Obstetric Ultrasound

Ultrasound is the single most important imaging modality in obstetrics — radiation-free, real-time, and the gold standard for dating, anomaly detection, placental localisation and foetal surveillance. NEET PG loves the hard numbers: dating parameters, cut-off values, placenta praevia grading, biophysical profile scoring, and the classic signs of ectopic and molar pregnancy.

First principles & safety

Obstetric USG uses 2–7 MHz transducers. Transabdominal (TAS) gives a panoramic view; transvaginal (TVS) gives higher resolution and is the workhorse of the first trimester. The ALARA principle (As Low As Reasonably Achievable) governs exposure. Two parameters define bio-effects:

• Thermal Index (TI): estimated tissue temperature rise.
• Mechanical Index (MI): risk of cavitation. Keep MI < 1.9 for diagnostic scans.

High-yield: Pulsed/colour Doppler is avoided in the first trimester (especially routine embryonic scanning) because the higher energy deposition raises the thermal index over a small, sensitive embryo. Use it only when clinically indicated.

When do structures appear? (TVS milestones)

A reliable mental timeline answers many NEET PG single-best-answer questions.

Structure	Earliest TVS appearance	Correlating β-hCG
Gestational sac	4.5–5 weeks	~1000–2000 mIU/mL (discriminatory zone)
Yolk sac	5–5.5 weeks (sac ≥ 8 mm)	—
Foetal pole / cardiac activity	6 weeks (sac ≥ 16–25 mm)	—
Cardiac activity always seen	CRL ≥ 7 mm	—

High-yield: The discriminatory zone is the β-hCG level above which an intrauterine gestational sac should be visible. TVS ≈ 1500–2000 mIU/mL; TAS ≈ 6000 mIU/mL. A β-hCG above this with an empty uterus suggests ectopic pregnancy until proven otherwise.

High-yield: No cardiac activity with CRL ≥ 7 mm, or no embryo with mean sac diameter ≥ 25 mm = definitive early pregnancy failure (missed abortion) by the Society of Radiologists in Ultrasound criteria.

The double decidual sac sign (two concentric echogenic rings) and an intradecidual sign favour a true intrauterine pregnancy over the pseudo-sac of an ectopic.

Gestational age estimation

Dating accuracy decreases as pregnancy advances because biological variation widens. Earlier is more accurate.

Parameter	Best window	Accuracy (±)
Mean sac diameter (MSD)	5–6 wk	~1 week
Crown–rump length (CRL)	7–13 wk	±5–7 days (most accurate of all)
BPD, HC, FL, AC composite	14–20 wk	±7–10 days
Composite (3rd trimester)	> 24 wk	±3 weeks

High-yield: CRL in the first trimester is the most accurate single parameter for dating an entire pregnancy. If LMP-based and CRL-based dates differ by > 5–7 days, the scan date is used.

The four biometric pillars of second/third-trimester dating and growth:

1. BPD (biparietal diameter) — outer edge of near parietal bone to inner edge of far bone, measured at the level of thalami and cavum septi pellucidi.
2. HC (head circumference) — same plane; least affected by head shape (dolichocephaly/brachycephaly), so more reliable than BPD.
3. AC (abdominal circumference) — at the level of the stomach bubble and the "J"-shaped left portal vein in the liver; the most sensitive single index of foetal growth and IUGR.
4. FL (femur length) — full ossified diaphysis.

Flow of growth assessment: measure BPD, HC, AC, FL → compute estimated foetal weight (EFW) (Hadlock formula) → plot on percentile chart → < 10th centile = small for gestational age; confirm IUGR with Dopplers.

High-yield: A low HC : AC ratio with relatively preserved head size = asymmetrical IUGR (placental insufficiency, head-sparing). A proportionate fall in all parameters = symmetrical IUGR (early insult: aneuploidy, TORCH, established by first trimester).

First-trimester aneuploidy screening

The 11–13⁺⁶ week scan is a fixed NEET PG favourite.

• Nuchal translucency (NT): the sonolucent space at the back of the foetal neck. Measured with CRL 45–84 mm. NT > 3.0–3.5 mm is abnormal and associated with trisomy 21, 18, 13, Turner syndrome (45,X — cystic hygroma) and cardiac defects.
• Nasal bone: absent/hypoplastic nasal bone increases the risk of trisomy 21.
• Combined test = NT + maternal age + serum free β-hCG (↑ in Down) + PAPP-A (↓ in Down).

High-yield: In trisomy 21, first-trimester serum shows ↑ free β-hCG and ↓ PAPP-A with ↑ NT. Cystic hygroma with grossly increased NT points to Turner syndrome (45,X).

