BACKGROUND Pregnancy is a particularly vulnerable time for exposure to indoor air pollutants, such as formaldehyde (FA), which is linked to spontaneous abortion, congenital malformations, and premature birth. OBJECTIVE To determine personal exposure to FA during pregnancy, and to identify the relationship between FA exposure levels and potential residential sources of FA. METHODS The study sample consisted of 140 pregnant women recruited from obstetrical clinics in Huntsville, Alabama. Formaldehyde exposure was measured by FA vapor monitor badges. Questionnaires were administered to participants to identify potential residential sources of FA. Urine cotinine, a surrogate for tobacco smoke exposure, was also used as an indicator of a possible source of residential exposure to FA. RESULTS The mean level of FA exposure by vapor monitor badge was 0.04 parts per million (ppm) (SD = 0.06; range 0.003-0.54 ppm). Minimum risk levels of 0.03 and higher were found in 36.4% of participants. Exposure levels of FA were higher in spring than winter (p < 0.001). Exposure levels of FA were correlated with indoor temperature of dwellings (p < 0.02), installation of new carpet within last 5 years (p < 0.04), and use of nail polish (p < 0.01). No relationship was found between FA exposure and urine cotinine levels. CONCLUSIONS Formaldehyde exposure may increase at various times in the lives of women; however, it is of particular concern during pregnancy because of perinatal risk to the exposed fetus.