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.
Syphilis screening is mandatory at booking and is one of the highest-yield interventions in obstetrics: untreated maternal syphilis causes stillbirth, prematurity and congenital syphilis, all preventable by timely penicillin. The SA system increasingly deploys point-of-care syphilis (and dual HIV/syphilis) tests to enable same-visit treatment, because referral-and-recall loses women. Hepatitis B surface antigen screening identifies neonates needing birth-dose immunoprophylaxis. Screening for these infections sits within the STI framework (infections-in-pregnancy, sti-pathology).
Maternal haematological and metabolic screening
Anaemia screening (Hb/FBC) at booking and again later in pregnancy is standard; anaemia is common in the SA population and is both a risk marker and a treatable contributor to adverse outcome. Blood group and antibody screening identifies RhD-negative women and atypical red-cell antibodies, the gateway to anti-D prophylaxis and to surveillance for haemolytic disease of the fetus (rh-isoimmunisation).
Gestational diabetes (GDM) screening is a recurring exam controversy. The approaches diverge: universal oral glucose tolerance testing versus risk-factor- based selective screening (obesity, previous GDM, previous macrosomia, family history, certain ethnic groups, glycosuria). NICE NG3 uses a risk-factor-based OGTT pathway; the diagnostic glucose thresholds themselves differ between IADPSG/ WHO and NICE criteria, which changes who is labelled diabetic. State that the specific OGTT thresholds and the universal-versus-selective choice depend on the guideline and local protocol, and verify the exact cut-offs against the current NDoH guideline before quoting a number. The downstream management belongs to medical-complications-in-pregnancy.
Fetal aneuploidy screening
Fetal aneuploidy screening estimates the risk of trisomy 21 (and 13/18) so that a woman can choose diagnostic testing. The tiers, in ascending performance:
- Maternal age alone — a weak screen; abandoned as a standalone because most affected pregnancies occur in younger women simply because they have more babies.
- Combined first-trimester test (~11–14 wk) — nuchal translucency on ultrasound + maternal serum βhCG + PAPP-A, combined with maternal age into a risk. This is the workhorse offered structural screening programmes.
- Cell-free DNA / NIPT — analyses placental cfDNA in maternal blood; the highest detection rate with the lowest false-positive rate for the common trisomies, but it remains a screen, not a diagnosis — a high-risk cfDNA result must be confirmed by an invasive diagnostic test before any irreversible decision.
- Diagnostic tests — chorionic villus sampling (~11–14 wk) or amniocentesis (from ~15 wk) give a karyotype/molecular result and carry a small procedure-related loss risk.
The counselling that wraps this — non-directive, informed, the meaning of a risk figure, and the implications of a confirmed diagnosis — is the examinable core, covered in down-syndrome-counselling and underpinned by principles-of-inheritance. In the SA public sector, second-trimester serum screening and first-trimester combined screening availability vary by level of care, and cfDNA is largely private-sector; the registrar must screen and counsel honestly about what is locally accessible rather than offer a test the woman cannot obtain.
Fetal structural and growth screening
The mid-trimester anomaly scan (~18–22 wk) screens for structural malformations and soft markers; its detection rate is operator- and machine-dependent and is not uniformly available across SA districts, a fact you must state explicitly rather than assume a UK programme. Growth screening uses serial symphysis-fundal height measurement as the universal first-line screen, escalating to ultrasound biometry and Doppler in those who screen positive (static or faltering SFH, risk factors). The interpretation of growth restriction and its Doppler-based surveillance is developed in intrauterine-growth-restriction and placental-insufficiency-response. Reduced fetal movements are a patient-reported screen for fetal compromise that must always trigger assessment (decreased-fetal-movements).
Assessment
Applying a screening protocol to the woman in front of you is a four-step clinical act.
1. Establish the baseline risk. A structured history stratifies her before any test is sent: age, parity, past obstetric history (loss, anomaly, GDM, hypertension, preterm birth), medical conditions, family/genetic history, medication and substance use (substance-use-in-pregnancy), and HIV status. This determines which risk-based screens apply and pre-sets the prior probability that multi-marker tests will modify. Confirm gestational age properly first — every gestation-bound test is wrong if the dates are wrong.
