- Drugs with molecular mass of less than 1500 easily cross the placenta, while drugs with a molecular mass of more than 1500 cross the placenta with difficulty,or not at all. Heparin is an example of the latter.
- Since the majority of drugs cross the placenta, most of this cannot be considered to be safe in pregnancy. Drugs should therefore be avoided as far as possible, especially during the first trimester. The classic teratogenic period is form day 31 after LNP in a 28-day cycle, to71 days from the LNP. During this critical period, the organs are being formed.
- However, the use of certain drugs may be essential, despite possible detrimental effects.
- Certain drugs are absolutely contra-indicated during pregnancy.
Drugs Which Are Normally Prescribed During Pregnancy
- Iron tablets (one to two per day) are normally prescribed to provide the additional iron requirements of pregnancy. It is however best avoided in the first trimester, as it may worsen nausea and vomiting, which occur commonly at this stage of pregnancy. Besides, iron requirements are negligible during the first trimester. The iron tablets should preferably be taken in the evening with a glass of water, as several nutrients may impede absorption (e g tea, cocoa, bran, wheat-germ, lentils, butterbeans, spinach) Nausea and Vomiting is also less likely to occur in the evening or at night.
- Folate tablets (one per day) are sufficient to provide the additional folate requirements of pregnancy. These are especially indicated for patient with an insufficient diet.
Drugs Which Are Sometimes Prescribed During Pregnancy
- Fluoride is sometimes given in solution to patients in area where the water has an inadequate fluoride content.
- Vitamin preparations are sometimes prescribed for patients with an insufficient diet. With a balance diet it is unnecessary. An overdose of vitamin A or D can be teratogenic.
Drugs Which May Be Necessary, Despite Possible Detrimental Effects
These are drugs required for serious material diseases. Where stopping the drug will endanger the mother’s life or seriously impair her health. Examples of these are hypotensive drugs. Systemic corticosteroids, anti-convulsive drugs, anti-thyroid drugs and cytotoxic drugs. Specialist supervision is necessary if these drugs are used during pregnancy.
Drugs Which Are Absolutely Contra-indicated
These are drugs where the risk of teratogenicity is too great, or where safer alternatives are available. Examples of these are as follows:
- Thalidomide (proven teratogenic effects).
- Folic acid antagonists e g methotrexate (proven teratogenic effects).
- Retinoids (proven teratogenic effects).
- Lodides and radio-active iodine (can cause foetal hypothyroidism).
- Live viral vaccines (cause foetal viraemia with possible teratogenic effects).
- Diethylstilboestrol (danger vaginal adenosis and clear-cell carcinoma of the vagina when a female foetus reaches adolescence).
- Androgens (virilisation of female feotuses).
- Danazol (virilisation of female feotuses).
- 19-Nortestosterone progestogen derivatives (virilisation of female fetuses).
- Estrogen/progestogen combinations (possibility of the VACTREL-syndrome, I e vertebral, anal, cardiac, tracheal, renal, oesophageal and limb abnormalities).
- Alcohol is best avoided, as the precise effects of social alcohol consumption on the foetus are known. A single episode of excessive alcohol consumption can cause the foetal brain damage. Prolonged, excessive alcohol consumption can cause the foetal alcohol syndrome, which is characterised by intra-uterine growth retardation, mental retardation, facial abnormalities e g micrognathia (abnormal congenital smallness of the lower jaw). A thin upper lip, hypoplasia of the maxilla, short palpebral fissures, epicanthic folds, ptosis, cardiac abnormalities, limb abnormalities and retarded neonatal development.
- Tetracyclines (may impede foetal bone development; can precipitate maternal liver failure-safer alternatives are available).
- Sulphonamides (displace bilirubin from albumin with increase in conjugated bilirubin and consequent danger of kernicterus-safer alternatives are available).
- Reserpine (if administered within 48 hours before delivery it can cause nasal congestion, hampered breathing, foetal bradycardia and hampered temperature regulation-safer alternatives are available).
Drugs And Breast Feeding
Most drugs taken by the lactating mother are excreted in the breast milk. Drugs which may have detrimental effects on the baby, e g alcohol, should therefore be avoided. If a drug which may have detrimental effects on the baby, is considered to be essential, e g cytotoxic drugs, breast feeding should be stopped.
Because of the large number of drugs available, you will usually not be expected to know the effects of all drugs in pregnancy. However, it is important to have a concept of principles of drug use during pregnancy. A knowledge of drugs which are absolutely contra-indicated is also important. Before prescribing a drug in pregnancy, it is advisable to consult manuals like the MDR (Mims Desk Reference, ed D Botha. MIMS Subscription Department, Pretoria) and/or the South African Medicines Formulary (eds EA Conradie and JL Straghan, Publications Division, Medical Association of South Africa, Pinelands).
The Author: Johan Theron Nel