Pharmacokinetics describes what the body does to a drug from the moment it is administered until it leaves the body. For midwives in Nigeria managing conditions like pregnancy-induced hypertension or malaria, understanding this concept is vital. It dictates how much medication reaches the target site, how long it stays active, and when a repeat dose is needed. The study of pharmacokinetics is broken down into four main processes: absorption, distribution, metabolism, and excretion—often remembered by the acronym ADME. Each of these phases changes significantly during pregnancy due to normal physiological adaptations.
Absorption is the movement of a drug from its site of administration into the bloodstream. Oral medications must cross the gastrointestinal tract and survive first-pass metabolism in the liver before reaching systemic circulation.
Once in the bloodstream, a drug travels throughout the body to various tissues, a process called distribution. Some drugs bind to plasma proteins like albumin, while others remain free. Only the unbound, free drug can produce clinical effects or cross biological membranes.
In pregnant women, the total volume of body water increases drastically, diluting water-soluble drugs. Furthermore, lipid-soluble drugs easily cross the placental barrier to reach the developing fetus. Midwives must remember that virtually all medications given to a mother will cross the placenta to some degree.
Metabolism, or biotransformation, chemically alters drugs so they can be easily removed from the body. The liver is the primary organ responsible for this process, using specific enzymes—most notably the cytochrome P450 system—to turn active drugs into inactive water-soluble metabolites. Some medications are administered as inactive precursors called prodrugs, which rely on liver metabolism to become active. During pregnancy, hormonal shifts can either speed up or slow down hepatic enzyme activity, altering how long a medication stays active in the maternal system.
Excretion removes the drug and its metabolites from the body. The kidneys are the main organs for excretion, filtering waste through the glomeruli into the urine. Other minor pathways include the bowels, sweat, lungs, and breast milk. Because renal blood flow and glomerular filtration rate increase significantly during pregnancy, drugs cleared entirely by the kidneys are eliminated much faster than in the non-pregnant state.
Midwives must appreciate that pharmacokinetic parameters are not fixed; they shift across trimesters and during the postpartum period. Increased plasma volume, delayed gastric emptying, altered liver enzyme activity, and elevated renal clearance all combine to change drug concentrations. A dose that works well for a non-pregnant woman may fail in pregnancy or, conversely, lead to toxicity if elimination is slowed. Postpartum, these physiological values gradually return to baseline, requiring careful medication review during the transition.