5.3 Biophysical Profile and Doppler

When the NST Is Not Enough

A nonreactive nonstress test (NST) is common and frequently benign, so it cannot stand alone as a reason to deliver. The biophysical profile (BPP) and umbilical artery Doppler add the ultrasound and blood-flow data needed to distinguish a sleeping or immature fetus from a truly compromised one. The C-EFM exam expects you to recall the five BPP components, the 2-or-0 scoring, the score cutoffs, and what absent or reversed Doppler flow means.

The Biophysical Profile (BPP)

The BPP combines the NST with four real-time ultrasound observations. It is essentially a fetal Apgar performed before birth, sampling both acute markers (breathing, movement, tone, reactivity - which disappear early with hypoxia) and a chronic marker (amniotic fluid - which reflects long-standing placental function).

The acute markers are organized by the gradual hypoxia concept: the fetal CNS centers mature in a fixed order during gestation, and they fail in reverse order with worsening hypoxia. Fetal tone (the earliest center to develop) is the last to disappear, while reactivity and breathing (later-developing) are lost first. A fetus that has lost tone is therefore more profoundly compromised than one that has merely lost reactivity - which is why the total score, not any single component, drives interpretation.

The Five Components

Scoring and Interpretation

Each component is scored 2 (present/normal) or 0 (absent/abnormal) - there is no score of 1. The maximum total is 10.

A persistent score of 4 or less, or a score of 0-2, signals likely fetal compromise and usually prompts delivery. Oligohydramnios (a 0 on amniotic fluid) is treated as concerning regardless of the total because it reflects chronic uteroplacental insufficiency and warrants further evaluation even with an otherwise high score.

The Modified Biophysical Profile

The full BPP is time-consuming, so the modified BPP is widely used as a faster screen. It combines:

1. The NST (an acute marker of current fetal status), plus
2. Amniotic fluid assessment - the amniotic fluid index (AFI) or, more commonly today, the deepest (single) vertical pocket.

The modified BPP is normal when the NST is reactive and the deepest vertical pocket is greater than 2 cm. An abnormal modified BPP (nonreactive NST or deepest pocket 2 cm or less) is followed by a full BPP or CST.

Umbilical Artery Doppler

Umbilical artery Doppler velocimetry measures resistance to blood flow in the umbilical artery and is used chiefly in fetal growth restriction (IUGR/FGR). As placental small-vessel disease worsens, downstream resistance rises and diastolic (between-beat) flow falls.

The classic index is the systolic/diastolic (S/D) ratio - peak systolic velocity divided by end-diastolic velocity. As placental resistance rises, end-diastolic flow drops and the S/D ratio climbs. The progression of severity is:

1. Elevated S/D ratio (above the 95th percentile for gestational age) - increased placental resistance
2. Absent end-diastolic flow (AEDF) - no forward flow during diastole; reflects roughly 60-70% obliteration of placental vessels
3. Reversed end-diastolic flow (REDF) - flow actually reverses in diastole; the most ominous finding, with sharply increased perinatal mortality

Absent or reversed end-diastolic flow indicates severe placental compromise and is associated with high perinatal morbidity and mortality; these findings typically prompt admission, intensive surveillance, and consideration of delivery depending on gestational age. (When end-diastolic flow is absent or reversed, the S/D ratio is immeasurable, so the finding is documented as AEDF or REDF rather than as a ratio.)

Worked Example - Scoring a BPP: A 38-week growth-restricted fetus is tested. The NST is reactive (2), one 35-second breathing episode is seen (2), four limb movements occur (2), and an extension-flexion movement is noted (2), but the deepest amniotic fluid pocket is only 1.5 cm (0). Add the components: 2 + 2 + 2 + 2 + 0 = 8 of 10. Although 8 is technically reassuring, the single deepest pocket under 2 cm is oligohydramnios, a chronic marker of uteroplacental insufficiency, so the team should pursue umbilical artery Doppler and consider delivery rather than treating the 8 as fully reassuring.