Clinical overview
Obstetric ultrasound is the single most important imaging tool in pregnancy and the technical skill the FCOG(SA) examiners expect every registrar to wield safely and interpret correctly. It is not one investigation but a sequence of trimester-specific examinations, each answering a defined clinical question: Is the pregnancy intrauterine, viable and correctly dated? Is the fetus structurally normal and the chromosomal risk acceptable? Is the fetus growing, well-oxygenated and surrounded by normal liquor? Is the placenta normally sited? And — in twins — what is the chorionicity, the single most prognostically important determination in multiple pregnancy.
In the South African public sector ultrasound is a rationed resource. The National Integrated Maternal and Perinatal Care Guideline (NDoH, 2024) builds antenatal care around a basic dating/booking scan and a structured second-trimester anomaly scan where capacity allows, with growth and Doppler surveillance reserved for high-risk pregnancies. Many district hospitals have a single machine and few accredited sonographers; the registrar must therefore know which scan adds clinical value and which is a luxury. This chapter teaches you to use ultrasound — to choose the right examination for the trimester, to perform liquor, placental and Doppler assessment, and to assess multiple pregnancy — rather than merely to list its features. The companion physics and safety material lives in Ultrasound knobology and Doppler safety, and the adnexal-mass interpretation in Ultrasound malignancy signs.
Core knowledge
Figure F14.1 — Obstetric ultrasound roadmap: ask the right question for the trimester — 1st (location/viability/dating/NT), 2nd (anomaly survey + biometry), 3rd (growth + Doppler) — and scan safely (ALARA; no routine pulsed Doppler in the first trimester).
Safety and the trimester framework
Ultrasound is non-ionising but not without bioeffect: tissue heating (thermal index, TI) and cavitation (mechanical index, MI). ISUOG safety guidance applies the ALARA principle — as low as reasonably achievable — and is most relevant to Doppler, which deposits far more energy than B-mode. Pulsed/spectral Doppler should be avoided in the first trimester for routine indications because the embryo is small and thermally vulnerable; keep TI < 1.0 and exposure time short when Doppler is genuinely indicated. This safety frame is examined repeatedly and detailed in Ultrasound knobology and Doppler safety.
Each trimester has a characteristic task list:
- First trimester (up to ~13⁺⁶ weeks): confirm intrauterine location, number, viability, accurate dating, and chorionicity in twins; offer aneuploidy screening.
- Second trimester (~18–22 weeks): detailed anatomical (anomaly) survey, placental localisation, liquor and early growth.
- Third trimester: growth (fetal biometry and estimated fetal weight), liquor volume, placental site confirmation and, where indicated, fetal Doppler.
Dating and biometry
Crown-rump length (CRL) in the first trimester is the most accurate dating measurement (classically accurate to within ~5 days when measured between roughly 7 and 13 weeks; standard teaching). Once CRL exceeds the validated range, dating shifts to a composite of biparietal diameter (BPD), head circumference (HC), abdominal circumference (AC) and femur length (FL). Dating becomes progressively less reliable with advancing gestation; a scan after ~22 weeks may carry an error of two weeks or more, which is why early dating is prioritised. Dating principles in depth are covered in Gestational age assessment.
Estimated fetal weight (EFW) is derived from HC, AC and FL (commonly the Hadlock formula; standard teaching), with AC the most sensitive single parameter for growth disturbance because it reflects hepatic glycogen and subcutaneous fat. Serial biometry — not a single measurement — defines growth velocity.
Liquor volume
Amniotic fluid reflects fetal urine output (and therefore renal and placental function) and fetal swallowing. Two semi-quantitative methods are used:
- Deepest vertical pool (DVP, also single deepest pocket): a vertical fluid column free of cord and fetal parts. Standard teaching: normal ~2–8 cm; oligohydramnios < 2 cm; polyhydramnios > 8 cm.
- Amniotic fluid index (AFI): the sum of the deepest pool in each of four uterine quadrants. Standard teaching: normal ~5–25 cm; oligohydramnios < 5 cm; polyhydramnios > 25 cm (mild/moderate/severe bands are used clinically).
Evidence (and current obstetric teaching) favours DVP over AFI for surveillance because AFI diagnoses more oligohydramnios without improving outcomes, leading to more intervention. Liquor pathology — its causes and significance — is the subject of Liquor volume abnormalities.