Prescribing in pregnancy

There are marked and progressive physiological changers during pregnancy which can alter pharmacokinetics. And also drug given in pregnancy can affect the baby. So special attention should be given to prescribing in pregnancy.

Alter pharmacokinetics

Absorption 

Delay absorption due to reduce gastrointestinal motility. So oral drug get some time to absorption. But no magor defect in drug absorption. In acute conditions intramuscular route can use because vasodilation lead to increase tissue perfusion.

Distribution

Total body water increase by to 8 liters. So volume of distribution increase in water soluble drugs.
Plasma albumin level get decrease and α₁ acid glycoprotein get increase. So free fraction of acidic drugs get increase and free fraction of basic drug get decrease.
Body fat increase by about 4kg. It act as a reservoirs for lipid soluble drug.

Metabolism

Hepatic microsomal enzymes undergo induction. so many drugs metabolise faster. The placenta also contribute to part of this.

Elimination

Renal plasma flow get doubles and so rapid elimination in renal eliminating drugs.
Eg:- Amoxicillin dose should double in systemic infection.

Affect the foetus

The lipid soluble drugs and drugs which have low molecular weight (less than 600) enter to foetal blood. But water soluble compounds and drugs which have high molecular weight does not enter to foetal blood. But in prolonged use any drug can enter to foetal blood to some extent.
During first trimester drugs can produce congenital malformation (teratogenesis). The greatest risk carried out from third week to eleventh week.
During second and third trimester drugs can produce growth or functional abnormalities in foetus.
Drugs give shortly before labour Can have adverse effect on labour.

Important
Avoid newer drugs, unless safety is clearly established.
Use the minimum effective dose.
Use drugs for the shortest period necessary.