2. Offer the test with informed choice. Screening is offered, not imposed (HIV is opt-out, but still consented). The woman must understand what the test looks for, that a positive screen means further testing not disease, the chance of false positives and negatives, and what a confirmed diagnosis would mean for her — including options she may not wish to exercise. This is the informed-consent backbone (informed-consent) and is delivered within respectful, non-coercive care (respectful-care). Document the offer and the decision.
3. Perform the test correctly and at the right gestation. A combined test sent outside 11–14 weeks, an OGTT mistimed, an SFH measured without an empty bladder or plotted on the wrong chart — each turns a valid screen into noise.
4. Interpret against the threshold and act. A screen-positive result is the beginning of a pathway, not an endpoint: it triggers a confirmatory/diagnostic step, counselling, and a management or surveillance plan. A screen-negative result is reassurance within the test's detection rate, not a guarantee — safety-netting and the repeat-screening schedule remain. The high-risk findings themselves feed into ongoing risk-based care (high-risk-pregnancy-risks).
The applied skill examiners test is closing the loop: who gets told, who gets referred, by when, and to which level of care. A point-of-care syphilis positive treated at the same visit is a screening programme; a lab syphilis positive with no recall system is a dead letter.
Figure I11.2 — Closing the loop after antenatal screening: baseline risk, informed offer, correct timing, interpretation and action.
Management
Acting on screen-positive results, by category
- HIV-positive — start TLD immediately, link to care, plan viral-load monitoring, mode of delivery and infant prophylaxis (hiv-in-pregnancy).
- Syphilis-positive — treat with penicillin without delay, ideally same-visit; treat the partner; screen for co-infection; follow titres.
- HBsAg-positive — flag for neonatal birth-dose immunoprophylaxis and maternal assessment.
- Anaemia — investigate cause and treat (iron the commonest); recheck.
- RhD-negative / antibody-positive — anti-D prophylaxis pathway and, if antibodies present, titre/MCA-PSV surveillance (rh-isoimmunisation).
- GDM screen-positive — confirm on OGTT per protocol; dietary/medical management (medical-complications-in-pregnancy).
- Aneuploidy high-risk — non-directive counselling and offer of diagnostic CVS/amniocentesis; never act irreversibly on a screen alone (down-syndrome-counselling).
- Structural anomaly — refer to fetal-medicine/tertiary care for diagnosis, counselling and planning (pregnancy-and-neoplasia is unrelated; route via the anomaly-scan referral pathway).
- SFH static/small — growth scan + Dopplers; manage per FGR pathway (intrauterine-growth-restriction).
Screening for the great obstetric syndromes
Two screening protocols deserve singling out because they prevent the SA killers. Pre-eclampsia: blood pressure and urinalysis at every antenatal contact is the universal screen, supplemented by a first-trimester risk assessment. Women identified as higher risk are offered low-dose aspirin from 12 weeks per NICE NG133 — this is screening that directly reduces a leading cause of maternal death, and the threshold for aspirin and the BP/proteinuria cut-offs are laid out there (hypertension-in-pregnancy, pre-eclampsia-and-hellp). PlGF-based testing (NICE DG49) can help rule out pre-eclampsia in symptomatic women but is an adjunct, largely outside routine SA public screening. Preterm birth: history and, where available, cervical-length screening identify women for progesterone or cerclage pathways (preterm-birth-and-pprom, cervical-cerclage).
The South African delivery model
Screening in SA is delivered through the level-of-care system: most screening is universal and primary-care-based (HIV, syphilis, Hb, BP/urine, group, SFH), with screen-positives and higher-tier tests (detailed ultrasound, OGTT interpretation, fetal-medicine referral) escalated to district, regional and tertiary facilities per the NDoH Maternity Guideline. The Essential Medicines List governs what you can treat with on the spot. The programme's success metric is not the test sent but the action delivered before the window closes — same-visit treatment, functioning recall, and clear referral criteria. Resource reality means you screen for what you can act on; offering a test with no downstream pathway is unethical and wasteful.
