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Ultrasound in pregnancy. Prepare a 6 mark answer for this topic. Mbbs answer. Use dc dutta book of obstetrics

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I now have enough information to write a comprehensive, structured MBBS 6-mark answer on Ultrasound in Pregnancy in the style of DC Dutta's Textbook of Obstetrics. Here is the answer:

Ultrasound in Pregnancy

(DC Dutta's Textbook of Obstetrics — MBBS 6-mark format)

Definition

Ultrasound (USG) in obstetrics uses high-frequency sound waves (3.5–7.5 MHz) to produce real-time images of the fetus, placenta, amniotic fluid, and uterus, aiding diagnosis and management throughout pregnancy.

Types of Ultrasound Used

TypeProbeUse
Transabdominal (TAS)3.5 MHzAfter 10–12 weeks
Transvaginal (TVS)5–7.5 MHzEarly pregnancy, cervix
Doppler (pulsed/colour)VariableFetal blood flow
3D / 4DVariableStructural anomalies

Indications / Uses (Trimester-wise)

🔹 First Trimester (< 14 weeks)

  1. Confirmation of intrauterine pregnancy — gestational sac visible by TVS at 4.5 weeks
  2. Fetal viability — cardiac activity seen from 6 weeks (CRL ≥ 7 mm = FHR expected)
  3. Gestational age / dating — Crown-Rump Length (CRL) is most accurate (± 5–7 days)
  4. Number of fetuses — diagnosis of twins; chorionicity and amnionicity
  5. Diagnosis of ectopic pregnancy
  6. Nuchal Translucency (NT) measurement — at 11–13+6 weeks; NT ≥ 3.5 mm → risk of Down syndrome / other aneuploidies
  7. Chorionic villus sampling (CVS) guidance
  8. Diagnosis of blighted ovum, missed abortion, molar pregnancy

🔹 Second Trimester (14–28 weeks)

  1. Fetal anomaly scan (Level II) — ideally at 18–20 weeks; detects ~60% of major structural anomalies
    • CNS: anencephaly, spina bifida, ventriculomegaly
    • Cardiac: four-chamber view, LVOT, RVOT
    • Abdominal: anterior wall defects (gastroschisis, exomphalos), renal agenesis
    • Skeletal dysplasias
  2. Fetal biometry — BPD, HC, AC, FL → confirms gestational age, detects IUGR
  3. Placental localisation — diagnosis of placenta praevia
  4. Amniotic fluid assessment — AFI (normal 5–25 cm); polyhydramnios / oligohydramnios
  5. Cervical length — TVS cervical length < 25 mm predicts preterm labour
  6. Amniocentesis guidance

🔹 Third Trimester (> 28 weeks)

  1. Fetal growth monitoring — serial biometry for IUGR / macrosomia
  2. Fetal presentation and lie
  3. Placental grading (Grannum grade 0–III) and localisation
  4. Amniotic fluid index (AFI)
  5. Doppler studies — umbilical artery, middle cerebral artery, ductus venosus → fetal well-being
  6. Biophysical profile (BPP) — 5 parameters (tone, movement, breathing, AFV, NST); score ≤ 4/10 → delivery
  7. Fetal weight estimation (Hadlock formula)
  8. Version guidance — ECV for breech

Biometric Parameters and Normal Values

ParameterSignificance
CRLDating in 1st trimester
BPD (Biparietal Diameter)Most used for GA in 2nd trimester
HC (Head Circumference)More accurate than BPD if head is dolichocephalic
AC (Abdominal Circumference)Best single parameter for fetal growth/weight
FL (Femur Length)Confirms GA; ↓ in skeletal dysplasia

Doppler Ultrasound

  • Umbilical artery S/D ratio: Absent or reversed end-diastolic flow → severe uteroplacental insufficiency; immediate delivery considered
  • Middle cerebral artery (MCA): Brain-sparing in IUGR; MCA PI ↓ with ↑ cerebral flow
  • Uterine artery: Notching at 20–24 weeks → predicts pre-eclampsia / IUGR

Safety

USG is considered safe in pregnancy. No confirmed teratogenic effects at diagnostic intensities. ALARA principle (As Low As Reasonably Achievable) should be followed. Thermal index (TI) and mechanical index (MI) should be kept low, especially in the first trimester.

Summary Box (High-Yield for Exams)

ScanTimingKey Purpose
Viability / dating scan7–10 weeksCRL, cardiac activity
NT scan + biochemistry11–13+6 weeksDown syndrome screening
Anomaly scan18–20 weeksStructural defects
Growth scan28–36 weeksIUGR, lie, Doppler
Reference: DC Dutta's Textbook of Obstetrics, 9th Ed., Chapter on Ultrasound in Obstetrics; Creasy & Resnik's Maternal-Fetal Medicine, 8th Ed.
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