Objective:To determine the levels in serum of tissue inhibitor of metalloproteinases (TIMP) in pregnancy and to examine the possibility of a time course in relation to parturition, both term and preterm.Methods:Serum tissue inhibitor of metalloproteinases was measured using an enzyme‐linked immunosorbent assay. A cross‐sectional study was conducted in 333 women during pregnancy, labor, and the postpartum period and in 27 nonpregnant volunteers. Longitudinal data were obtained from 22 women who provided a sample at term, during labor, and in the postpartum period.Results:In uncomplicated pregnancies, serum TIMP levels were low from the onset of pregnancy until 37 weeks' gestation, in comparison to levels in nonpregnant women (P< .001). During the final weeks of pregnancy, levels rose and at 37‐42 weeks were similar to nonpregnant levels. The levels did not change with the onset of labor. Serum concentrations of TIMP obtained during preterm labor were elevated compared to a control group of patients at a similar gestation who subsequently delivered at term (P< .01). Serum TIMP levels were significantly higher during the postpartum period than at all other times (P< .001).Conclusions:Changes in serum TIMP levels during and after pregnancy may parallel the remodeling of the extracellular matrix that takes place throughout this period. Further work is necessary to evaluate the prognostic value of TIMP for preterm labor.(Obstet Gynecol 1994;83:532‐7)