Pentraxin 3 (PTX3) is an inflammatory molecule that has been reported to be a promising early biomarker for subsequent preeclampsia. The levels of PTX3 vary during pregnancy and it is thus a need to establish reference intervals during normal pregnancy. Repeated blood samples were collected from 52 healthy pregnant females. The samples were divided according to collection time into the following groups: week 7-17, week 17-24, week 24-28, week 28-31, week 31-34, week 34-38, before delivery and after delivery. The samples were analyzed for PTX3 with a sandwich ELISA and the 2.5 and 97.5 percentiles for each sample period was calculated. There was a continuous increase of serum PTX3 as pregnancy progressed. The increase was most evident after week 31 with the highest levels just before delivery.