns, fetal movement, and tone, in both animals and humans. Most likely, oligohydramnios results from decreased fetal urine production, which is seen with fetal hypoxemia as a result of blood flow redistributed away from the fetal kidneys and viscera in favor of the brain, heart, and adrenal glands.

Each of the movements evaluated in the BPP results from efferent signals originating in different CNS centers, which mature at different gestational ages (see Table 2). When activities known to originate from each of these oxygen-dependent centers are observed, it can be assumed that brain function is normal and systemic hypoxia is not present. Conversely, if one or more of the BPP activities is not observed within the prescribed observation period of 30 minutes, hypoxemia must be assumed to be the cause of the absence of that activity.