Obstetric Ultrasound Basics
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Direct answer
Between 8 weeks and 13 weeks plus 6 days, the crown-rump length is the most accurate way to date a pregnancy (within about 5-7 days) and anchors every later decision; before that, gestational sac and yolk sac measurements apply, and after 14 weeks dating shifts to a composite of biparietal diameter, head circumference, abdominal circumference and femur length, accurate only to about 7-10 days in the second trimester and two to three weeks by the third. The first-trimester scan also carries the nuchal translucency screen at 11 to 13 weeks plus 6 days (raised above 3-3.5 mm suggests aneuploidy, and the nasal bone is assessed alongside), the anomaly scan is timed at 18-22 weeks (Indian practice commonly 18-20 weeks), and the third trimester contributes growth scans with estimated fetal weight by Hadlock's formula, amniotic fluid assessment (index or deepest pocket) and umbilical artery Doppler. Ultrasound uses non-ionising sound waves — no radiation burden at any stage of pregnancy, which is why liberal use is safe, but scans are performed only when clinically indicated.
What you must remember
- Dating ladder: gestational sac then yolk sac (about 5 weeks), crown-rump length 8 to 13+6 weeks — the gold standard (within roughly a week); beyond 14 weeks, the quad-parameters composite (BPD, HC, AC, FL); a first-trimester-established due date is never changed by later scans.
- First-trimester milestones: cardiac activity by about 6 weeks (discriminatory zone: maternal beta-hCG around 1500-2000 IU/L should show an intrauterine sac on transvaginal scan — the ectopic-pregnancy decision number); nuchal translucency at 11-13+6 weeks with nasal bone, raising aneuploidy risk above 3-3.5 mm.
- Anomaly scan (18-22 weeks): systematic head-to-toe fetal survey — intracranial structures, spine, four-chamber cardiac view and outflow tracts, abdominal wall, stomach bubble, kidneys and bladder, limbs, cord insertion and placental position; detects the majority of major structural anomalies.
- Placenta praevia logic: a low-lying placenta at 20 weeks is common and usually migrates (lower-segment formation) — rescan at 32-34 weeks before labelling praevia; a praevia with prior caesarean raises accreta suspicion needing colour Doppler.
- Growth and weight: estimated fetal weight uses Hadlock's combination of HC, AC and FL; serial growth (interval every 2-3 weeks) beats single estimates; abdominal circumference is the earliest growth-restriction clue and the last to recover.
- Fluid measures: amniotic fluid index (four-quadrant deepest pockets; normal roughly 5-24 cm) or single deepest vertical pocket (normal 2-8 cm).
- Doppler assignments: umbilical artery for growth-restriction surveillance (raised S/D ratio, then absent and reversed end-diastolic flow), middle cerebral artery peak systolic velocity for fetal anaemia, uterine artery notching at 20-24 weeks as a pre-eclampsia screen in selected practice.
- Route: transvaginal wins in early pregnancy (location, viability, cervical length under 25 mm) while transabdominal serves the second and third trimesters and the obese abdominal wall.
- Cervical length: measured on transvaginal scan in threatened preterm birth and history-indicated screening — under 25 mm at 20-24 weeks marks high risk.
One scan per trimester, used well
Sequence the standard Indian antenatal schedule. First trimester (8-13+6 weeks): confirm intrauterine site and viability by crown-rump length, count fetuses and — the step candidates forget — establish chorionicity in twins now, because the lambda-versus-T sign is unreadable later. Nuchal translucency rides the 11-13+6 window with the nasal bone and, where chosen, first-trimester serum screening (PAPP-A, free beta-hCG).
Second trimester (18-22 weeks): the anomaly survey, fetal echocardiography where indicated (diabetic mothers, prior congenital heart disease), placental location with a praevia tagged for a 32-34 week rescan, and cervical length where risk warrants. Third trimester (28 weeks onward, then 32 and 36): growth scans with estimated fetal weight and abdominal circumference trend, fluid measurement, presentation check before term (breech at 36 weeks triggers external cephalic version counselling), and Dopplers restricted to indicated cases — umbilical artery for the small fetus, not every pregnancy. The discipline the exam rewards is matching question to window: dating accuracy decays after 13+6 weeks, NT outside 11-13+6 is meaningless, and an anomaly scan before 18 weeks misses late-manifesting cardiac and renal findings.
What the exam repeats
Four numbers are asked as recall: 13+6 (end of CRL/NT dating), 3-3.5 mm (NT cut-off), 18-22 weeks (anomaly scan), 25 mm (cervical length) — with 1500-2000 IU/L beta-hCG as the ectopic-algorithm hinge (no sac at that level means ectopic or early failing pregnancy until proven otherwise). The recurring trap is re-dating a pregnancy by a third-trimester scan; established first-trimester dates stand. And the safety one-liner: obstetric ultrasound is non-ionising — the answer whenever radiation fear appears in a stem.
Frequently asked questions
What is the most accurate gestational age parameter and its window?
The crown-rump length between 8 and 13 weeks 6 days, accurate to within about a week — later scans only confirm, never override, this dating.
At what gestation is the anomaly scan performed?
Between 18 and 22 weeks (commonly 18-20 weeks in Indian practice), surveying fetal structures from intracranium to limbs with cardiac outflow views.
Why is a low-lying placenta at 20 weeks rescanned at 32-34 weeks?
Because lower-segment development effectively lifts most low-lying placentas; praevia is confirmed only on the repeat scan before planning delivery.
What beta-hCG level should show an intrauterine sac on transvaginal scan?
Around 1500-2000 IU/L — the discriminatory zone; absence of a sac at this level demands evaluation for ectopic pregnancy.
How is amniotic fluid quantified on ultrasound?
By the amniotic fluid index (sum of deepest vertical pockets in four quadrants, normal roughly 5-24 cm) or the single deepest vertical pocket (normal 2-8 cm).