Clinical overview
Substance use is one of the most under-asked, under-documented and clinically consequential exposures in South African antenatal care. Tobacco, alcohol, prescribed and over-the-counter medication, and illicit drugs each alter the maternal-placental-fetal unit through distinct mechanisms, yet they cluster together — the woman who smokes is more likely to drink, the woman who uses methamphetamine ("tik") is more likely to use tobacco and cannabis — and they cluster with poverty, intimate-partner violence, untreated mental illness and HIV. South Africa carries one of the highest documented prevalences of fetal alcohol spectrum disorder (FASD) in the world, with community studies from the Western Cape winelands reporting rates an order of magnitude above global estimates. The Saving Mothers reports (NCCEMD) remind us that substance use rarely appears as a direct cause of maternal death but sits behind many — through late booking, non-attendance, trauma, and the medical complications it accelerates.
Your task as a registrar is not to moralise but to explain the effects accurately to the woman and the team, and to advise on cessation with the specific, evidence-based, non-judgemental approach that actually changes behaviour. Detection requires you to ask everyone, every booking, in a way that invites honesty. The single most powerful obstetric intervention here is helping a woman stop smoking; the single most preventable cause of lifelong neurodevelopmental disability is prenatal alcohol exposure. Both are missed when we do not ask. This chapter sits alongside Antenatal booking, High-risk pregnancy factors and GBV and mental health in pregnancy.
Core knowledge
Tobacco / smoking
Cigarette smoke delivers nicotine (a potent vasoconstrictor and addictive stimulant) and carbon monoxide (which binds haemoglobin as carboxyhaemoglobin, shifting the oxygen dissociation curve left and reducing fetal oxygen delivery), among thousands of other toxins. The dominant obstetric mechanism is chronic uteroplacental hypoperfusion and relative fetal hypoxia, compounded by direct toxic effects on trophoblast invasion.
Consequences, dose-dependent across all of these (standard teaching, with effect sizes broadly consistent across NICE NG201 and SA guidance):
- Fetal growth restriction and reduced birthweight — the most reproducible effect; see Intrauterine growth restriction.
- Preterm birth and preterm prelabour rupture of membranes (Preterm birth and pprom).
- Placental abruption and placenta praevia — smoking is a classic modifiable risk factor for both (Antepartum haemorrhage).
- Miscarriage, ectopic pregnancy (tubal ciliary toxicity) and stillbirth.
- Sudden infant death syndrome (SIDS) postnatally, increased by both antenatal and environmental (passive) smoke.
- A paradoxical, non-protective small reduction in pre-eclampsia incidence is described but must never be presented as a benefit — outcomes are uniformly worse.
Quitting at any gestation helps; quitting before or early in pregnancy largely normalises birthweight. The benefit is immediate and continuous, which is the message to give.
Alcohol
Ethanol and its metabolite acetaldehyde are direct teratogens and neurotoxins that cross the placenta freely, reaching fetal concentrations approximating maternal levels; the fetus clears them slowly. Alcohol disrupts neuronal proliferation and migration, causes oxidative stress and apoptosis in developing neural tissue, and interferes with cell adhesion — producing the midline craniofacial dysmorphology and diffuse central-nervous-system injury that define the spectrum.
Fetal alcohol spectrum disorder (FASD) is the umbrella term; fetal alcohol syndrome (FAS) is the most severe, classically defined by three features (standard diagnostic teaching): the characteristic facies (short palpebral fissures, smooth philtrum, thin vermilion upper lip), pre- and/or postnatal growth restriction, and CNS/neurodevelopmental abnormality, in the context of confirmed or probable prenatal alcohol exposure. The neurodevelopmental harm — intellectual disability, executive-function deficits, behavioural and attentional disorders — is the lifelong burden and is the most common preventable cause of intellectual disability.
Critically, no safe threshold and no safe trimester have been established. Binge drinking is especially harmful, but harm is documented at lower exposures and the first trimester (organogenesis and early brain development) is high-risk while the third trimester (rapid brain growth) is also vulnerable. The honest advice is therefore complete abstinence throughout pregnancy and when trying to conceive. South African prevalence makes this a national priority.