Ultrasonography in Biophysical Profile 

The biophysical profile (BPP) is a noninvasive test that predicts the presence or absence of fetal asphyxia and, ultimately, the risk of fetal death in the antenatal period.^[1] When the BPP identifies a compromised fetus, measures can be taken to intervene before progressive metabolic acidosis leads to fetal death.^{[2, 3, 4]}

The BPP combines data from 2 sources (ie, ultrasonographic imaging and fetal heart rate [FHR] monitoring). Dynamic realtime B-mode ultrasonography 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.^{[5, 6]}

Originally described by Manning and colleagues,^{[7, 8]} 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) ultrasonographic measurement of the AFV, (3) observation of the presence or absence of fetal breathing movements, (4) gross body movements, and (5) tone. Table 1, below, 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 of the ultrasonographic 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 (see the graph below). If 1 or more ultrasonographic 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 usable standard.^[9] The BPP allows 2 points for each parameter that is present, yielding a maximum score of 10; however, if all ultrasonographic 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 1 or more ultrasonographic variables are abnormal, the NST should be performed.^[10]

Table 1. Criteria for Coding Fetal Biophysical Variables as Normal or Abnormal (Open Table in a new window)

Each of the movements evaluated in the BPP results from efferent signals originating in different central nervous system (CNS) centers, which mature at different gestational ages (see Table 2).

Table 2. Maturation of Central Nervous System Regulatory Centers (Open Table in a new window)

Table 3 shows a variety of factors, other than hypoxemia, that have been shown to affect different BPP parameters.

Table 3. Factors Affecting the Biophysical Profile (Open Table in a new window)

The ultrasonographic portion of the biophysical profile (BPP) should begin with notation of the starting time.^[2] The profile may be completed when all of the variables have been observed; however, a full 30 minutes must elapse before the profile is judged to be abnormal. The scan should start with a general survey, noting the position of the fetus and the presence of cardiac activity. Although not part of the BPP, surveying the placental position and grade and the fetal morphology is common practice during observation of fetal activity.

Amniotic fluid volume (AFV) is assessed as normal if 1 or more pockets of fluid are detected that measure at least 2 cm along the vertical axis. Oligohydramnios is present if the largest pocket measures less than 2 cm.

The pressure applied to the transducer by the sonographer is inversely proportional to the depth of the fluid pocket.^[11] Careful attention to transducer pressure is required to avoid a false diagnosis of oligohydramnios, which can result from excessive transducer pressure compressing the maternal abdomen.

Fetal movements are judged as normal or abnormal according to the criteria established in Table 1. The first video below shows breathing movements, while the second video shows movements that can be mistaken for fetal breathing.

The following videos demonstrate gross movements of the lower and upper body and tone.

The video below demonstrates the amniotic fluid index (AFI) by measuring 4 pockets of fluid.

The AFI is a semiquantitative method for evaluating the AFV. The AFI is derived by adding the largest vertically measured fluid pocket from each uterine quadrant. This method appears to be at least as accurate as the largest–pocket-of-fluid method and can reasonably be substituted as an alternative method for evaluating AFV in the biophysical profile (BPP). Using this method, oligohydramnios is defined as an AFI of less than 5.

To obtain an AFI, the mother must be in the supine position and the linear ultrasonographic probe must be parallel to the maternal spine and perpendicular to the floor for all measurements. The abdomen is divided into 4 quadrants, with the umbilicus delineating the upper and lower halves and the linea nigra delineating the left and right halves.

The largest pocket of fluid in each quadrant is measured along the vertical dimension, which is the dimension perpendicular to the ultrasonographic probe. The pockets must be free of umbilical cord or fetal extremities, although brief appearances of these are acceptable. The video below demonstrates measurement of the AFI.

A modified biophysical profile (BPP), consisting of a nonstress test (NST) and an amniotic fluid index (AFI), is used widely.^[12] If either the NST or the AFI is abnormal, a complete BPP or a contraction stress test (CST) is performed. The modified BPP, CST, and complete BPP have similarly low false-negative mortality rates, defined as the number of fetal deaths within 1 week of a normal test result.^[13] Nevertheless, no clear evidence exists that the 2 variables used in the modified BPP are better predictors than the other variables omitted from the BPP.^[14] Furthermore, this method requires 2 modalities for fetal evaluation, while normal ultrasonographic findings in a BPP eliminate the need for an NST.

Antepartum testing using the biophysical profile (BPP) or any other method should not be performed earlier than the gestational age at which extrauterine survival or active intervention for fetal compromise is possible. Furthermore, no indications exist for testing in a fetus at term when likelihood of successful induction is high or when vaginal delivery is contraindicated for obstetric reasons. For patients with a low probability of successful induction, the BPP is a useful tool that can be used while waiting for cervical ripening. In these patients, the purpose of the BPP is to avoid the maternal morbidity resulting from failed induction followed by cesarean delivery.

The frequency of testing varies according to the clinical variables in each patient. In most high-risk pregnancies, testing plans start with weekly testing,^[3] although twice-weekly testing is the standard for pregnancies beyond 42 weeks and for patients with insulin-dependent diabetes.^[15] Frequency of testing increases in direct proportion to the severity of the maternal or fetal condition.

In unstable pregnancies with progressive deterioration of the fetal condition, abnormal umbilical cord blood flow patterns occur first. Subsequently, FHR variation is reduced, followed by loss of breathing movements, while general fetal movements and tone are the last parameters to demonstrate abnormal results. Frequent assessment of fetal BPP movements may help prolong the pregnancy in fetuses with a marginally reduced FHR variation.

An abnormal BPS should prompt further evaluation or intervention depending on the circumstances. If an abnormal score occurs in a term fetus, preparation should be made for delivery. An abnormal score in a fetus remote from term requires conservative management, since the risk of fetal death is similar to the neonatal mortality rate resulting from prematurity. In these patients, daily testing often is performed. Table 4 provides general guidelines for treatment following the BPS.

The biophysical profile (BPP) is a reliable method of predicting fetal survival. Data have been collected on this and other antepartum testing procedures for more than 20 years. Testing methods usually are evaluated by comparing the false-negative mortality rate for each method. The false-negative mortality rate is defined as the number of fetal deaths, corrected for lethal congenital anomalies and unpredictable causes of demise, that occur within 1 week of a normal test result.

The BPP has a false-negative mortality rate of 0.77 deaths per 1000 tests. Furthermore, the BPS highly correlates with the antepartum fetal umbilical venous cord pH level.

In a study, cordocentesis performed immediately following a BPP demonstrated that a poor BPS was always associated with a pH of less than 7.20, while a score of 10 of 10 always yielded a pH of greater than 7.20.^[16]

The false-negative mortality rate for the nonstress test (NST) alone is 1.9 per 1000 tests, more than twice that of the BPP. The modified BPP has a mortality rate of 0.8. The low false-negative rates of these testing methods depend on an appropriate response to an abnormal result. Intervention and retesting are the usual responses.