Role of usg in pregnancy

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Role of Ultrasonography (USG) in Pregnancy

Ultrasound is the cornerstone of obstetric imaging. It uses no ionizing radiation and provides real-time information about the fetus, placenta, and uterus throughout all three trimesters.

Types of Obstetric Ultrasound Examinations

According to the CPT Code framework described in Creasy & Resnik's Maternal-Fetal Medicine, the major categories are:

CPT Code	Examination Type	When Used
76801	First trimester (< 14 weeks), transabdominal	Routine first-trimester screening
76805	Standard midtrimester anatomy scan	18-22 weeks, most pregnancies
76811	Detailed maternal and fetal evaluation (Level II)	High-risk, suspected anomaly
76816	Follow-up/limited scan	Interval growth check

Roles by Trimester

First Trimester (< 14 weeks)

Confirm intrauterine pregnancy - differentiates IUP from ectopic pregnancy, especially in women presenting with bleeding or pain
Gestational age dating - crown-rump length (CRL) is the most accurate dating parameter; CRL between 45-84 mm is the ideal window
Fetal viability - cardiac activity, yolk sac, fetal pole
Multiple pregnancy - determines chorionicity and amnionicity, which drives all subsequent management
Nuchal translucency (NT) measurement (11-14 weeks):
- NT is the subcutaneous fluid behind the fetal neck, measured in a strict midline sagittal view
- NT combined with maternal age, PAPP-A, and hCG forms the "first trimester combined screen" for trisomy 21 (Down syndrome), trisomy 18, and trisomy 13
- Detection rate for Down syndrome: 77-82% at a 5-8.3% false-positive rate
- NT must be measured when CRL is 45-84 mm; transabdominal route succeeds in ~95% of cases
Uterine anatomy - fibroids, adnexal masses, bicornuate/septate uterus

Second Trimester (14-28 weeks) - The Anatomy Scan

The standard midtrimester anatomy scan (18-22 weeks) is the most clinically important ultrasound of pregnancy. It evaluates:

Fetal head and brain:

Cerebral ventricles (ventriculomegaly)
Choroid plexus
Posterior fossa (Dandy-Walker, cerebellar vermis)
Midline falx, cavum septi pellucidi
Neural tube defects (anencephaly, spina bifida)

Face:

Profile, orbits, lips (cleft lip)
Nasal bone - a soft marker for Down syndrome

Chest:

Cardiac activity, four-chamber view
Outflow tracts (aorta, pulmonary artery)
Lung echogenicity

Abdomen:

Stomach, bowel, abdominal wall (gastroschisis, omphalocele)
Kidneys and bladder (renal agenesis, pelviectasis)
Umbilical cord insertion

Spine:

Longitudinal and transverse views at cervical, thoracic, lumbar, and sacral levels

Limbs:

All four long bones (femur, humerus, tibia, fibula, radius, ulna)
Hands, feet, posture

Placenta and fluid:

Placental location (low-lying placenta, placenta previa)
Amniotic fluid index (AFI) or maximum vertical pocket (MVP)
Umbilical cord vessels (two arteries + one vein)

Second-trimester soft markers for aneuploidy: Echogenic intracardiac focus, choroid plexus cysts, short femur/humerus, renal pelviectasis, echogenic bowel, single umbilical artery - each adjusts risk for chromosomal abnormalities.

Third Trimester (> 28 weeks)

Fetal growth surveillance - biometry (BPD, HC, AC, FL) with estimated fetal weight (EFW); detects intrauterine growth restriction (IUGR) and macrosomia
Amniotic fluid assessment - oligohydramnios (AFI < 5 cm) or polyhydramnios (AFI > 25 cm)
Placental assessment - location, maturity, abruption
Fetal presentation - cephalic, breech, transverse
Biophysical profile (BPP) - fetal breathing, body movements, tone, amniotic fluid volume; each 2 points, max 10; score < 6 suggests fetal compromise

Doppler Ultrasound

Doppler USG assesses blood flow waveforms in fetal and uteroplacental vessels:

Umbilical artery (UA) Doppler - the primary surveillance tool for IUGR. As placental resistance rises, end-diastolic flow progressively decreases, then becomes absent (AEDF), then reversed (REDF). AEDF/REDF indicates high risk of perinatal mortality and is an indication for delivery timing discussions.
Middle cerebral artery (MCA) Doppler - peak systolic velocity (PSV) > 1.5 MoM indicates fetal anemia (from Rh isoimmunization, parvovirus)
Ductus venosus (DV) - absent or reversed a-wave signals severe cardiac compromise
Uterine artery Doppler - high resistance with notching at 20-24 weeks predicts pre-eclampsia and placental insufficiency

Emergency / Special Indications

In the emergency setting, pelvic USG is used to:

Confirm IUP or diagnose ectopic pregnancy in any woman with first-trimester bleeding or pain
Identify free fluid in suspected ectopic rupture
Evaluate subchorionic hematoma in threatened abortion

3D/4D Ultrasound

3D ultrasound reconstructs surface and volumetric images. Limitations include dependence on amniotic fluid for acoustic windows and fetal position. It is most useful for evaluating facial anomalies (cleft palate, micrognathia), limb defects, and neural tube defects where standard 2D planes may be insufficient.

Cervical Length Measurement

Transvaginal cervical length (TVCL) at 18-24 weeks is used to predict preterm birth risk. A cervical length < 25 mm is the standard threshold for increased risk in singleton pregnancies with prior preterm birth; management with vaginal progesterone or cerclage follows.

Safety

USG uses non-ionizing sound waves and has an excellent safety profile. No confirmed adverse fetal effects have been documented with diagnostic levels, making it the preferred imaging modality throughout pregnancy.

Key sources: Creasy & Resnik's Maternal-Fetal Medicine: Principles and Practice; Rosen's Emergency Medicine

Make a presentation summarizing USG findings by trimester

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Nth scan in first trimester

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