Prescription ,Over the Counter (OTC) and Complementary Medication /

General considerations

01 In Brief

Any drug or chemical taken during pregnancy is able to cross the placenta from the mother to the fetus from as little as five weeks after conception. Between 17 to 70 days post conception is the most dangerous period, where the teratogenic effects of drugs can result in major malformations of your baby. However the nervous system continues to develop in a complex way throughout pregnancy and the effects on neurological function can occur beyond 70 days until delivery and are often subtle e.g. intellectual, learning and behavioural problems.

02 What Do I Need To Know?

• Any drug or chemical substance given to the mother during pregnancy is able to cross the placenta to some extent, depending on its chemical size and lipid solubility. These include: 
  • Prescription medicines;
  • Over the counter medicines;
  • Nutrition supplements e.g. vitamins; and
  • Complementary therapies e.g. herbal remedies.

TIMING OF EXPOSURE TO MEDICATION IN PREGNANCY

• Transport across the placenta of medications ingested by the mother to the fetus and the fetus to the mother is established by 5 weeks of gestation. During the first two weeks of development, from conception until the date of the first missed period, the fetus is thought to be resistant to the teratogenic effects of medication.
• The critical time is when the organs start to develop; about 17 days post conception until completion by 60-70 days. Exposure to certain medication during 17-70 days can cause major birth defects. Exposure beyond 70 days is not associated with major birth defects.
• Some medication beyond 70 days can cause functional problems especially in the nervous system; which continues to develop in a complex way. Damage may still occur in the second and third trimester, such as intellectual disability, learning and behavioural problems, deafness and cerebral palsy.

 HOW DOES MEDICATION AFFECT THE DEVELOPING FETUS

• How medications cause teratogenic effects in humans is not well understood but may be related to the type of drug, how it is taken, the dose, frequency, whether it is used with other medications, the timing in relation to gestation and other factors such as an individual response to the medication.  
• They may cause harm in variety of ways. They may:    
  
  • Affect the maternal receptors and hence have an indirect effect on the fetus;
  • Have a direct effect on embryonic development resulting in specific abnormalities e.g. thalidomide;
  • Affect the nutrition of the fetus by blocking nutrients;
  • Have subtle effects on intellectual, social and functional development;
  • Have delayed effects e.g. adenocarcinoma of the vagina in young females whose mothers took Diethylstilbestrol (a synthetic oestrogen that was used to treat symptoms in early pregnancy). 

• When medication is prescribed in pregnancy, the treating health professional must consider the balance between the alleviation of maternal suffering and the therapeutic benefits of the drug to the mother against the potential for harm to the fetus.

Last updated: 27/07/2011 by Dr Elizabeth Hallam*/Claire Galea

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