General information on drug use in pregnancy: introduction

A drug can have more than one harmful effect on the fetus. Individual effects depend on the time of fetal exposure to the drug.

During the first 2 weeks after fertilisation and before full implantation, the embryo is thought to be resistant to any teratogenic effects of drugs. This is because there is no direct communication between maternal and embryonic tissue until after the placenta starts to form.

The critical period for teratogenic effects is during organogenesis. This starts at about 17 days after conception and is complete by 60 to 70 days. Exposure to certain drugs during this period (17 to 70 days) can cause major birth defects.

Some drugs can interfere with functional development of organ systems (eg central nervous system, integumentary system, cardiovascular system) in the second and third trimesters and produce serious consequences.

A woman may not be aware of her pregnancy until after the early stages of organogenesis. For this reason, drugs in the most severe category of risk (X in the Australian categorisation) should not be prescribed to a woman of childbearing potential, unless a pregnancy test is negative and she is using an effective method of contraception.

There are several conditions, however, in which long-term medication will be necessary in a woman of childbearing potential despite known harms of the drugs. At the time of initial prescribing in any such situation, the prescriber should discuss the desirability of reviewing medication requirements well before conception. For some disorders, it may be possible to change to a different category of drug. If a patient conceives while on medication and there has been no opportunity for earlier discussion with the prescriber, her medication should be reviewed as soon as possible.

The following check list may assist in deciding whether to prescribe a particular drug during pregnancy:

Nonpharmacological treatment: Is such a treatment available and likely to be successful? Would such treatment be reasonable at least until the first trimester is complete? Most pregnant women strongly favour this type of treatment and compliance is likely to be high.

Harm–benefit analysis (see also Box 11.1 for psychotropic drugs): For the particular drug under consideration, what are the potential harms and benefits to the mother and harms to the fetus of prescribing? What are the harms and benefits (for each) of not prescribing?

Incidence of spontaneous congenital abnormality: When drugs cannot be avoided, it may be appropriate to discuss the incidence of non–drug-related, spontaneous abnormalities. This is often underestimated. The incidence in Australia of significant congenital abnormality is 2% to 4% of live births, and minor abnormalities are recognised in approximately 15% of newborns.

Education, documentation and communication: Has the education of the woman and her partner regarding harms and benefits been properly documented in the patient's notes? Have those health professionals involved in obstetric management been informed? It may be appropriate to discuss the use and limitations of available antenatal screening to detect abnormalities in the fetus. Couples will need to give some consideration to the consequences of an abnormal result.

Routine review later in the pregnancy includes consideration of whether dose alteration is indicated during delivery to avoid neonatal problems such as respiratory depression.