Clinical Deep-Dive · Obstetrics

Obstetric Ultrasound: A Systematic Deep-Dive

From the first-trimester dating scan to the fetal anatomy survey, biometry, amniotic fluid, placenta and the obstetric Doppler suite. Threshold-driven, referenced to ISUOG, ACOG/AIUM, the SRU/NEJM viability panel and the seminal Doppler literature — organised the way you actually work a scan.

Educational reference only. This page summarises published thresholds and guideline positions for teaching. It is not a clinical protocol and does not replace ISUOG/ACOG/AIUM guidance, local protocols, or the judgement of a credentialled sonologist. Cut-offs (e.g. gestational-sac and CRL viability criteria) are deliberately conservative to avoid harm to a potentially viable pregnancy; always apply the version adopted by your unit and confirm findings with a second measurement/scan where indicated.

1First trimester 2Anatomy survey 3Biometry & growth 4Fluid & placenta 5Dopplers 6Anomalies & red flags

The first-trimester scan answers three questions in order: Is it intrauterine? Is it viable? How far along? — then adds early anatomy and aneuploidy risk. The discriminatory thresholds below are the conservative SRU (2013) / NEJM criteria adopted internationally to avoid terminating a potentially viable pregnancy.

Dating — Crown–Rump Length (Robinson)

CRL is the most accurate biometric for gestational age (GA) up to ~14 weeks (CRL ≈ 84 mm), using the Robinson & Fleming charts. Measure in a true mid-sagittal plane, neutral fetal position (not flexed or hyperextended), maximum straight length excluding the yolk sac and limbs.

Dating rule: Where CRL dating differs from the LMP-based EDD by more than a threshold (ACOG: >5 days when CRL < 9⁺⁰ wk, >7 days at 9⁺⁰–13⁺⁶ wk), re-date by the scan. First-trimester CRL is more accurate than any later biometry.

Below ~6 weeks (before a measurable embryo), gestational-sac mean sac diameter (MSD) can approximate GA but is less reliable and should not be used to override viability decisions.

Viability milestones & discriminatory criteria

Structures appear in a predictable order. The gestational sac is seen first (~4.5–5 wk), then the yolk sac, then the embryo with cardiac activity.

Schematic order of appearance. A yolk sac should be visible by MSD ~8–10 mm (transvaginal); cardiac activity should be present once the embryo reaches CRL 7 mm.

Diagnostic of failed early pregnancy (SRU 2013 / NEJM):

MSD ≥ 25 mm with no embryo (empty sac).
CRL ≥ 7 mm with no cardiac activity.
No embryo with heartbeat ≥ 2 weeks after a scan showing a sac without a yolk sac.
No embryo with heartbeat ≥ 11 days after a scan showing a sac with a yolk sac.

Suspicious but NOT diagnostic (repeat scan needed): CRL < 7 mm with no heartbeat; MSD 16–24 mm with no embryo; absent yolk sac; small sac relative to embryo (MSD − CRL < 5 mm); embryonic bradycardia. The conservative cut-offs raise the empty-sac threshold from the older 16 mm to 25 mm to eliminate false-positive miscarriage diagnoses.

Ectopic pregnancy — signs

Rule: an empty uterus with a positive β-hCG above the discriminatory zone should prompt an active search for ectopic. There is no reliable single β-hCG level that guarantees an intrauterine pregnancy will be seen.

Look for

Tubal ring / “bagel” sign — echogenic ring adnexal to the ovary, moving separately from it on probe pressure.
Adnexal mass distinct from the corpus luteum (which sits within the ovary).
Live extrauterine embryo — definitive.
Free fluid / echogenic haemoperitoneum in the pouch of Douglas / Morison’s pouch (suggests rupture).
Pseudogestational sac — central, no double-decidual sign, no yolk sac.

Pitfalls

Interstitial/cornual ectopic — eccentric sac with a thin (<5 mm) myometrial mantle; risk of catastrophic bleeding.
Caesarean-scar & cervical ectopics.
Heterotopic pregnancy (IUP + ectopic) — higher risk with ART.
Do not mistake a corpus luteum for a tubal ring — the CL is intra-ovarian and has a “ring of fire” low-resistance flow.

Multiples — chorionicity is a first-trimester priority

Determining chorionicity and amnionicity is most reliable in the first trimester and dictates the entire pregnancy’s risk profile and surveillance schedule. Once past ~14 weeks it becomes unreliable.

