Diagnosis of Pregnancy

Ultrasonography

TVS (Transvaginal Sonography): 3–5 MHz (preferred in 1st trimester)

TAS (Transabdominal Sonography): 5–10 MHz

Critical hCG Levels for Visualization

ModalityhCG ThresholdTVS1500 mIU/mLTAS2500–6000 mIU/mL

Earliest Structures Seen on Ultrasound

StructureTVSTASGestational sac4 weeks 5 days5 weeks 5 daysYolk sac5 weeks6 weeksFoetal pole6 weeks7 weeksFoetal cardiac activity6 weeks7 weeks

First sign of intrauterine gestation: Double bubble sign

1st Trimester Scan

• Purpose: Dating scan / Viability scan
• CRL → EDD ±5–7 days
• 2nd Trimester → BPD ±2 weeks
• 3rd Trimester → FL ±3 weeks

Mean Sac Diameter (MSD)

MSD = (Length + Height + Width) / 3

Gestational Age = MSD + 30 days (in mm)

MSD > 25 mm, CRL > 7 mm, no cardiac activity → Missed Abortion

Nuchal Translucency (NT) Scan

• Done at 11–13⁶ weeks
• CRL: 45–84 mm
• Measures thickness behind fetal spine

NT Interpretation:

• < 3 mm: Normal
• ≥ 3 mm: Abnormal (Down’s > Turner’s > Cardiac anomalies)

Scan done in midsagittal section with neutral head position and visible nasal bone.

Anomaly Scan / Level 2 Scan / TIFA

Targeted Imaging for Foetal Anomalies

When: 18–22 weeks

3rd Trimester

Interval scan / Growth scan

Checks fetal nutrition via Abdominal Circumference (AC)

Sequence of Imaging

Dating → NT → TIFA → Interval