Women can be prescribed a variety of drugs for treating a medical condition or a disease without considering the possibility of pregnancy. For example, a sexually-active young female suffering from recurrent convulsions are usually prescribed an antiepileptic drug-to be taken continuously. It is quite possible for such a woman to become pregnant while taking the anti-epileptic drug. Since most of the antiepileptic drugs are teratogenic, serious damage to the conceptus as well as a significant psychological burden on the mother may occur.
In general, the anticipated effect(s) of a drug use during pregnancy can be divided according to the fetal age, as follows:
From the 1st day until the 20th day after fertilization: Interestingly, teratogenesis is unlikely to occur during this period. However, the effect(s) of any harmful drug use during pregnancy in this period could lead either to the death of the conceptus or to no adverse effects at all, which is commonly known as the all-or-nothing effect,
From the 21st day until the 56th day after fertilization: The hallmark of this fetal period is characterized by organogenesis. Therefore, teratogenesis is most likely to occur at this stage of conception. The spectrum of harmful effects of a drug use during pregnancy in this period could range from spontaneous abortion, through teratogenicity (revealed as a gross anatomical defect), to a hidden embryopathy (like a permanent subtle functional or metabolic defect that may become noticeable later in life). Furthermore, the harmful effect(s) of a drug use during pregnancy in this period may result in increased rates of childhood malignancies (for example, a child of a mother treated, during pregnancy, with radioactive iodine for thyroid malignancy),
From the start of the second trimester until delivery: The exposure of the conceptus to a drug use during pregnancy in this period is unlikely to cause teratogenic effects because the stage of fetal organogenesis has completed. However, the exposure of the fetus to certain drugs during this period may lead to a change in the growth and / or function of the normally formed fetal tissues and organs. It is worth mentioning that at this stage, the contribution of the placenta in the metabolism of drugs rises. Therefore, for fetal toxicity to occur, higher doses are required to be used by the mother.
In general, the developmental and reproductive toxicological effects of a drug use during pregnancy on the conceptus vary depending on multiple factors. The fetal age at the time of exposure, which is described above, represents only one of the factors that determine the effect of the given drug on the fetus. Another important factor is related to a drug use during pregnancy itself, in terms of the dose, the duration, and the route of administration.
In addition, the factors related to the mother also play crucial roles in governing the occurrence and / or the severity of adverse events in the conceptus. For example, nausea, and vomiting associated with pregnancy may decrease the absorption and bioavailability of drugs taken orally.
Therefore, the same dose of a certain drug that is ingested by two different women, at the same gestational age, could lead to the development of adverse event(s) in one fetus but not the other.