The amount or levels of some antidepressant medications in the blood (e.g. selective serotonin reuptake inhibitors or SSRIs) seem to be decreased during pregnancy. If the levels of SSRIs in the blood are low, the medication may not be adequately treating depression. The objective of our study is to evaluate whether pregnant women have decreased levels of SSRIs and a resultant decreased improvement of depression with SSRIs. We also want to evaluate whether variations in the genes related to how the body eliminates the SSRIs or how the SSRIs work influence the improvement in the depression pregnant women experience from these drugs. We will conduct a study in 100 pregnant women who are receiving fluoxetine (n = 25) or sertraline (n = 25) for the treatment of depression, who are not receiving any medication for depression (n = 25), or who are pregnant no history or treatment for depression (n= 25). Participants will be enrolled any time before 26 weeks of pregnancy. Blood samples will be collected for genetic analysis in all subjects, and for drug level measurements during pregnancy, at delivery, and post partum in the subjects taking antidepressant medication. We will also be monitoring monthly platelet serotonin levels in all groups. In addition, the scores of a rating scale to measure the severity of depression and two rating scales to measure the undesirable effects of the treatment will be obtained on enrollment and monthly for up to 12 weeks after delivery. Maternal and umbilical cord blood samples will be obtained at delivery for determination of SSRI levels, platelet serotonin levels (a marker that SSRIs are working), and for collection of neonatal DNA. After delivery all mothers will complete a survey about how she is feeling about herself and the baby, and the baby's behavior will be observed for a 1 hour time interval. Babies of mothers taking medication will have a blood sample drawn at 2-3 months after birth to measure the drug level as compared with cord blood sample at delivery. We plan to collect a breastmilk sample from the mothers in the medicated arms that have chosen to breastfeed. Clinical data related to pregnancy and neonatal health will be obtained from the prenatal and neonatal charts.