Taking medicines during pregnancy

Medicines taken during pregnancy have the potential to do harm to both the foetus and the mother. They may also display unpredictable efficacy because of altered handling compared with medication taken outside of pregnancy (Table 1).

Table 1

The effects of pregnancy on pharmacokinetics/ pharmacodynamics
Effects Drugs affected (levels change)
Absorption Reduced gastrointestinal motilityIncreased gastric pH None known
Distribution Increased volume of distributionDecreased albumin Sodium valproate (↓)
Metabolism Altered liver enzyme activity (cytochrome 450s and UGT) Metoprolol (↓), phenytoin (third trimester) (↓), morphine (↓), lamotrigine (↓), caffeine (↑)
Excretion Increased renal secretion peaking in second trimester beta-lactams (↓), Low molecular weight heparins (↓), aminoglycosides (↓)

Sodium valproate – anticonvulsivant medicine

Metoprolol –antihypertensive medicine, beta-blocker

Phenytoin – anticonvulsivant medicine

Morphine – painkiller medicine, opioids

Lamotrigine – anticonvulsivant medicine

Beta-lactams – antibiotibiotics class; e.g.: penicillin, amoxicillin, ampicillin, etc.

Low molecular weight heparins – anticoagulant medicine

Aminoglycosides – antibiotics class: kanamicin, streptomicin, amicacin, etc.

Any decision to use drug therapy should, where possible, be made in collaboration with the mother, ensuring that she is informed of any known risks or benefits of both taking and not taking the medication.

Where drug therapy is deemed necessary, the chosen therapy should be the least likely to cause harm, used at the lowest possible dose; taking into consideration maternal, foetal and drug factors (Table 2). In some instances, it may be feasible to withhold treatment at certain times (for example, in the first trimester during organogenesis to reduce the risk of congenital abnormalities, or prior to delivery to avoid the problem of withdrawal).

Table 2

Factors to consider when prescribing in pregnancy
Maternal factors Foetal factors Drug factors
Implications of not taking the drugMaternal choiceGestationComorbidities  Risk of congenital malformations (especially weeks one to eight)Risk of organ toxicity (e.g. renal toxicity)Withdrawal postpartum  Altered absorption, distribution, metabolism, excretionNarrow therapeutic indexSafer alternativeAbility to cross placentaTopical versus systemic

Adverse effects

Any woman known to have been exposed to a known or potential teratogen (Table 3) during pregnancy will require referral to an obstetrician for further management. This may involve more intense monitoring with additional scans to identify any possible congenital malformations.

Table 3

Examples of drugs known to be teratogenic
Drug Teratogenic effect
Anticonvulsants Microcephaly, major malformations
Sodium valproate Spina bifida
Retinoic acid, isotretinoin Intellectual disability
Thalidomide Phocomelia
Warfarin Chondrodysplasia punctate
Methotrexate Severe malformations (e.g. cranial defects)
Lithium Cardiac anomalies

Microcephaly – medical condition in which the circumference of the head is smaller than normal because the brain has not developed properly or has stopped growing.

Spina bifida – birth defect where there is incomplete closing of the backbone and membranes around the spinal cord. There are three main types:

  • spina bifida occulta
  • meningocele
  • myelomeningocele

Phocomelia – is an extremely rare congenital disorder involving malformation of the limbs (dysmelia) (Fig. 1).


Figure. 1: The effects of thalidomide administration during pregnancy

Chondrodysplasia punctata – clinically and genetically diverse group of rare diseases, first described by Erich Conradi (1882–1968), that share the features of stippled epiphyses and skeletal changes.

When you meet with your doctor to confirm you’re pregnant, ask what meds are OK to take and what meds you need to find alternatives for. Your health careprovider will weigh the risks and benefits to help you know what’s safe.

Also, tell your doctor about any alternative medicines or supplements you take, even if the label says “natural.” And if you get any new prescriptions while you’re pregnant, make sure the people who prescribe them know that you’re pregnant.

Some supplements/aromatherapy essential oils (based on some specific plant extracts) are dangerous!

Avoid these oral supplements: Arbor vitae, beth root, black cohosh, blue cohosh, cascara, chaste tree berry, Chinese angelica (dong quai), cinchona, cotton root bark, feverfew, ginseng, golden seal, juniper, kava kava, licorice, meadow saffron, pennyroyal, poke root, rue, sage, St. John’s wort, senna, slippery root, tansy, white peony, wormwood, yarrow, yellow dock, and vitamin A (large doses can cause birth defects).

Avoid these aromatherapy essential oils: Calamus, mugwort, pennyroyal, sage, wintergreen, basil, hyssop, myrrh, marjoram, and thyme.

To sum up, it is vital to use safe sources to be informed and to discuss any doubt concerning the pregnancy with our healthcare providers: gynecologist, general physician, pharmacist.

Follow our articles about pregnancy and consider every detail concerning the mother’s and the fetus’s health.

The Pharmacists


The pharmaceutical journal

ANMDM (Agenția Națională a Medicamentului și a Dispozitivelor Medicale)


National Institute of Neurological Disorders and Stroke

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