Signs

Dichorionic (DCDA) — thick inter-twin membrane with the lambda / twin-peak sign (triangular placental tissue projecting into the membrane base).
Monochorionic diamniotic (MCDA) — thin membrane meeting the placenta at a right angle (T-sign).
Monochorionic monoamniotic (MCMA) — no dividing membrane; cord entanglement risk.

Why it matters

Monochorionic twins share a placenta → risk of TTTS, TAPS, sIUGR, discordant anomalies → fortnightly scanning from ~16 wk.
Label twins consistently (site + cord insertion) for the whole pregnancy.
Date from the larger CRL by convention.

Early anatomy & the 11–13⁺⁶ aneuploidy screen

The first-trimester screen combines nuchal translucency (NT), maternal serum markers and maternal age. Increasingly NIPT (cell-free DNA) is used, but NT retains value for structural anomalies and rarer chromosomal issues missed by NIPT.

Nuchal translucency: measured 11⁺⁰–13⁺⁶ wk (CRL 45–84 mm), mid-sagittal, neutral neck, on-to-on caliper placement. NT ≥ 3.0 mm sits between the 95th–99th centile; NT ≥ 3.5 mm ≈ the 99th centile and is a widely used referral threshold associated with markedly raised risk of aneuploidy, cardiac and other structural anomalies.

Combined test markers

NT ↑ with aneuploidy.
Free β-hCG — ↑ in trisomy 21.
PAPP-A — ↓ in trisomy 21 (and a marker of placental function).
Combined with maternal age → risk for trisomy 21, 18, 13.

Additional first-trimester markers

Nasal bone — absent/hypoplastic in ~60–70% of T21.
Ductus venosus a-wave — reversed/absent raises risk.
Tricuspid regurgitation.
Early structural clues: acrania/exencephaly, megacystis, absent stomach, omphalocele, single umbilical artery.

The mid-trimester anatomy survey (typically 18–22 wk, per AIUM/ISUOG) is a systematic head-to-toe examination. Work it the same way every time so nothing is skipped. Below is a working checklist grouped by system, followed by the “soft markers” that adjust aneuploidy risk.

Systematic checklist

Head, brain & face

Skull shape & ossification (no lemon sign)
Lateral ventricles — atrium < 10 mm
Cavum septi pellucidi present
Midline falx; symmetric hemispheres
Posterior fossa — cerebellum & cisterna magna (2–10 mm); no banana sign
Nuchal fold < 6 mm (measured ≤ 22 wk)
Orbits; profile
Upper lip & alveolar ridge — exclude cleft

Spine

Sagittal + transverse + coronal sweeps
Intact overlying skin
Normal vertebral alignment; no splaying

Thorax & heart

Situs — stomach & heart both left
Four-chamber view (size, axis, symmetry)
LV & RV outflow tracts
3-vessel & 3-vessel-trachea view
Lungs symmetric; no effusion/mass
Intact diaphragm (heart & stomach not co-planar)

Abdomen

Stomach — present, left-sided, normal size
Bowel echogenicity (not > bone)
Both kidneys; renal pelves < 4 mm (2nd tri)
Bladder present
Abdominal wall & cord insertion intact
Cord vessels — 3 (two arteries, one vein)

Limbs

Three long bones per limb
Hands & feet — position & orientation
No fixed flexion/clubbing

Placenta, cord & cervix

Placental location & relation to internal os
Cord insertion (placental & fetal)
Number of cord vessels
Cervical length (TVS if short/at risk)
Amniotic fluid volume (subjective + AFI/SDP)

General

Fetal number, presentation, cardiac activity
Biometry (BPD, HC, AC, FL) — see tab 3

Soft markers Overview

Soft markers are minor, often transient findings that modestly adjust aneuploidy risk (usually via a likelihood ratio applied to the background/screen risk). In the NIPT era, an isolated soft marker in a low-risk, screen-negative pregnancy usually warrants no action.

Soft marker	Main association	Note
Echogenic intracardiac focus	T21 (weak)	Very common, usually LV; isolated → low significance
Echogenic bowel	T21, CF, CMV, swallowed blood, IUGR	Only if ≥ bone echogenicity
Mild pyelectasis	T21	Renal pelvis ≥ 4 mm (2nd tri)
Short femur / humerus	T21, skeletal dysplasia, FGR	Below expected for BPD/GA
Thickened nuchal fold	T21 (stronger)	≥ 6 mm — significant marker
Choroid plexus cyst	T18	Look hard for other T18 features
Absent/hypoplastic nasal bone	T21	Powerful marker
Single umbilical artery	Aneuploidy, cardiac, renal	Prompt careful anatomy + growth surveillance

Four standard biometric measurements drive gestational-age assessment (after the first trimester) and estimated fetal weight (EFW). Interpretation is by centile on a validated chart and — crucially — by interval growth across serial scans.

