Objective: To compare heparin sodium (100 United States Pharmacopeia U/mL) with 0.9% sodium chloride for use in the maintenance of intermittent intravenous (IV) devices during pregnancy. Methods: Women at 26-34 weeks' gestation who required serial phlebotomy were assigned randomly to heparin or normal saline flush, administered in a double-blind fashion. Catheter sites were examined and flushed with the study solution at least once every 6 hours. Partial thromboplastin times (PTTs) were measured at catheter insertion and 48 hours later. Statistical analysis was performed with Student t test, Mann-Whitney U test, Fisher exact test, log-rank, and χ2 analysis, as appropriate. Results: There was a significant increase in catheter patency rate at 48 and 72 hours in the heparin group (26 of 31 versus 17 of 33, and 21 of 31 versus nine of 33, respectively; P < .01). In addition, there was a significantly lower rate of catheter complications in the heparin group (four of 31 versus 13 of 33; P < .01). There were no differences in PTTs. Conclusion: During pregnancy, dilute heparin flush to maintain patency of intermittent IV site devices results in the following: a greater catheter patency rate at 48 and 72 hours after insertion of the catheter, a lower rate of catheter complications requiring therapy, and no alteration in PTT.