Prescribing for the pregnant patient

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ABSTRACTPrescribing in pregnancy can be challenging for providers facing insufficient information about drug safety, overestimation of the risk of medications by both the patient and the care provider, and increasing litigation costs. This article provides key concepts to consider when prescribing for a pregnant patient and offers practical advice for choosing the safest possible drug treatments.


  • There is no protective physiologic barrier between the maternal and fetal environments.
  • The gestational stage may determine the effect of a medication on the fetus.
  • The physiologic changes of pregnancy affect the pharmacokinetics of medications.
  • Sole reliance on the US Food and Drug Administration’s pregnancy safety category may be inadequate.
  • Key questions: Is the problem self-limited or amenable to nonpharmacologic management? How do the patient’s (and provider’s) presumptions affect decisions about this medication in pregnancy? How does pregnancy affect the problem, and how does the problem affect pregnancy?



Primum non nocere: First, do no harm—a principle taught across the world to all medical students. It reminds the health care provider to consider the possible harm that any intervention might produce. Never is it more relevant in the mind of a clinician than when prescribing a medication for a pregnant woman. We are, after all, brought up in a society averse to medical risk.

When managing a pregnant patient, should the baby be the highest priority, whatever the mother may face? Or to take the extreme opposite position, should the mother be treated with the best possible options and the baby ignored?

And what about the views of the patient? There is a widespread cultural belief about the vulnerability of the mother and fetus during pregnancy. Therefore, when faced with the decision of whether to use a medication or not, what is the best recourse for the pregnant patient? Should she be the “good mother” and avoid all risk to the baby, or should she be the “responsible mother” who follows medical advice and takes treatment as recommended?

In truth, the path to safe management of a pregnant patient is rarely so dichotomous. In most cases, what is best for the mother is also best for the baby. However, caring for a pregnant or lactating woman can be challenging for clinicians facing insufficient information regarding medication safety, overestimation of the risk of medication by both the patient and the care provider, and increasing litigation costs.

This article provides key principles to guide clinicians caring for pregnant patients, as we find ourselves increasingly dependent on pharmacotherapy. It also includes sources of information clinicians can turn to when they need additional pregnancy safety data about a certain drug and when they want advice about conditions commonly seen in pregnancy and medications that can be justifiably used in those circumstances.


The following concepts are key to prescribing for a pregnant patient:

No protective barrier exists between the maternal and fetal environments

The placenta contains a semipermeable membrane that selectively allows some substances to pass from the maternal to the fetal blood and excludes others. However, it is not really a “protective mechanism” when it comes to medications. Assume that the fetus will have exposure, at least to some degree.

In general, drugs that are lipophilic, of a low molecular weight, or not ionized at physiologic pH cross the placenta more efficiently than others. Heparin and insulin are notable exceptions to the rule that most drugs cross the placenta. They do not.

The gestational stage may determine the effect of a medication on the fetus

In animals and in humans, exposure of the embryo or fetus to a teratogen may produce a permanent abnormality of structure or function.

First-trimester exposures are most worrisome for structural malformations. However, fetal neurologic and behavioral development, fetal survival, and function of specific organs can be affected even after the first trimester. For example, while first-trimester exposure to angiotensin-converting enzyme inhibitors has been linked to a slight increase in congenital heart defects, exposure in the second or third trimester can result in fetal oligohydramnios, neonatal anuria, pulmonary hypoplasia, intrauterine growth restriction, and fetal death.

Physiologic changes of pregnancy affect the pharmacokinetics of medications

Pregnancy is associated with increased plasma volume, increased glomerular filtration rate, and dilutional hypoalbuminemia, which can all affect the bioavailability of medications. Absorption of oral agents also may be affected by slowed gastric motility in pregnancy.

Although these physiologic alterations do not routinely warrant a change in drug dosage, they may be important considerations when choosing an appropriate agent. For example, medications taken in multiple doses per day are more likely to have a sustained effect than once-daily medications, which would be rapidly cleared in a pregnant patient.

Sole reliance on the FDA pregnancy safety category may be inadequate

To help clinicians prescribe medications for pregnant women, the US Food and Drug Administration (FDA) assigns medications to one of five categories of risk (A, B, C, D, or X) (Table 1). Unfortunately, this classification system has several shortcomings:

  • The categories are often seen as a grading system in which the risk increases from the lowest in category A to highest in category X, and the safety information in the accompanying narrative is not always appreciated by prescribers.
  • Clinicians incorrectly assume that drugs in a particular category carry a similar risk. However, 65% to 70% of all medications are in category C. This category includes medications with adverse animal data or no animal data at all. In addition, adverse animal data may vary in severity from decreased fetal weight to major structural malformation and fetal loss, indicating a difference in expected risk.
  • Most of the data on medication safety in pregnancy comes from animal studies, case reports, case series, case-control studies, or pregnancy registries, and each of these sources has significant limitations.
  • The categories do not distinguish between supporting data from animal studies and human studies. For instance, a category-B drug may have animal studies that show no risk but no adequate human studies, or may have animal studies showing risk but human studies that do not.

Looking at the pregnancy risk classifications used in the United States (ie, the FDA system), Australia, and Sweden, researchers compared the classification of 236 drugs between the three systems and found that only one in four drugs was similarly classified into the same risk category. This discrepancy further brings into question the usefulness and reliability of these classifications.1

Finally, none of the classification systems tells us the potential harm from withholding a medication in pregnancy.

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