The four measurements

Parameter	Plane / how	Landmarks
BPD	Axial, thalamic level	Symmetric hemispheres, midline falx, CSP, thalami; outer-to-inner (or outer-to-outer per chart)
HC	Same plane as BPD	Ellipse around outer skull table; unaffected by head shape (dolicho/brachycephaly)
AC	Axial abdomen	Stomach bubble + portal sinus (J-shape / “hockey stick”), ribs symmetric; outer skin ellipse
FL	Long axis of femur	Full ossified diaphysis, beam ~perpendicular; exclude epiphyses

AC is the single most sensitive parameter for fetal growth and the first to fall in asymmetric (placental) FGR. HC is preferred over BPD for dating when the head shape is abnormal.

Estimated fetal weight — Hadlock

EFW is calculated from a regression formula combining biometrics. The most widely used is Hadlock (commonly BPD + HC + AC + FL, or subsets), with a typical random error of roughly ±15% (~2 SD). EFW is plotted on a population or customised centile chart.

Growth definitions (by EFW or AC centile):

SGA / small — EFW or AC < 10th centile.
Fetal growth restriction (FGR) — Delphi/ISUOG consensus: EFW/AC < 3rd centile, or < 10th centile with abnormal Doppler (UA or uterine artery) or reduced interval growth.
Macrosomia / LGA — EFW > 90th centile; absolute EFW > 4000–4500 g flags shoulder-dystocia risk.

Symmetric vs asymmetric FGR

Symmetric (global)

All parameters proportionately small (HC:AC preserved).
Early insult — aneuploidy, congenital infection (TORCH), early placental disease.
Often earlier onset, worse prognosis if genetic/infective.

Asymmetric (head-sparing)

AC falls first; HC/FL relatively preserved → raised HC:AC and FL:AC ratios.
Uteroplacental insufficiency (late-onset placental FGR).
Managed by Doppler + growth surveillance (see tab 5).

Interval-growth rule: to reliably assess growth velocity, space serial biometry by at least ~2–3 weeks (measurement error otherwise swamps true change). A single small measurement is “small”; a falling centile trajectory across intervals is “growth-restricted”.

Amniotic fluid reflects fetal urine production (renal/urinary tract, placental function) and swallowing (GI/CNS). The placenta and cord assessment screens for praevia, accreta spectrum and vasa praevia — the “cannot-miss” haemorrhage risks.

Quantifying amniotic fluid — AFI vs SDP

Two methods; the single deepest (vertical) pocket (SDP/DVP) is increasingly preferred as it produces fewer false-positive diagnoses of oligohydramnios (and hence fewer inductions) with equivalent perinatal outcomes.

AFI (four-quadrant sum)

Oligohydramnios — AFI ≤ 5 cm.
Normal — AFI 5–24 cm.
Polyhydramnios — AFI > 24 cm (≥25 in some references).

SDP / deepest vertical pocket

Oligohydramnios — SDP < 2 cm.
Normal — SDP 2–8 cm.
Polyhydramnios — SDP > 8 cm.

Measure a pocket free of cord and fetal parts, perpendicular to the floor. Oligohydramnios DDx: ruptured membranes, uteroplacental insufficiency/FGR, renal agenesis / bilateral obstruction (e.g. PUV), post-dates. Polyhydramnios DDx: maternal diabetes, GI/CNS anomaly impairing swallowing (atresia, anencephaly), fetal hydrops, TTTS (recipient), idiopathic.

Placenta praevia & accreta spectrum

Praevia / low-lying

Praevia — placenta covers the internal os.
Low-lying — placental edge < 20 mm from, but not covering, the os.
Confirm with TVS (safe, more accurate than TA); many low-lying placentas “migrate” up with lower-segment growth → re-scan ~32 wk.

Accreta spectrum (PAS)

Risk: praevia plus prior caesarean/uterine surgery.
Signs: loss of the retroplacental clear (hypoechoic) zone; placental “lacunae” (Swiss-cheese lakes) with turbulent flow; myometrial thinning (< 1 mm); bladder-wall interface interruption; bridging vessels on colour Doppler.