The anomaly scan (TIFFA / 18–20 weeks)

The mid-trimester Targeted Imaging for Foetal Anomalies (TIFFA) scan, ideally at 18–22 weeks, systematically screens organ systems. Classic NEET PG soft markers and signs:

Sign / marker	Association
Lemon sign (scalloped frontal bones)	Open spina bifida / Chiari II
Banana sign (cerebellum wrapped around brainstem)	Open spina bifida
Strawberry-shaped head	Trisomy 18
Double bubble sign	Duodenal atresia (assoc. trisomy 21)
Snowstorm / honeycomb uterus	Hydatidiform mole
Echogenic intracardiac focus, echogenic bowel, pyelectasis	Soft markers for aneuploidy
Dangling choroid plexus, colpocephaly	Ventriculomegaly / hydrocephalus

Other must-know findings: anencephaly (absent calvarium — detectable as early as the first trimester), holoprosencephaly (monoventricle), cystic hygroma (nuchal, septated), and omphalocele vs gastroschisis (omphalocele is midline, membrane-covered, cord inserts on the sac; gastroschisis is right-of-cord, no covering membrane).

Placental localisation — placenta praevia

Placenta praevia = placenta implanted over or near the internal cervical os. TVS is safe and more accurate than TAS for assessing the lower segment. The classic four-grade system:

Grade	Description
I (low-lying)	Encroaches lower segment, does not reach os
II (marginal)	Reaches edge of internal os
III (incomplete/partial central)	Covers os partially (asymmetrically)
IV (complete/central)	Completely covers the internal os

High-yield: Placenta praevia presents as painless, causeless, recurrent, apparently external bleeding in the 3rd trimester. Diagnostic investigation of choice = TVS. Never do a per-vaginal/per-speculum digital exam before excluding praevia ("never PV until you exclude praevia").

A low-lying placenta in the second trimester often "migrates" upward as the lower segment forms; reassess at ~32 weeks. Placenta accreta spectrum (accreta/increta/percreta) is suggested by loss of the retroplacental clear zone, placental lacunae ("Swiss cheese"), and bladder-wall interface disruption — strongly linked to prior caesarean + anterior praevia.

Abruptio placentae

Premature separation of a normally situated placenta. Clinically: painful bleeding, tense/woody uterus, foetal distress. USG is insensitive for abruption (a normal scan does NOT exclude it) because acute haemorrhage is iso-/hyperechoic and easily missed.

High-yield: Abruptio placentae is primarily a CLINICAL diagnosis — ultrasound is used mainly to exclude praevia, not to confirm abruption. A retroplacental haematoma when seen is diagnostic but only present in a minority.

Feature	Placenta praevia	Abruptio placentae
Bleeding	Painless, external, recurrent	Painful, may be concealed
Uterus	Soft, relaxed	Tense, tender, "woody"
Foetal distress	Late/uncommon	Early/common
USG	Diagnostic (TVS)	Often normal; clinical Dx
Shock	Proportionate to visible loss	Out of proportion (concealed)

Doppler in foetal surveillance

Doppler interrogates resistance in feto-placental circulation; rising resistance reflects placental insufficiency. Indices: S/D ratio, Resistance Index (RI), Pulsatility Index (PI).

Sequence of Doppler deterioration in IUGR (memorise the order):

Umbilical artery (↑ resistance → absent → reversed EDF) → Middle cerebral artery (↓ PI = brain-sparing) → Ductus venosus (absent/reversed a-wave) → Umbilical vein pulsations (pre-terminal).

Vessel	Abnormal finding	Meaning
Umbilical artery	Absent or reversed end-diastolic flow (AREDF)	Severe placental insufficiency; ominous
MCA	Low PI / cerebroplacental ratio < 1	"Brain-sparing" redistribution
Ductus venosus	Absent/reversed a-wave	Cardiac decompensation — delivery indicated
MCA peak systolic velocity (PSV)	↑ MCA-PSV > 1.5 MoM	Foetal anaemia (Rh isoimmunisation)

High-yield: Reversed end-diastolic flow in the umbilical artery is an ominous sign demanding urgent delivery. MCA peak systolic velocity > 1.5 MoM is the non-invasive test of choice for foetal anaemia, having replaced amniocentesis/Liley charts.

Biophysical profile (BPP)

The Manning biophysical profile combines a 30-minute real-time USG with a non-stress test (NST). Five components, each scored 0 or 2 (never 1):

1. Foetal breathing movements
2. Gross body movement
3. Foetal tone
4. Amniotic fluid (single deepest pocket > 2 cm)
5. Reactive NST (cardiotocography)

Mnemonic — "Test the BABY MAN": Breathing, Amniotic fluid, Body movement, Y (tone), MANning's NST. (Or simply: Breathing, Movement, Tone, Fluid, NST.)

