Suggestions for Prescribing Medication in Pregnant Women

    When possible:

• Use medication only if absolutely indicated.
• Avoid initiating therapy during the first trimester.
• Select a medication with a proven track record in human pregnancy.
• Use a single-agent.
• Use the lowest effective dose.
• Discourage the use of over-the counter drugs that might interact with prescription medications.

  Modified from:  Norwitz ER, Greenberg JA.Antibiotics in pregnancy: are they safe? Rev Obstet Gynecol. 2009
   Summer;2(3):135-6. PMID: 19826569

Factors that Appear to Determine the Extent of Placental Transfer of Drugs Administered to the Mother [1-5]

• The molecular weight of the drug; Drugs with molecular weights (MW)  less than 500 D appear to cross the placenta easily, drugs with MW greater than 500  have  incomplete transfer across the human placenta. Drugs with very high MW , such as insulin with a M.W ~ 5800 do not normally cross the placenta.
• The pKa (pH at which the drug is 50% ionized); Strongly dissociated acid drug molecules should have an incomplete transfer, but this does not seem to be an absolute rule
  
• Physiological properties of the placenta (blood flow, gestational age)
• Expression and activity of metabolizing enzymes and efflux transporters (e.g. ATP-binding cassette (ABC) transporters ). For example placental 11beta-hydroxysteroid dehydrogenase 2 (11beta-HSD2) enzyme inactivates prednisolone by interconversion into
  prednisone , and ABC transporters pump tenofovir disoproxil fumarate  from trophoblast back to maternal circulation.
• The extent of drug binding to plasma protein does not appear to influence the type of drug transfer across the human placenta to a significant degree.

Prescription Drugs of Choice During Pregnancy -perinatology.com
      Medications recommended as first line agents during pregnancy by various organizations and authorities .

Over-the Counter Medications of  During Pregnancy

Drugs in Pregnancy and Lactation: A Reference Guide to Fetal and Neonatal Risk Classic text by  Briggs GG, Freeman RK, Yaffe S.J.