Vasa praevia & cord

Vasa praevia — fetal vessels running through the membranes across/near the internal os, unsupported by cord or placenta. Rupture in labour is rapidly fatal to the fetus. Look for a velamentous cord insertion or a succenturiate/bilobed lobe; confirm with TVS colour + pulsed Doppler showing a fetal arterial waveform fixed over the os.

Cord insertion — central, marginal (“battledore”), or velamentous (into membranes; associated with FGR and vasa praevia).
Single umbilical artery — one artery + one vein; prompt anatomy/growth surveillance.
Nuchal cord — common, usually not clinically actionable in isolation.

Obstetric Doppler interrogates fetal and uteroplacental circulations to grade placental insufficiency, detect fetal anaemia and inform delivery timing. Report indices (PI, RI, S/D ratio) against gestation-specific reference ranges, and always angle-correct velocity measurements (MCA-PSV).

Umbilical artery (UA) — placental resistance

The workhorse in FGR surveillance. As placental villous vascular resistance rises, diastolic flow falls. Sample a free loop of cord (fetal-end shows the earliest changes).

Progression of severity:

Raised PI / RI / S/D ratio (> gestation-specific centile, typically >95th) — early placental insufficiency.
Absent end-diastolic flow (AEDF) — significant; ≥ ~70% of placental villous vasculature obliterated.
Reversed end-diastolic flow (REDF) — ominous; strongly associated with perinatal mortality/acidosis → urgent senior review, corticosteroids, expedited delivery per protocol/GA.

Middle cerebral artery (MCA)

MCA-PSV — fetal anaemia

Angle-corrected peak systolic velocity rises in anaemia (lower blood viscosity, hyperdynamic flow). Sample the proximal MCA near its origin from the circle of Willis, angle ~0°.

MCA-PSV > 1.5 MoM for gestation → predicts moderate-to-severe fetal anaemia (Mari 2000: 100% sensitivity at ~12% false-positive). Non-invasively replaced serial amniocentesis for alloimmunisation/parvovirus surveillance.

MCA-PI & brain-sparing

In hypoxia, cerebral vasodilatation lowers MCA-PI (“brain-sparing”) — a compensatory redistribution.

Cerebroplacental ratio (CPR) = MCA-PI ÷ UA-PI. A low CPR (< ~1.0 / < 5th centile) flags redistribution and adverse outcome, even when individual UA/MCA values are borderline — useful in late-onset FGR and the SGA-vs-FGR distinction.

Uterine artery & ductus venosus

Uterine artery

Reflects maternal-side placentation.
Raised PI and/or an early-diastolic “notch” (esp. bilateral, persisting beyond ~24 wk) → high-resistance placentation.
Predicts pre-eclampsia and early-onset FGR; used in first-trimester PE screening algorithms.

Ductus venosus (DV)

Reflects fetal cardiac (right-heart) function/acidaemia.
Normal: forward flow throughout, positive a-wave.
Absent or reversed a-wave → late, ominous sign in FGR (myocardial compromise) — a key delivery-timing trigger in early FGR (e.g. TRUFFLE).
Also a first-trimester aneuploidy marker.

Putting the Dopplers together — FGR surveillance logic

No single index acts alone. The pattern and gestation guide monitoring intensity and delivery timing.

Finding	Meaning	Typical response
Raised UA-PI, +ve EDF	Early placental insufficiency	Increase surveillance (e.g. 1–2 weekly), serial growth
Low CPR / raised UtA-PI	Redistribution / high-resistance placentation (late-onset FGR)	Closer monitoring; informs term/late-preterm delivery timing
Absent EDF (AEDF)	Advanced placental compromise	Admit/steroids, intensive monitoring, plan delivery per GA
Reversed EDF (REDF) / abnormal DV a-wave	Pre-terminal fetal compromise	Urgent MFM review; expedited delivery per protocol/GA
MCA-PSV > 1.5 MoM	Fetal anaemia (distinct pathway from FGR)	Refer for IUT / cause workup (alloimmunisation, parvovirus)

Early-onset (< 32 wk) FGR is monitored primarily with UA + DV (TRUFFLE); late-onset FGR relies more on CPR/UtA and MCA, because UA can stay normal despite significant placental disease.

A high-yield reference table of key structural anomalies by system, with the sonographic signs that clinch (or first raise) the diagnosis. This is a pattern-recognition aide — full workup, karyotype and MDT counselling always follow.