Emergency escalation drill
Screening occasionally surfaces an emergency, and the drill must be reflexive. A booking or routine BP of ≥160/110 mmHg, or any raised BP with symptoms (headache, visual disturbance, epigastric pain) or significant proteinuria, is not a "screen result to file" — it is severe pre-eclampsia until proven otherwise. Act immediately:
- Call for help / senior + escalate level of care — this woman is not for the routine queue.
- Control BP — antihypertensive per protocol; do not let severe hypertension stand.
- Magnesium sulphate for severe pre-eclampsia/eclampsia (seizure prophylaxis/treatment), per NICE NG133 / SA Maternity Guideline.
- Stabilise, monitor, plan delivery as the definitive treatment, and transfer to the appropriate level of care.
The full regimen and dosing are in pre-eclampsia-and-hellp — verify magnesium and antihypertensive doses against the current guideline before administration. The principle for screening: a number that crosses an emergency threshold converts an asymptomatic screen into a resuscitation.
Figure I11.3 — Severe blood-pressure screening drill: red flags that convert routine antenatal screening into emergency pre-eclampsia care.
Red flags / pitfalls
- Treating a screen as a diagnosis. The cardinal error — never act irreversibly (termination, major intervention) on a screen-positive aneuploidy or anomaly result without diagnostic confirmation.
- False reassurance from a screen-negative. Detection rates are never 100%; safety-net, keep screening on schedule, and respond to new symptoms regardless of an earlier "normal".
- Mistiming gestation-bound tests — combined test outside 11–14 wk, OGTT at the wrong gestation, anomaly scan too early. Confirm dating first.
- Importing a foreign programme wholesale. NICE/RCOG schedules assume universal cfDNA and anomaly scanning that SA cannot uniformly deliver. Apply the NDoH pathway; offer only tests the woman can actually access and act upon.
- Failing to close the loop — a positive syphilis or HIV result with no same-visit treatment or recall is the commonest system failure and a direct contributor to preventable stillbirth and vertical transmission.
- Not repeating HIV testing — a single negative at booking misses seroconversion; repeat per protocol.
- Ignoring the BP/urine screen — the cheapest, most universal screen prevents the disease that kills; never let it become a tick-box.
- Quoting thresholds from memory. GDM glucose cut-offs, aspirin risk criteria and magnesium doses differ between guidelines — verify against the current source before you act on a number.
- Coercive or undocumented offers — screening must be consented and recorded, delivered respectfully even when opt-out.
Evidence anchors
- National Integrated Maternal and Perinatal Care Guideline, 5th edition (2024), NDoH — the SA source of truth for the antenatal screening schedule, level-of-care pathways and what is universal versus risk-based.
- Saving Mothers / NCCEMD reports (latest triennium) — justify the priority given to HIV, hypertension and haemorrhage screening in the SA programme.
- South African National HIV/ART Consolidated Guidelines (2023) and SAHCS 2023 Adult ART Guidelines — opt-out repeat HIV testing and immediate TLD on a positive screen.
- NICE NG201 — Antenatal care (2021) — the comparator screening schedule; note divergence from the SA pathway in availability of cfDNA and anomaly scanning.
- NICE NG3 — Diabetes in pregnancy — risk-factor-based GDM screening and OGTT pathway (confirm exact thresholds against the current guideline).
- NICE NG133 — Hypertension in pregnancy (2019) — first-trimester pre-eclampsia risk screen, low-dose aspirin from 12 weeks, magnesium sulphate for severe disease; NICE DG49 — PlGF-based testing to rule out pre-eclampsia.
- RCOG GTG 31 — Small-for-Gestational-Age and Growth-Restricted Fetus and ISUOG Doppler standards — escalation from SFH to biometry/Doppler; RCOG GTG 57 — Reduced Fetal Movements for the patient-reported screen.
- WHO HIV Testing Services Guidelines and WHO STI Treatment Guidelines — international anchors for repeat HIV testing and syphilis screen-and-treat.