8 - Drugs in early pregnancy

Summary

Introduction

Although most women are screened in the community and referred to highrisk consultant-led antenatal clinics if required, clinicians are faced with early pregnancy drug-exposure dilemmas when managing acute gynaecology emergencies. This chapter provides evidence-based knowledge regarding drug exposure in early pregnancy and help in improving the care of women in the emergency gynaecology setting (Box 8.1).

Pregnant women on average take between one and four medications during pregnancy, not including vitamins or minerals. About 80% of women use some form of medication during the first trimester of pregnancy. Nearly 70% of these are over-the-counter medications and 30% are prescriptions.

Before prescribing medicines or over-the-counter drugs to pregnant women, it is important to understand the physiological changes during pregnancy that have a marked impact on the pharmacokinetics and placental transfer of drugs. Increased cardiovascular output and expansion of plasma volume reduce drug concentrations and the drug-binding capacity of the circulating proteins. Other changes include increased renal and endocrine function, reduced lung capacity and gastrointestinal motility and changes in liver metabolism. Finally, as the placenta is primarily a lipid barrier, many of the administered drugs cross the placenta by passive diffusion and reach the fetus. Low-molecular-weight, lipid-soluble, non-ionised drugs cross the placenta more readily. The effects of drug exposure during early pregnancy depend on the developmental stage of the embryo or fetus. There are three main stages in the development of the conceptus:

• Predifferentiation or pre-embryonic stage (from conception until 17 days postconception).