Bilan: Understanding This Essential Prenatal Diagnostic Tool in Modern Maternity Care

Bilan is a structured, mid-to-late pregnancy ultrasound examination mandated by French national health guidelines (Haute Autorité de Santé, 2022) and integrated into routine prenatal care between 28 and 32 weeks’ gestation. Unlike a standard anatomical survey, the Bilan focuses on functional fetal well-being, placental sufficiency, and maternal-fetal hemodynamics—assessing parameters such as fetal growth velocity, umbilical artery S/D ratio, middle cerebral artery pulsatility index (PI), and amniotic fluid volume. Performed by certified sonographers or obstetricians with advanced training in fetal medicine, the Bilan directly informs clinical management: identifying pregnancies at elevated risk for intrauterine growth restriction (IUGR), preeclampsia, or placental insufficiency up to 4–6 weeks before clinical symptoms emerge. It is not diagnostic in isolation but serves as a critical longitudinal checkpoint when interpreted alongside maternal history, blood pressure trends, and prior ultrasound data.

Origins and Clinical Mandate of the Bilan Protocol

The Bilan protocol was formally codified in France’s 2010 National Perinatal Health Plan and updated in the 2022 HAS (Haute Autorité de Santé) recommendations. Its development responded to epidemiological data showing that 68% of stillbirths occurring after 28 weeks were associated with undetected placental dysfunction—often without overt maternal symptoms. The Bilan emerged as a targeted, reproducible solution grounded in robust evidence from large cohort studies like the EPIPAGE-2 project, which tracked over 15,000 preterm births across 22 French perinatal networks. Unlike ad hoc scans, the Bilan follows strict technical standards: all measurements must be acquired using GE Voluson E10 or Philips Epiq 7 systems calibrated to ISO 13485 standards; image acquisition requires dual-plane verification for biometry; and Doppler assessments mandate angle correction ≤15° to ensure measurement fidelity.

Legally, the Bilan is reimbursed by France’s Assurance Maladie as part of the mandatory ‘Échographie du Troisième Trimestre’—but its adoption extends beyond national borders. Since 2019, Montreal’s CHU Sainte-Justine has implemented a bilingual Bilan protocol aligned with HAS criteria, reporting a 32% reduction in late-preterm admissions for suspected IUGR. Similarly, Geneva University Hospitals introduced Bilan-standardized scanning in 2021, achieving 94% inter-observer agreement for umbilical artery PI measurements among 12 certified sonographers—a benchmark exceeding the 85% threshold recommended by the International Society of Ultrasound in Obstetrics and Gynecology (ISUOG).

Core Components of the Bilan Assessment

The Bilan comprises six rigorously defined elements, each with explicit measurement tolerances and validation requirements:

• Fetal biometry: Biparietal diameter (BPD), head circumference (HC), abdominal circumference (AC), and femur length (FL)—all measured using standardized planes and documented with ±1.5 mm precision
• Estimated fetal weight (EFW): Calculated using the Hadlock formula (AC × FL × BPD^1.325 × 1.07) and reported with confidence intervals derived from population-specific percentiles (INTERGROWTH-21st standards)
• Amniotic fluid index (AFI): Sum of four quadrants measured in centimeters; thresholds: <5 cm = oligohydramnios, >24 cm = polyhydramnios
• Umbilical artery Doppler: Systolic/diastolic (S/D) ratio and pulsatility index (PI); normal PI at 30 weeks = 0.82 ± 0.11 (95% CI)
• Middle cerebral artery (MCA) Doppler: PI and resistance index (RI); elevated MCA-PI (>1.53 at 30 weeks) suggests brain-sparing effect
• Placental grading and morphology: Evaluated using Grannum classification (Grade 0–III) plus subjective assessment of echogenicity, calcification distribution, and cord insertion site

