Pre-conception and Early Pregnancy Impacts
The negative impact of toxic substances can begin long before actual pregnancy occurs. Mutagenic action is characterized by direct damage to the genetic apparatus (DNA). This effect impacts the germ cells of the partners before conception and may also extend to somatic cells.
Upon successful fertilization, the period of potential embryotoxic action begins (derived from the Greek word embryon, meaning 'germ' or 'embryo'). This type of toxic influence spans the first twelve weeks of the first trimester, but its specificity is most pronounced in the earliest stages—from the first to the third week of pregnancy.
During this timeframe, corresponding to the blastocyst stage, the fertilized egg (zygote) is located within the lumen of the fallopian tubes and uterine cavity until implantation is complete. Drug exposure during this period follows the strict 'all-or-none' principle: gross developmental disruptions inevitably lead to embryonic death, or, if damage is avoided, the embryo continues to develop absolutely normally. Drugs with proven embryotoxic effects include antimitotic agents (e.g., colchicine) and antimetabolites (mercaptopurine, fluorouracil).
Teratogenic Action and Organogenesis Impairment
The term teratogenic action originates from the Greek root teras, meaning 'monster' or 'malformation'. This effect represents a gross disruption of organ and system formation—organogenesis. As a result of such interference, severe congenital skeletal malformations and major anomalies in the internal organs of the embryo arise.
The physiological context of this phenomenon is that the critical period for teratogenic effects occurs between the third and twelfth weeks, with the absolute peak of danger at weeks 4–8 of pregnancy. It is during this temporal window that internal organs actively develop and skeletal elements form, making embryonic tissues maximally susceptible to damaging factors.
The high-risk group includes antineoplastic agents, specifically folic acid antagonists (a typical representative is methotrexate). Special attention should be paid to ethanol: its consumption is extremely hazardous, especially if it coincides with the moment of conception.
Historical Example: The Thalidomide Tragedy
The most famous, large-scale, and tragic example of delayed delayed embryotoxic catastrophe (type D) was the 'thalidomide disaster', which unfolded between 1956 and 1962.
The drug thalidomide was widely prescribed to pregnant women in early pregnancy as an effective antiemetic and sedative (sleeping aid) agent. Its use spanned Western Europe, the Americas, and Australia. The consequence of taking this medication was the birth of many children with specific congenital limb malformations.
This pathology received the medical name phocomelia (derived from the Greek words phoke meaning 'seal', and melos meaning 'limb' or 'body part'). In phocomelia, a child's arms and legs remain underdeveloped and visually resemble seal flippers.
Fetotoxic Action in Later Stages
As the critical first trimester and basic formation of all anatomical structures conclude, the fetal period begins. It starts after the twelfth week of pregnancy and continues until delivery.
The negative impact of medications at this stage is classified as fetotoxic action. Its key distinction is the absence of gross anatomical malformations, as the fetal organs have already formed. Instead, drugs provoke functional disruptions of organ systems or significantly impair the maturation processes of fully formed fetal organs.