Clinical overview
Antenatal screening is the systematic application of tests to an apparently well, asymptomatic pregnant population to identify those at higher risk of a maternal or fetal disorder, so that confirmatory testing, treatment or planning can follow. The verb in this objective is apply — you are expected not merely to list tests but to deploy the right test, to the right woman, at the right gestation, with the right interpretation and the right downstream action. That is the difference between a registrar who knows the booking bloods and one who can run a screening programme across a district.
The discipline rests on a single distinction that examiners probe relentlessly: screening is not diagnosis. A screen-positive result identifies a woman who needs a diagnostic test; it does not label her as diseased. Conflating the two causes the two great harms of screening — needless intervention in the false-positive (a woman terrified by a "high-risk" Down syndrome result who miscarries a normal fetus after an unnecessary amniocentesis) and false reassurance in the false-negative. Every screening decision is governed by the classic Wilson–Jungner logic: the condition must be important, have a recognisable latent stage, an acceptable accurate test, an agreed treatment or intervention, and a programme whose benefit outweighs its harm and cost. In the South African setting the cost and capacity arms of that calculus are load-bearing — a test that cannot be reliably delivered or acted upon at a district hospital is not a screening programme, it is a liability. This chapter frames maternal and fetal screening as a single, gestation-anchored protocol you apply from booking (Antenatal booking) through to term, grounded in the NDoH Integrated Maternal and Perinatal Care Guideline and the SA syllabus (SA maternity guidelines).
Core knowledge
The arithmetic of a screening test
You must be fluent in the operating characteristics, because the whole of applied screening is the manipulation of these numbers (the formal treatment lives in Biomedical statistics):
- Sensitivity — proportion of truly affected correctly screen-positive (detection rate). Drives how many cases you catch.
- Specificity — proportion of unaffected correctly screen-negative. Drives your false-positive rate (FPR = 1 − specificity).
- Positive predictive value (PPV) — of those screen-positive, the proportion truly affected. PPV is prevalence-dependent: the same test has a far worse PPV in a low-prevalence population, which is why we screen-then-confirm.
- Likelihood ratios allow a prior risk (maternal age, history) to be multiplied into a posterior risk — the engine of multi-marker aneuploidy screening.
A screening test is "positive" when it crosses an agreed threshold/cut-off chosen to balance detection against false positives — moving the cut-off trades sensitivity against specificity. Examiners love to ask why a cut-off was set where it is: the answer is always the downstream harm-benefit and capacity trade-off, never an intrinsic biological line.
What we screen for, and when
Antenatal screening clusters into maternal-disease screening, fetal-aneuploidy screening, fetal-structural screening, fetal-growth and wellbeing screening, and intrapartum screening. The unifying scaffold is gestational age, so accurate dating is the prerequisite for every gestation-bound screen (Gestational age assessment).
| Domain | Test | Timing (standard teaching) |
|---|---|---|
| Maternal infection | HIV, syphilis, hepatitis B | Booking; HIV re-test through pregnancy |
| Maternal haematology | Haemoglobin/FBC, blood group + antibody | Booking, repeat later pregnancy |
| Maternal metabolic | Glucose (risk-based or universal OGTT) | Booking risk-screen; ~24–28 wk |
| Maternal renal/BP | BP + urine protein every visit | Every contact |
| Fetal aneuploidy | Combined test / cfDNA (NIPT) | ~11–14 wk |
| Fetal structure | Detailed anomaly ultrasound | ~18–22 wk (where available) |
| Fetal growth | SFH, serial growth scans if indicated | From ~24 wk |
| Fetal wellbeing | Movements, CTG/Doppler if indicated | Third trimester |
Figure I11.1 — Gestation-anchored antenatal screening runway showing the right test, right timing and immediate action pathway.
The exact thresholds, intervals and which tests are universal versus risk-based differ between the NDoH guideline and NICE NG201, and between levels of care. Lead with the SA pathway and flag international divergence — that is the registrar move.
Maternal infection screening — the SA backbone
South Africa's maternal mortality profile makes HIV the single most important antenatal screen. Saving Mothers (NCCEMD) has repeatedly placed non-pregnancy-related infection, dominated by HIV, among the leading causes of maternal death alongside obstetric haemorrhage and hypertension. The programme is therefore opt-out, repeated, and treat-on-the-spot. All women are tested at booking; HIV-negative women are re-tested through pregnancy and into the breastfeeding period to catch seroconversion, because acute maternal infection carries the highest vertical-transmission risk. A positive screen triggers immediate first-line ART — TLD (tenofovir + lamivudine + dolutegravir) per the SA ART guidelines — with viral-load-driven decisions about mode of delivery and infant prophylaxis. The detail of the cascade lives in HIV in pregnancy and the counselling discipline in HIV counselling; for screening, the principle is universal opt-out testing with repeat testing and same-visit linkage to care.