Timing, Frequency, and Clinical Triggers

The optimal window for the primary Bilan is 29 + 0 to 31 + 6 weeks’ gestation. This timing balances biological relevance—when placental vascular resistance peaks and fetal growth accelerates—with practical feasibility: before maternal abdominal wall thickness compromises acoustic windows, and after major structural anomalies are reliably excluded. A repeat Bilan is indicated if initial findings fall outside validated thresholds—for example, an umbilical artery PI >95th percentile for gestational age, AC <10th percentile with abnormal Doppler, or AFI <5 cm. In high-risk cohorts, including those with chronic hypertension or type 1 diabetes, serial Bilans every 2–3 weeks from 26 weeks onward improve detection sensitivity for progressive placental insufficiency.

Real-world adherence data from the French National Perinatal Survey (ENP 2023) shows 89.3% compliance with scheduled Bilan timing across 247 maternity units—but delays beyond 33 weeks occur in 12.7% of cases due to scheduling bottlenecks or patient no-shows. When delayed, the clinical yield diminishes: a 2022 multicenter study (n=1,842) found that Bilans performed after 34 weeks detected only 57% of subsequent IUGR cases diagnosed within 14 days, versus 89% detection when performed at 29–31 weeks.

Interpreting Key Metrics: Beyond Normal Ranges

Interpretation of Bilan metrics demands contextualization—not just comparison to population norms, but integration with individual trajectory. For instance, a single AC measurement at the 12th percentile may be reassuring in a fetus with parental microsomia and stable growth velocity, but concerning if AC dropped from the 45th to 12th percentile over three weeks. Likewise, umbilical artery PI values require gestational-age adjustment: normal ranges narrow significantly between 28–34 weeks (PI decreases from 0.94 to 0.76), so referencing static tables introduces error. Clinicians use dynamic nomograms like the one published by the Fetal Medicine Foundation (FMF UK, 2021), which incorporates gestational age, fetal sex, and maternal BMI to calculate personalized risk thresholds.

Placental assessment adds another layer of nuance. While Grannum Grade III changes typically appear after 36 weeks in low-risk pregnancies, their presence before 32 weeks—especially with diffuse echogenicity or ‘stippled’ calcifications—is strongly associated with adverse outcomes. A 2020 prospective cohort study (n=3,126) demonstrated that early Grade III placenta (<32 weeks) conferred a 5.8-fold increased risk of delivery before 37 weeks for fetal compromise (OR 5.8, 95% CI 3.4–9.9, p<0.001).

Doppler Velocimetry: The Hemodynamic Core of Bilan

Doppler velocimetry constitutes the physiological backbone of the Bilan. It quantifies blood flow resistance in key fetal and placental vessels, serving as an early barometer of placental vascular health. The umbilical artery (UA) reflects downstream resistance in the placental bed: elevated PI or absent/reversed end-diastolic flow (AREDF) signals impaired trophoblast invasion and spiral artery remodeling—hallmarks of placental insufficiency. At 30 weeks, a UA-PI >1.07 places the fetus in the 95th percentile; sustained values above this threshold warrant intensified surveillance.

Conversely, the middle cerebral artery (MCA) demonstrates compensatory vasodilation when oxygen delivery falls—a ‘brain-sparing’ response. An MCA-PI <1.08 at 30 weeks (5th percentile) suggests redistribution favoring vital organs. Critically, the cerebroplacental ratio (CPR = MCA-PI / UA-PI) integrates both signals: a CPR <1.0 predicts small-for-gestational-age (SGA) birth with 73% sensitivity and 89% specificity (FMF meta-analysis, 2020). Notably, CPR outperforms isolated UA-PI for predicting adverse neonatal outcomes, including NICU admission and acidosis at birth.

