Clearance in pregnancy

Clearance in pregnancy

The physiological changes in pregnancy will lead to changes in a drugs clearance. The increase in renal blood flow leads to an increase in the glomerular filtration rate and renal drug elimination is generally enhanced. Hepatic drug metabolism may either increase, decrease or remain unchanged. These changes occur early in gestation but are most pronounced in the third trimester of pregnancy. The changes in metabolism and excretion are shown in the next slides.





The pharmacokinetics of tobramycin (an aminoglycoside antibiotic) displays alterations in clearance over pregnancy:

  • clearance is higher in mid-trimester with a corresponding shorter half-life
  • clearance is lower in the third trimester with a corresponding longer half-life

A woman is treated for years with the same daily oral dose of lithium with controlled release. The lithium plasma-concentration is 0.7-0.9 mmol/l. She becomes pregnant. Lithium does not bind to protein and is not metabolized by the liver. Her lithium plasma level will…..


A woman becomes pregnant. She uses the same medication as before pregnancy. You establish that the clearance of the drug doubles and that the volume of distribution triples compared to their levels before pregnancy. What do you expect regarding the half-life? It will