Pregnancy: Other considerations

Last updated: Tuesday, September 27, 2022

It is easy to focus solely on the potential harm to the embryo or fetus when advising on the use of medicines in pregnancy, but there are other important aspects of prescribing in pregnancy to be aware of. Monitoring of any chronic medical condition should be intensified during pregnancy because the pattern of disease may change (improve, worsen) as well as the response to medicines.

In particular, drug pharmacokinetics will change in the mother. For example, increases in plasma volume result in lower serum concentrations of drugs that are predominantly held in the plasma, that is those with a low volume of distribution (e.g. aspirin, phenytoin). A reduction in serum albumin concentrations may result in higher levels of the free fractions of some protein bound drugs (e.g. phenytoin, diazepam). Increases in renal function may affect the clearance of drugs excreted by the kidney (e.g. ampicillin, gentamicin).

It is important to remember that some of these parameters will quickly revert back to their pre-pregnancy levels and that dose adjustments may be required soon after delivery. For example the changes in drug metabolism that require lamotrigine doses to be significantly increased during pregnancy rapidly return to normal postpartum, requiring close monitoring and prompt dose reductions to avoid toxicity.

When advising on a medicine for use in pregnancy don’t forget the 'normal' contraindications and precautions as they apply to the mother (e.g. avoid recommending labetalol for hypertension in a pregnant patient with asthma). Preventable side effects from medication can reduce maternal compliance, but major side effects might also threaten the pregnancy.

Finally, note that all women should take folate supplements from the time pregnancy is planned and for the first 12 weeks of pregnancy to reduce the risks of neural tube defects. Most women should take 400 micrograms daily, but there are exceptions. For example, women taking antiepileptic medication, those on proguanil for malaria prophylaxis, women with diabetes or who are obese, and women who have previously had a child with neural tube defects should take 5mg daily.

You can read more about folic acid use in pregnancy on the bumps website and read about other dietary advice in pregnancy on the NHS website.

Paternal exposure to medicines

You may be asked about paternal exposure to medicines and the risks this may pose to conception or the development of the embryo or fetus. Common scenarios include the time to wait between stopping a drug and trying to conceive or the risks to an unplanned pregnancy if the father is taking a particular medicine. There are several key points to consider, including:


Most medicines probably don’t pose a significant risk but there are exceptions including cytotoxic agents. If a patient has been exposed to such a medicine then in theory he should wait about 6 months (2 spermatogenic cycles) before conception is planned.

It is theoretically possible that embryonic or fetal exposure to a drug or its active metabolites may occur through transfer via semen during sexual intercourse to the pregnant mother. The drug or its metabolites may have a direct effect on the uterus, or the embryo or fetus may be exposed through the systemic circulation if the drug is absorbed through the vaginal mucosa. However, studies suggest that transfer of an agent through the cervix is unlikely and any uptake via the vaginal mucosa is expected to be extremely low and below clinically meaningful levels.