A woman with generalized seizures well controlled with lamotrigine becomes pregnant and has breakthrough seizures. What is most likely happening?

During pregnancy, various physiological changes can significantly alter the pharmacokinetics of medications, including antiseizure drugs like lamotrigine. As a woman’s body changes to support the developing fetus, several factors influence drug metabolism.

In the case of lamotrigine, pregnancy tends to increase the volume of distribution and enhance the metabolic clearance of the drug. This means that, although a patient may have had well-controlled seizures prior to pregnancy, the body metabolizes lamotrigine more rapidly during pregnancy, which can lead to decreased plasma concentrations of the drug. Consequently, these lower levels of lamotrigine can result in breakthrough seizures, despite the patient's prior control over her epilepsy with the same medication dosage.

Thus, the scenario indicates that the lamotrigine concentrations are decreasing due to the physiological changes related to pregnancy, leading to less effective seizure control and the emergence of breakthrough seizures.