Key anomalies & their sonographic signs

System	Anomaly	Sonographic sign(s)	Associations / notes
CNS	Anencephaly / acrania	Absent cranial vault above orbits; “frog-eye” face; exposed brain (exencephaly) earlier	Detectable in first trimester; ↑ AFP; poly (impaired swallowing)
	Open spina bifida	Lemon sign (scalloped frontal bones) + banana sign (cerebellum wrapped, effaced cisterna magna) — the Chiari II cranial signs; defect on spine views	Ventriculomegaly; ↑ AFP; folate-related
	Ventriculomegaly	Atrium ≥ 10 mm (mild 10–12, severe > 15 mm)	Aetiologically broad — infection, aneuploidy, obstruction, haemorrhage
	Dandy-Walker / posterior fossa	Enlarged cisterna magna > 10 mm; vermian defect; 4th ventricle communicating with cisterna magna	Aneuploidy, syndromes
Thorax	Congenital diaphragmatic hernia (CDH)	Abdominal contents (stomach/bowel) in the chest; mediastinal shift; heart displaced (usually left CDH → heart pushed right)	Lung-to-head ratio (LHR) prognosticates; ~left-sided
Thorax	CPAM / lung lesion	Cystic or echogenic lung mass; possible mediastinal shift, hydrops	May regress; feeding vessel (sequestration)
Abdo wall	Gastroschisis	Free-floating bowel loops, no covering membrane; defect right of a normal cord insertion	Usually isolated; ↑↑ AFP; growth surveillance
Abdo wall	Omphalocele	Midline, membrane-covered sac at cord insertion (contains bowel ± liver)	Strong aneuploidy/syndrome (T18, Beckwith-Wiedemann) & cardiac link
GU	Bilateral renal agenesis	Absent kidneys + empty bladder + anhydramnios; adrenals “lying down”; absent renal arteries on colour	Potter sequence — lethal (pulmonary hypoplasia)
	Posterior urethral valves (PUV)	Dilated thick-walled bladder + “keyhole” posterior urethra; bilateral hydroureteronephrosis; oligo/anhydramnios	Male; renal dysplasia; renal cortical cysts poor sign
	Multicystic dysplastic kidney	Non-communicating cysts of varying size, no normal parenchyma/no reniform shape	Usually unilateral; check contralateral kidney
Skeletal	Skeletal dysplasia	Severe long-bone shortening (FL < 5th centile / >3 SD below mean); bowing/fractures; small thorax (thoraco-abdominal ratio) → lethality; hypomineralisation	Lethal forms: thanatophoric, achondrogenesis, OI type II
Hydrops	Hydrops fetalis	Fluid in ≥ 2 compartments: ascites, pleural/pericardial effusion, skin oedema (>5 mm), placentomegaly, polyhydramnios	Immune (alloimmunisation → check MCA-PSV) vs non-immune (aneuploidy, cardiac, infection, TTTS, chest mass)
Cardiac	Structural CHD	Abnormal 4-chamber view (asymmetry, single ventricle, AVSD), abnormal outflows / 3-vessel-trachea, abnormal axis	Commonest major anomaly group; check for aneuploidy, situs, extracardiac anomalies

Cannot-miss red flags: anhydramnios (renal agenesis/PUV), hydrops, REDF in the umbilical artery, absent/reversed DV a-wave, vasa praevia over the os, a lethal skeletal dysplasia with a hypoplastic thorax, and any anomaly clustering suggestive of aneuploidy — each warrants urgent senior/MFM referral.

ProGo deeper with High Yield Pro

The tabs above are the free clinical core. Pro unlocks the interactive tools and worked cases that turn thresholds into reporting fluency:

Interactive gestational-age & EFW calculator (Robinson CRL, Hadlock) with centile plotting

MCA-PSV & UA/CPR MoM calculator with gestation-indexed reference curves

Guided viability decision tree (SRU criteria) with “diagnostic vs suspicious” logic

Structured report templates: dating, anatomy survey, growth & Doppler

40+ annotated anomaly cases with cine clips & measurement callipers

Amniotic-fluid & placenta praevia/PAS reporting checklists

What Pro will include

Free content stays free. Pro supports independent, guideline-referenced ultrasound education.

