Drugs in Pregnancy, Lactation & Special Populations

A drug given to a pregnant woman reaches the fetus through the placenta, a drug taken while breastfeeding appears in milk, a child's immature liver handles medicines differently from an adult's, and an elderly patient's failing kidneys clear drugs slowly. Special-population pharmacology adjusts every prescription for these realities: avoiding teratogens that deform the fetus, choosing drugs with low breast-milk transfer, dosing children by weight, and reducing doses as kidney function declines with age.

This matters because the wrong drug in the first trimester can cause lifelong birth defects, while under-treating a mother's epilepsy or infection harms two patients at once. Getting these adjustments right is core safe prescribing.

Key Concepts

• Teratogenicity: the ability to cause birth defects; risk is highest during organogenesis, roughly weeks 3 to 8 of gestation.
• Proven teratogens: thalidomide (limb defects), isotretinoin (severe malformations), warfarin (nasal and bone defects), valproate (neural tube defects), ACE inhibitors in later pregnancy (kidney failure in the fetus), tetracyclines (teeth discoloration).
• Safer in pregnancy: penicillins, most cephalosporins, paracetamol, insulin for diabetes, levothyroxine, and folic acid supplementation.
• Relative infant dose: the infant's drug intake via milk as a percentage of the maternal dose; below 10 percent is generally considered compatible with breastfeeding.
• Paediatric dosing: calculated by body weight in mg per kg; Clark's rule (child weight divided by 70, times adult dose) and Young's rule (age divided by age plus 12, times adult dose) are classic approximations.
• Geriatric changes: reduced kidney and liver function, lower body water, higher body fat, and polypharmacy; the Beers criteria list drugs to avoid in the elderly, such as long-acting benzodiazepines.

Formulas

• Cockcroft-Gault (creatinine clearance, mL/min): (140 minus age) times weight in kg, divided by (72 times serum creatinine in mg/dL); multiply by 0.85 for women.
• Clark's rule: child dose = (weight in kg divided by 70) times adult dose.
• Paracetamol (child): 15 mg per kg per dose, every 4 to 6 hours, maximum 4 doses per day.

Worked Example

A 70-year-old man weighing 70 kg has a serum creatinine of 1.2 mg/dL and needs a renally cleared antibiotic. Step 1: numerator = (140 minus 70) times 70 = 70 times 70 = 4,900. Step 2: denominator = 72 times 1.2 = 86.4. Step 3: clearance = 4,900 divided by 86.4 = 56.7 mL/min. A healthy young adult would be near 100 to 120 mL/min, so this patient's kidneys clear drugs at roughly half speed and the dose must be reduced or the interval extended.

Common Misconceptions

• "No medicine is safe in pregnancy, so stop everything." Untreated maternal disease harms the fetus more than most drugs; the art is choosing the safest effective drug, not abandoning treatment.
• "Any medicine means I must stop breastfeeding." Most drugs transfer in tiny amounts with a relative infant dose below 10 percent; only a few (anticancer drugs, radioisotopes, amiodarone) truly require stopping.
• "Children are just small adults." Children differ in metabolism, not just size: some drugs need higher per-kg doses because children clear them faster, and aspirin is avoided because of Reye's syndrome.

Real-World Applications

• Antenatal care: folic acid 5 mg for women on antiepileptics, switching valproate to safer alternatives before conception.
• Gestational diabetes and hypertension: insulin and methyldopa or labetalol as the safe choices.
• Neonatal care: vitamin K at birth, and weight-based antibiotic dosing in sepsis.
• Geriatric wards: renal dose adjustment of digoxin, aminoglycosides and allopurinol using Cockcroft-Gault.

Pregnancy demands avoiding proven teratogens while still treating maternal disease, lactation is compatible with most drugs at low relative infant doses, children need weight-based doses, and the elderly need kidney-based reductions via Cockcroft-Gault.