Any medication given to the mother can have immediate or future effects on the fetus. The same drug will act simultaneously on two people whose physiological and metabolic capacities are totally different.

For example, the fetus does not have, unlike the adult, a hemoencephalic barrier that protects the brain from pathogenic or medicinal substances. In addition, its ability to break down drugs is significantly reduced, which makes the dosage very dangerous.

An ineffective barrier

However, there is indeed a “barrier” at the level of the placenta, but its role is in practice rather minimal in terms of limiting the passage from mother to child. Only large molecules such as heparin or insulin do not cross it. Other drugs are slowed down: it takes about an hour for the fetus to reach 10% of the maternal level of a drug.

The first trimester and the law of all or nothing

During the first 8 days of gestation, either the drug has no effect or it causes the death of the embryo. It is the law of all or nothing.

Subsequently and until the second trimester, the risk of inducing congenital malformations is major: the drug is then said to be teratogenic. Overall, it is possible to schematize 5 situations, categorized from A to E:

• A- Controlled studies in women have shown that drugs in this class do not present any risk during the 1st trimester. However, these studies are few in number and do not eliminate the risks for the other quarters.
• B- Animal studies have not shown any risks but there are no studies in women. This is the case with most recent drugs that have been the subject of regulatory studies in animal toxicology.
• C- This class includes drugs for which there are no studies in animals or in women. These are usually old drugs that should only be prescribed if the expected benefit outweighs the risk to the fetus.
• D- For drugs in this class, there is evidence of a risk to the human fetus but the expected benefit is often acceptable in relation to the risk.
• E- Drugs not to be prescribed: there is evidence of a fetal risk (teratogenic) and the therapeutic benefit is lower than the risk.

The rest of the pregnancy

Some medicines have well-known side effects in the fetus throughout pregnancy and should not be given to pregnant women.

Examples:

• tetracyclines bind to bones and dental blanks inducing dental abnormalities;
• aminoglycosides are toxic and cause deafness problems;
• hormones entail risks of inducing endocrine pathology;

At birth, the child can keep drugs in his body that are administered shortly before childbirth. However, without the mother’s immunity, he will no longer have the means to eliminate them. They are likely to cause serious side effects during the first days of life.

What if treatment is unavoidable?

If a prescription is essential, it must be adjusted according to the stage of pregnancy and the benefit / risk ratio assessed. When possible, it is essential to prescribe well-known drugs for which we have a certain distance.

If a drug was taken without knowledge of the pregnancy, it is possible to find information on the risk incurred by contacting a Pharma-covigilance center.

Finally, it should be noted that the side effects of all drugs tend to increase in pregnant women. The physiological changes inherent in pregnancy alter the body’s response to drugs. The increase in cardiac output, the increase in fat mass, or the secretion of additional hormones mean that the normal doses of drugs (including those of banal substances) are no longer suitable.