References & further reading

Doubilet PM, Benson CB, Bourne T, Blaivas M, et al. Diagnostic Criteria for Nonviable Pregnancy Early in the First Trimester. N Engl J Med 2013;369:1443–51 (SRU multispecialty panel). nejm.org
Society of Radiologists in Ultrasound (SRU) 2013 consensus — early first-trimester diagnosis of miscarriage and exclusion of a viable IUP.
ISUOG Practice Guidelines: performance of first-trimester fetal ultrasound scan. Ultrasound Obstet Gynecol 2013;41:102–113. isuog.org
ISUOG Practice Guidelines (updated): performance of the routine mid-trimester fetal ultrasound scan. Ultrasound Obstet Gynecol 2022. isuog.org
ISUOG Practice Guidelines: use of Doppler ultrasonography in obstetrics. Ultrasound Obstet Gynecol 2013;41:233–239. Wiley
ISUOG Practice Guidelines: diagnosis and management of small-for-gestational-age fetus and fetal growth restriction. Ultrasound Obstet Gynecol 2020;56:298–312.
AIUM–ACR–ACOG–SMFM–SRU Practice Parameter for the Performance of Standard Diagnostic Obstetric Ultrasound Examinations. J Ultrasound Med 2018. aium.org
ACOG Committee Opinion 700 (reaffirmed): Methods for Estimating the Due Date. First-trimester CRL is the most accurate dating tool.
Robinson HP, Fleming JEE. A critical evaluation of sonar crown–rump length measurements. Br J Obstet Gynaecol 1975;82:702–710.
Hadlock FP, Harrist RB, Sharman RS, et al. Estimation of fetal weight with the use of head, body, and femur measurements. Am J Obstet Gynecol 1985;151:333–337.
Nicolaides KH. Nuchal translucency and other first-trimester sonographic markers of chromosomal abnormalities. Am J Obstet Gynecol 2004;191:45–67 (Fetal Medicine Foundation).
Fetal Medicine Foundation — 11–13⁺⁶ week scan & NT standards. fetalmedicine.org
Mari G, et al. Noninvasive diagnosis by Doppler ultrasonography of fetal anemia due to maternal red-cell alloimmunization (MCA-PSV >1.5 MoM). N Engl J Med 2000;342:9–14.
Mari G. Middle cerebral artery peak systolic velocity for the diagnosis of fetal anemia: the untold story. Ultrasound Obstet Gynecol 2005;25:323–330. Wiley
Martinez-Portilla RJ, et al. Performance of fetal MCA-PSV for prediction of anemia: systematic review & meta-analysis. Ultrasound Obstet Gynecol 2019. Wiley
Baschat AA. Cerebroplacental Doppler ratio and adverse perinatal outcome; late-onset FGR surveillance.
Lees CC, et al. TRUFFLE study — ductus venosus and computerised CTG monitoring for timing of delivery in early FGR. Lancet 2015;385:2162–72.
Gordijn SJ, et al. Consensus definition of fetal growth restriction: a Delphi procedure. Ultrasound Obstet Gynecol 2016;48:333–339.
Nabhan AF, Abdelmoula YA. Amniotic fluid index versus single deepest vertical pocket as a screening test (Cochrane). Oligohydramnios AFI ≤5 cm / SDP <2 cm; polyhydramnios AFI >24 cm / SDP >8 cm.
Kehl S, et al. SAFE trial: single deepest vertical pocket vs AFI. Ultrasound Obstet Gynecol 2016. PubMed
RCOG Green-top Guideline No. 27a: Placenta Praevia and Placenta Accreta — diagnosis and management. 2018.
Jauniaux E, et al. FIGO consensus guidelines on placenta accreta spectrum disorders. Int J Gynaecol Obstet 2018.
Ruiter L, et al. Systematic review of accuracy of ultrasound in the diagnosis of vasa previa. Ultrasound Obstet Gynecol 2015.
Nicolaides KH, et al. Multicenter study of first-trimester screening for trisomy 21 (combined test: NT, free β-hCG, PAPP-A).
SMFM Consult Series — soft markers for aneuploidy in the NIPT era.
Radiopaedia.org — obstetric ultrasound articles (early pregnancy failure criteria, lemon/banana sign, gastroschisis vs omphalocele, MCA-PSV, placenta accreta). radiopaedia.org
Reproductive Health Access Project — Using ultrasound to diagnose early pregnancy loss (SRU criteria summary). reproductiveaccess.org

High Yield Imaging is an independent educational resource. Thresholds cited reflect published guidelines current at the time of writing; verify against your unit’s adopted versions before clinical use.

Obstetric Ultrasound