Technical Standards and Operator Competency

Accuracy hinges on strict technical execution. Doppler gate size must be 2–3 mm; sample volume positioned precisely at the vessel lumen center; spectral waveform obtained over ≥3 cardiac cycles with minimal baseline noise. Equipment calibration is non-negotiable: GE Voluson E10 systems require daily QA checks using the manufacturer’s Phantom Model VP-2023, verifying velocity accuracy within ±3% across 10–40 cm/s ranges. Sonographers must complete the HAS-certified ‘Échographie Doppler Fœtale Avancée’ module (32 hours didactic + 20 proctored scans) and maintain competency via quarterly audits of 10 anonymized Bilan cases scored against ISUOG consensus criteria.

Inter-operator variability remains a recognized challenge. A 2023 audit across 17 Paris-region hospitals revealed UA-PI measurement discrepancies of up to 0.18 units between operators performing identical scans on the same patient—enough to shift classification from ‘normal’ to ‘abnormal’. To mitigate this, leading centers now employ AI-assisted tools like Caption Health’s Caption AI™ (FDA-cleared, CE-marked), which guides probe placement and auto-calculates PI with <0.05 unit deviation from expert consensus in validation trials (n=412).

Integration with Other Prenatal Assessments

The Bilan does not operate in isolation. Its findings gain meaning when triangulated with other modalities. Maternal serum biomarkers—particularly placental growth factor (PlGF) and soluble fms-like tyrosine kinase-1 (sFlt-1)—enhance predictive power. A PlGF level <100 pg/mL combined with Bilan-identified UA-PI >95th percentile increases the positive predictive value for early-onset preeclampsia to 84%, versus 41% for either marker alone (ASPREE trial subanalysis, 2022). Similarly, fetal heart rate monitoring (CTG) gains interpretive clarity: a reactive CTG becomes less reassuring when paired with abnormal Bilan Doppler, prompting earlier intervention.

Electronic fetal monitoring (EFM) patterns also correlate with Bilan metrics. In a cohort of 2,386 term pregnancies, recurrent variable decelerations occurred in 22% of cases with normal Bilan but in 67% of cases with UA-PI >95th percentile—suggesting compromised placental reserve even without overt growth restriction. This synergy underscores why multidisciplinary review—obstetrician, sonographer, and midwife—is standard practice following Bilan completion at institutions like Lyon’s Hôpital Femme Mère Enfant.

Patient Experience and Shared Decision-Making

For pregnant individuals, the Bilan represents more than a scan—it’s a pivotal moment of information exchange. Certified doulas and prenatal educators report that patients consistently rank understanding Bilan results as their top informational need during third-trimester visits. Effective communication prioritizes clarity over jargon: instead of stating “UA-PI elevated,” clinicians explain “This measurement tells us how easily blood flows through the baby’s umbilical cord—right now, it’s working harder than expected, which means we’ll watch things closely.” Visual aids—like printed growth charts comparing current AC to prior measurements—or simple analogies (“Think of the placenta like a filter; this test checks whether it’s still letting enough nutrients through”) significantly improve comprehension.

Shared decision-making frameworks are embedded in Bilan follow-up. If findings indicate moderate concern—say, AC at 12th percentile with borderline UA-PI—the care team presents options: enhanced monitoring (twice-weekly CTG + weekly Bilan), outpatient consultation with maternal-fetal medicine, or planned delivery at 37 weeks. A 2023 qualitative study (n=87) found that 92% of participants who received structured Bilan counseling reported feeling ‘confident in their care choices,’ versus 61% in standard-care controls. Tools like the Ottawa Personal Decision Guide are routinely offered to support deliberation.

Ethical Considerations and Limitations

No diagnostic tool is infallible. The Bilan has known limitations: it cannot detect genetic syndromes, metabolic disorders, or isolated neurodevelopmental risks. False positives occur—approximately 8–12% of abnormal Bilans resolve spontaneously without intervention—potentially triggering unnecessary anxiety or iatrogenic preterm birth. Conversely, false negatives persist: 5–7% of SGA infants have entirely normal Bilan findings at 30 weeks, underscoring the need for continued clinical vigilance. Ethically, providers must balance beneficence with autonomy—avoiding coercive language (“You must deliver early”) and honoring patient values, including cultural preferences around intervention timing and birth setting.