Score	Interpretation	Action
8–10	Normal	Routine care
6	Equivocal	Repeat in 24 h / deliver if term
≤ 4	Abnormal (foetal asphyxia)	Deliver

High-yield: The first BPP parameter lost in hypoxia is the NST/reactivity, then breathing; tone is the last to disappear (it develops earliest in the foetus — "first to develop, last to go"). Oligohydramnios (deepest pocket < 2 cm) is the most ominous chronic marker.

Amniotic fluid is quantified by the Amniotic Fluid Index (AFI) — sum of deepest pockets in four quadrants. Oligohydramnios AFI < 5 cm; polyhydramnios AFI > 24–25 cm (or single deepest pocket > 8 cm).

Ectopic pregnancy

USG is central. TVS is the investigation of choice.

Approach: positive UPT → β-hCG above discriminatory zone (~1500–2000) → TVS shows empty uterus → search adnexa.

Classic findings:

• Empty uterus with positive pregnancy test.
• "Bagel" / "tubal ring" / "blob" sign — an adnexal echogenic ring separate from the ovary (most specific).
• Pseudogestational sac — a centrally located fluid collection in the uterine cavity (vs the eccentric true sac).
• "Ring of fire" on colour Doppler — peripheral hypervascularity around the ectopic (also seen with corpus luteum).
• Free fluid in the pouch of Douglas — echogenic free fluid suggests haemoperitoneum/rupture.

High-yield: In a suspected ectopic, a β-hCG that fails to rise by ≥ 53% over 48 hours (abnormal doubling) plus an empty uterus strongly supports ectopic. The most common site is the ampulla of the fallopian tube.

Molar pregnancy (gestational trophoblastic disease)

• Complete mole: "snowstorm" / "bunch of grapes" / honeycomb appearance — multiple anechoic vesicles filling the uterus, no foetal parts, often bilateral theca lutein cysts and a uterus large for dates. β-hCG markedly elevated (often > 100,000).
• Partial mole: focal cystic placental changes with a (usually non-viable, triploid) foetus or foetal parts.

High-yield: The snowstorm appearance with grossly elevated β-hCG and theca lutein cysts = complete hydatidiform mole. Karyotype: complete mole 46,XX (paternal, diploid); partial mole triploid (69,XXY).

Recently asked / exam angle

• Most accurate USG parameter for gestational age → CRL (first trimester).
• Best parameter to detect IUGR / earliest to be affected → abdominal circumference (AC).
• Investigation of choice for placenta praevia → transvaginal ultrasound.
• Lemon and banana signs → open neural tube defect / Chiari II.
• Most ominous umbilical artery Doppler finding → reversed end-diastolic flow.
• Non-invasive test for foetal anaemia → MCA peak systolic velocity > 1.5 MoM.
• First BPP component affected in hypoxia → NST reactivity; last → tone.
• Discriminatory zone (TVS) → β-hCG ≈ 1500–2000 mIU/mL.
• Snowstorm / honeycomb uterus + theca lutein cysts → complete hydatidiform mole.
• Double bubble sign → duodenal atresia (assoc. Down syndrome).
• Distinguishing omphalocele vs gastroschisis and abruption (clinical) vs praevia (USG) appear repeatedly in image-based and one-liner stems.

Rapid revision

1. CRL = most accurate gestational age estimator (7–13 weeks, ±5–7 days).
2. AC = most sensitive index of foetal growth/IUGR; measured at stomach + left portal vein "J".
3. HC is more reliable than BPD because it is independent of head shape.
4. Cardiac activity is expected when CRL ≥ 7 mm; missed abortion if absent.
5. MSD ≥ 25 mm with no embryo = definitive early pregnancy failure.
6. NT > 3.5 mm + ↑β-hCG + ↓PAPP-A = trisomy 21 risk; cystic hygroma = Turner (45,X).
7. Lemon + banana = open spina bifida; strawberry skull = trisomy 18.
8. Placenta praevia — painless recurrent bleed; TVS is safe and diagnostic; no PV exam.
9. Abruptio placentae is a clinical diagnosis; USG is often normal.
10. Reversed umbilical artery EDF = ominous; MCA-PSV > 1.5 MoM = foetal anaemia.
11. BPP: 5 components × 2; score ≤ 4 = deliver; tone lost last, NST lost first.
12. Ectopic = empty uterus + bagel/ring-of-fire sign + free fluid; complete mole = snowstorm + huge β-hCG + theca lutein cysts.