Ethanol, Nicotine and Substance Use
Start from one idea: a pregnant woman and her fetus share a chemistry set, but only one of them can metabolise and excrete what enters it. Whatever crosses the placenta reaches a fetus whose liver, kidney and brain are immature, who cannot decide its own exposure, and who is building organs and neural circuits on a fixed developmental timetable. Every problem in this chapter is a consequence of that single asymmetry. Once you hold it, the rest is just naming the molecule, asking how much and how often, and following the chain from drug to receptor to placenta to fetus.
From that first principle, three questions organise the whole topic. What is the molecular target? (where the drug acts — a receptor, an enzyme, an ion channel, oxygen carriage). What is the exposure pattern? (dose, frequency, route, timing in pregnancy, and how often use is interrupted by withdrawal). What is the maternal-fetal consequence? (placental perfusion, organ formation, growth, neonatal adaptation, breastfeeding, long-term child health, and the maternal illness around the substance). The Primary task is to reason confidently along that chain — molecular target plus exposure pattern to maternal and fetal outcome — while using language that keeps the patient engaged in care.
That last clause is not decoration. Use non-stigmatising terms: substance use, substance use disorder, dependence, prenatal exposure, person who uses opioids, or patient with alcohol use disorder. Avoid identity labels that blame the person rather than naming the health condition. Stigma worsens concealment, late booking, poor follow-up and unsafe withdrawal, and concealment is itself a clinical harm because it hides exposures you could otherwise mitigate. Compassion is not permissiveness; it is a clinical tool that improves detection and treatment.
This chapter gives the pharmacology and the O&G relevance — the mechanism layer. Detailed management pathways, drug regimens, local referral processes and antenatal protocols belong to the linked clinical chapters and current South African guidance, and are signposted rather than duplicated here.
Core Pharmacology Principles
Most psychoactive substances share features that make pregnancy important: they are small enough to cross biological barriers, active in the central nervous system, and capable of changing maternal behaviour, physiology or dependence pathways.
| Principle | Meaning | O&G implication |
|---|---|---|
| Placental transfer | Many substances cross by passive diffusion or transport | Fetal exposure can occur before pregnancy is recognised |
| Fetal metabolic immaturity | Fetal liver, kidney and brain handling differ from adults | Exposure can be prolonged or developmentally amplified |
| Timing vulnerability | Organogenesis, growth, brain development and neonatal adaptation have different windows | "No anomaly on scan" does not exclude neurodevelopmental harm |
| Dose and pattern matter | Binge, daily use, polysubstance use and withdrawal cycles differ | Ask quantity, frequency, route and last use |
| Maternal disease pathway | Dependence, overdose, malnutrition, violence, infection and poor sleep harm pregnancy | Treat the whole context, not only the molecule |
| Withdrawal can be dangerous | Abrupt cessation may destabilise mother and fetus for some substances | Plan treatment rather than demanding immediate unsupported stopping |
| Breastmilk transfer | Lipophilic drugs and active metabolites can expose infant | Infant sedation, feeding, growth and neurodevelopment may matter |
The mechanism-to-clinical chain is:
substance exposure -> maternal pharmacodynamic effect -> placental/fetal exposure or maternal illness pathway -> fetal growth, development or neonatal adaptation effect -> family and public-health consequences
Polysubstance use is common. Alcohol, tobacco, cannabis, opioids, stimulants, sedatives, antiretrovirals, TB treatment, psychiatric medicines and over-the-counter products may coexist. The exam trap is to attribute every outcome to one substance when social determinants and co-exposures are part of the causal pathway.
Two Foundations: Crossing the Placenta and the Timing Window
Before naming individual substances, fix the two pieces of basic science that explain almost every fetal effect. The first is how a molecule reaches the fetus; the second is when in development it arrives.
Placental transfer is mostly physics. The placenta is not a filter or a barrier in the everyday sense; in late pregnancy only a thin layer of trophoblast and basement membrane separates maternal blood in the intervillous space from fetal capillary blood, and uteroplacental blood flow rises across gestation (roughly tenfold from early to late pregnancy). Most psychoactive drugs cross by passive diffusion down a concentration gradient, and the rate depends on predictable properties:
| Property | Effect on transfer | Why it matters in this chapter |
|---|---|---|
| Low molecular weight | Small molecules cross easily | Ethanol, nicotine, THC and opioids are all small |
| Lipid solubility | Lipophilic drugs partition through membranes | THC and many sedatives are highly lipophilic and persist in tissue |
| Low ionisation at blood pH | Un-ionised fraction crosses; ionised fraction is trapped | Weak bases can become "ion-trapped" in the slightly more acidic fetal compartment |
| Low protein binding | Only the free (unbound) drug diffuses | Conditions that lower binding raise the transferable fraction |
| Placental and fetal metabolism | The placenta and immature fetal liver handle drugs differently from the adult | Fetal clearance is limited, so exposure is prolonged |
The take-home is mechanistic, not moralistic: the placenta does not protect the fetus from small, lipophilic, weakly bound psychoactive molecules. Saying "the placenta filters it out" is wrong for ethanol, nicotine, THC, opioids and sedatives alike.
Timing decides the kind of harm. Teratogenesis means structural or functional disruption of fetal development, and the same agent can produce different injuries depending on when it acts. Major organs are laid down during organogenesis, roughly the period from a few weeks after conception through about the first 10 weeks after the last menstrual period; an insult here can cause structural malformation. After organ formation, the developing fetus — especially the brain, which keeps forming and wiring connections into the third trimester and beyond — is more likely to suffer functional and neurobehavioural injury without an obvious anatomical defect. Two consequences follow that the exam rewards:
- Exposure before pregnancy is recognised still matters, because organogenesis begins before many people know they are pregnant. This is why periconception counselling and avoidance are emphasised, and why dating the exposure (rather than panicking about it) is the right clinical move.
- A normal anatomy scan does not exclude harm. Neurodevelopmental injury from prenatal exposure is, by definition, often invisible on a structural scan. "Scan was normal" reassures about gross structure, not about wiring.