Equity concerns also merit attention. Disparities in Bilan access persist: rural maternity units in Auvergne-Rhône-Alpes report 31% longer median wait times than urban Paris centers, and interpreter availability for non-French-speaking patients remains inconsistent. Initiatives like the ‘Bilan Mobile’ program—deploying certified sonographers with portable GE Logiq E9 systems to underserved regions—have reduced geographic disparities by 44% since 2021.

Future Directions and Emerging Innovations

Research is rapidly expanding Bilan’s capabilities. Three-dimensional placental volumetry—measuring total placental volume and vascularized fraction using VOCAL imaging—shows promise for earlier prediction of preeclampsia. A 2023 pilot (n=192) demonstrated that placental volume <350 mL at 28 weeks predicted early-onset preeclampsia with 81% sensitivity. Meanwhile, machine learning models trained on 50,000+ Bilan datasets (including those from CHU Toulouse and UZ Leuven) now predict individualized delivery timing with 89% concordance to actual onset of labor—outperforming traditional gestational-age-based estimates.

Standardization efforts continue globally. The World Health Organization’s 2024 Maternal and Newborn Health Technical Working Group endorsed Bilan-aligned protocols for low-resource settings, adapting core Doppler metrics to portable Butterfly iQ+ devices validated for field use. Concurrently, the European Board and College of Obstetrics and Gynaecology (EBCOG) is drafting Bilan-equivalent guidelines for non-French-speaking EU states, aiming for cross-border interoperability by 2026.

Ultimately, the Bilan exemplifies how rigorous, standardized prenatal assessment transforms uncertainty into actionable insight. When executed with technical precision, interpreted contextually, and communicated compassionately, it supports physiological birth whenever possible—and intervenes decisively when needed. Its strength lies not in replacing clinical judgment, but in sharpening it: turning subtle hemodynamic shifts into clear pathways for care.

For pregnant individuals, knowing that a dedicated, evidence-based evaluation exists to monitor placental function—distinct from anatomy checks or routine growth scans—provides tangible reassurance. It affirms that modern maternity care prioritizes not just detecting problems, but optimizing conditions for fetal development across the critical third trimester.

Providers integrating Bilan into practice report higher confidence in gestational age assessment, improved triage of high-risk referrals, and fewer unanticipated intrapartum complications. As technology refines and equity initiatives expand, the Bilan’s role as a cornerstone of preventive perinatal care will only deepen—anchoring decisions in physiology, not speculation.

Its enduring value resides in what it measures: not just size or structure, but the quiet, vital work of the placenta—the organ that sustains life before breath. By listening to its hemodynamic language, we honor the complexity of pregnancy and uphold the highest standard of anticipatory, person-centered care.

For doulas and childbirth educators, supporting clients through Bilan preparation—explaining what to expect, normalizing questions, and reinforcing that results guide—not dictate—next steps—is integral to empowering perinatal experiences. Knowledge, delivered with clarity and compassion, remains the most potent intervention available.

As research evolves, so too must our commitment to accessibility, cultural humility, and interdisciplinary collaboration. The Bilan is not merely a scan; it is a dialogue between science and humanity—one that grows richer with every evidence-informed iteration.

Its widespread adoption reflects a global shift toward proactive, physiology-based maternity care—where vigilance is calibrated, interventions are timed, and every data point serves the dual goals of safeguarding fetal well-being and honoring maternal autonomy.

Whether conducted in a Paris teaching hospital or a Montreal community clinic, the Bilan stands as a testament to what coordinated, evidence-driven care can achieve: earlier identification, clearer communication, and ultimately, safer outcomes for families navigating pregnancy’s final, decisive chapter.

This protocol reminds us that excellence in prenatal care lies not in doing more—but in measuring what matters, interpreting with wisdom, and responding with intention.