In one line
HIV in pregnancy is managed as one problem with two patients: the mother, whose own survival depends on lifelong dolutegravir-based ART and aggressive screening for tuberculosis, and the infant, whose freedom from infection is bought almost entirely by driving and holding the maternal viral load below detection across pregnancy, delivery and breastfeeding. Prevention of vertical transmission (PVT — the term that replaced PMTCT in South African practice) is, at its core, viral-load control plus risk-stratified infant prophylaxis, and South Africa has taken vertical transmission from roughly one in four exposed infants to under one in a hundred near birth on exactly that principle.
The mechanics of a positive antenatal HIV test, the basic ART regimen and routine booking bloods are assumed groundwork — covered as a booking diagnosis at the Intermediate level. The consultant layer is reasoning about viral-load–driven risk: the woman who arrives in labour unbooked, the unsuppressed mother near term, infant feeding under suppressive ART, and how to argue any of it from the data. The gynaecological HIV interface — genital TB, cervical disease, opportunistic infection — sits in Genital TB and HIV in gynaecology.
Mechanism & pathophysiology
Vertical transmission happens across three windows, and each maps to a different mechanism, which is why a single intervention does not cover all of them.
Intrauterine transmission (roughly the minority of in-utero infections, occurring mostly in late pregnancy) is transplacental: cell-free and cell-associated virus crosses a placenta whose integrity is itself degraded by maternal viraemia, chorioamnionitis, placental malaria and co-infections. Intrapartum transmission — historically the largest single contributor in a non-breastfeeding, untreated population — is exposure of the infant to maternal blood and genital-tract secretions during labour and delivery, amplified by ruptured membranes, instrumentation, and a high genital-tract viral load. Postnatal transmission is through breast milk, a continuous low-grade exposure that accumulates over the whole duration of breastfeeding; in a breastfeeding population it becomes the dominant route, and in South Africa more than 80% of the residual transmissions now occur during the first six months of breastfeeding.
The single variable that runs through all three windows is the maternal HIV viral load. Risk is not a step function tied to a diagnosis; it is continuous and steeply dose-dependent on the number of circulating virions the infant is exposed to. The 2025 Lancet meta-analysis of 147 studies quantified this with precision: pooled perinatal transmission was 0.2% with a maternal viral load below 50 copies/mL, 1.3% at 50–999, and 5.1% at ≥1000 copies/mL — an adjusted relative risk of 22.5 for the highest band versus the suppressed band. The same gradient holds postnatally, where monthly breastfeeding transmission was 0.1% with a recent viral load under 50 and 0.5% at ≥50. This is the molecular logic of PVT in one sentence: drive the viral load down and hold it there, and the exposure across every window collapses toward zero.
That gradient also explains why timing of suppression matters as much as the fact of it. A woman established on ART and virally suppressed before conception enters pregnancy with the placenta never exposed to high-titre virus; the Lancet analysis found zero perinatal transmissions among 4675 women on pre-conception ART with a viral load below 50 near birth — the strongest available evidence that the "undetectable equals untransmittable" principle extends to pregnancy and birth. The newly-diagnosed woman starting ART in the third trimester, by contrast, may not reach undetectable before she delivers, so her placenta and genital tract have been exposed to viraemia for months and she may still be unsuppressed intrapartum. Two women with the same final regimen can carry an order-of-magnitude difference in transmission risk purely on when suppression was achieved. The postnatal window is the one place where even suppression is not demonstrably zero-risk: the Lancet pooled estimate at recent viral load <50 was very low but not nil, and the authors were explicit that current data — largely from cohorts without frequent viral-load monitoring or modern regimens — are insufficient to claim true U=U during breastfeeding. That residual uncertainty is precisely what the infant-prophylaxis layer exists to cover.
The immunological backdrop is that pregnancy is a state of relative immune modulation, and untreated HIV strips the maternal reserve further. A low CD4 count is less about transmission risk per se (viral load drives that) and more about the mother's own vulnerability — to tuberculosis above all, but also to pneumonia, cryptococcal disease and the opportunistic infections that have made non-pregnancy-related infection the leading driver of HIV-associated maternal death in South Africa. Treating the mother and protecting the infant are therefore not competing goals; the same suppressive ART does both.
One mechanistic subtlety has practical force at the breastfeeding interface: breast inflammation raises the milk viral load even when the plasma viral load is suppressed. Mastitis, a cracked nipple, a breast abscess or subclinical inflammation locally disrupts the blood–milk barrier and lets cell-associated virus into the milk compartment, which is why breast pathology in a lactating woman living with HIV is treated promptly rather than conservatively, and why mixed feeding is discouraged — non-human feeds injure the infant gut mucosa and create a portal for whatever virus is present. Suppression lowers the milk viral load; protecting the breast and the infant gut closes the remaining gaps.
Assessment
The assessment separates cleanly into screening the population, and then placing the individual woman onto one of three management pathways.
Universal antenatal testing, and re-testing through the at-risk window.
- HIV testing is offered to every woman at the first antenatal visit (provider-initiated, opt-out), with same-day result and same-day ART for those who test positive.
- In a high-incidence setting a single negative booking test is not enough — maternal seroconversion during pregnancy or breastfeeding is a recognised and high-risk transmission route (the mother seroconverts with a very high viral load and no prophylaxis in place). South African practice therefore re-tests the HIV-negative woman at every subsequent antenatal visit and three-monthly throughout breastfeeding. A woman who tested negative at booking but presents in labour or postnatally must be re-tested.
- The negative rapid test also has a false-negative trap in early seroconversion (window period) and in advanced disease; clinical suspicion overrides a single reassuring result.
Staging the individual: viral load and CD4 carry different information.
- The viral load is the transmission-risk variable and the treatment-success variable. It is checked at the start of ART, then on the schedule below, and it is the number that decides intrapartum and infant-feeding plans. A viral load ≥50 copies/mL near delivery flags a higher-risk infant, and a viral load ≥1000 additionally flags likely treatment failure or non-adherence.
- The CD4 count stages the mother's own immune risk, not the infant's transmission risk. A CD4 below 200 mandates a deliberate search for, and prophylaxis against, opportunistic infection — cotrimoxazole prophylaxis, a reflex cryptococcal antigen test at the relevant threshold, and heightened TB vigilance.
- Tuberculosis screening is non-negotiable at every visit. TB is the leading infectious driver of maternal death in HIV-positive South African women; every pregnant woman living with HIV is symptom-screened (cough, fever, night sweats, weight loss) at each contact, investigated where positive, and offered TB preventive therapy where active disease is excluded.
- Co-infection screening completes the picture: syphilis (RPR, with the renewed national concern over congenital syphilis — a co-epidemic that shares the same antenatal-screening machinery as HIV and is folded into the triple-elimination agenda), hepatitis B (HBsAg — relevant to both the maternal regimen, since tenofovir and lamivudine are active against hepatitis B, and the infant's birth-dose vaccine), and cervical screening, since HIV accelerates cervical carcinogenesis.
- Advanced HIV disease (CD4 <200, or a WHO clinical stage 3/4 condition) reframes the woman as critically ill, not merely pregnant. She needs the advanced-disease package — cryptococcal antigen screening with pre-emptive treatment where positive, cotrimoxazole prophylaxis, intensified TB investigation including urine lipoarabinomannan where available — alongside her ART, because the immediate threat to her life is the opportunistic infection, not the obstetrics. The danger is anchoring on the pregnancy and missing the failing immune system behind it.