20.1 Biophysical Profile & Fetal Surveillance

The Biophysical Profile (BPP)

The biophysical profile (BPP) is the standard sonographic tool for assessing fetal well-being in the second half of pregnancy. A complete BPP combines the non-stress test (NST) from electronic fetal heart rate monitoring with four parameters observed in real time with ultrasound: fetal breathing movements (FBM), gross fetal body movement (FM), fetal tone (FT), and amniotic fluid volume (AFV). Each of the five components is scored 2 when normal and 0 when abnormal, producing a score out of 10 (out of 8 when the NST is omitted, as many protocols allow when all four sonographic parameters are normal).

Scoring criteria for each component

Physiologic basis: acute versus chronic hypoxia

The parameters are not interchangeable. Four of them reflect acute central nervous system function and are suppressed by hypoxia in a predictable order that mirrors the oxygen sensitivity of each CNS center: the NST is lost first, then fetal breathing movements, then gross body movement, and finally fetal tone. Recovery runs in the opposite direction: fetal tone — governed by the earliest CNS centre to develop (about 7.5 to 8.5 weeks) and therefore the last parameter lost — is the first to return once the fetus is reoxygenated, while FHR reactivity, governed by the last centre to mature (about 24 to 28 weeks), is the first lost and the last to return. The mnemonic first to go is the last to return applies to reactivity, not to tone. Amniotic fluid volume is the only chronic marker: sustained hypoxemia redistributes fetal cardiac output toward the brain, heart, and adrenal glands and away from the kidneys, so fetal urine output falls and oligohydramnios develops over days. This is why a fetus with normal tone, movement, and breathing but a small fluid pocket is managed differently from a fetus showing the reverse pattern.

The sonographic observation window is 30 minutes, and each parameter must be demonstrated within that window to earn its 2 points. A full BPP is generally considered reliable from about 26 to 28 weeks of gestation onward, once fetal behavioral states have matured enough to be interpreted.

Interpretation and management of the score

• 10/10 or 8/8 with normal fluid: reassuring; perinatal mortality is very low; repeat testing on the schedule dictated by the indication (weekly or twice weekly).
• 8/10: generally reassuring if fluid is normal; an 8/10 caused by oligohydramnios alone is more concerning than one caused by a missing acute parameter and may prompt delivery at term.
• 6/10: equivocal. Repeat the BPP within 24 hours; delivery is usually recommended if the pregnancy is at term, if oligohydramnios persists, or if the repeat score remains equivocal.
• 4/10 or less: abnormal. A score of 4 usually leads to delivery planning once the fetus is viable, and scores of 0 to 2 carry an extremely high perinatal mortality and mandate urgent evaluation, usually with immediate delivery in a viable fetus. The classic Vintzileos data showed perinatal mortality climbing steeply as the score falls below 6.

Indications for surveillance

Common indications include pregestational and gestational diabetes, chronic hypertension and preeclampsia, suspected fetal growth restriction (FGR) - also called intrauterine growth restriction (IUGR) - decreased fetal movement, post-term pregnancy (testing commonly begins at 41 weeks), isoimmunization or other risk of fetal anemia, prior stillbirth, preterm premature rupture of membranes, and multiple gestation with growth discordance. High-risk pregnancies are typically tested weekly or twice weekly.

The modified BPP

The modified biophysical profile pairs the NST with the amniotic fluid index (AFI), the sum of the deepest vertical fluid pocket in each of the four uterine quadrants. A normal AFI is roughly 5 to 25 cm; an AFI of 5 cm or less defines oligohydramnios, and an AFI above 24 to 25 cm defines polyhydramnios. The modified BPP predicts fetal well-being nearly as well as the full BPP with far less scanning time, so it is the most common first-line surveillance test; an abnormal result triggers a full BPP or contraction stress testing.

Relationship to Doppler and growth surveillance

The BPP is one piece of a surveillance package. In FGR, umbilical artery Doppler is the best-validated tool for distinguishing the constitutionally small fetus from the growth-restricted fetus with true placental insufficiency, and integrating it into management reduces perinatal mortality. Serial biometry (biparietal diameter, head circumference, abdominal circumference, femur length) is repeated no more often than every 3 to 4 weeks, because expected interval growth is small relative to measurement error. A typical high-risk protocol therefore combines interval growth scans, umbilical artery (and, when indicated, middle cerebral artery) Doppler, fluid assessment, and NST or BPP. Importantly, a normal BPP does not erase the concern raised by severely abnormal Doppler findings such as reversed end-diastolic flow - those fetuses need daily or inpatient surveillance regardless of the biophysical score.

Worked example. At 36 weeks, a fetus of a mother with gestational hypertension shows 4 body movements, a hand that extends and re-flexes, one 45-second breathing episode, a deepest pocket of 2.8 cm, and a reactive NST: 10/10, reassuring. The same fetus one week later with a deepest pocket of 1.4 cm scores 8/10 with fluid 0 - and that isolated oligohydramnios, the chronic marker, is exactly the finding that may tip management toward delivery at term even though every acute parameter is intact.