The aim of this study is to determine whether blood levels of lithium or sertraline are affected by different phases of the menstrual cycle and whether there is an effect on psychiatric symptoms. Subjects are seen for two visits: one visit during the luteal phase and one visit during the follicular phase of the menstrual cycle. On each visit, they will fill out a depression, anxiety and mania rating scale. Also at each visit a 20mL blood sample will be drawn to measure progesterone level and either a lithium or sertraline level, depending on which medication the patient takes. The primary hypothesis in this study is that blood levels of lithium and sertraline will be significantly lower in women during the luteal phase of the menstrual cycle than during the follicular phase. Examination will also be made of whether symptoms will increase in severity during the luteal phase as compared to the follicular phase. The investigators expect a negative linear association between symptom severity and blood level, i.e. expect symptom severity to worsen as blood levels of lithium or sertraline decrease.