Objective: For unknown reasons, a previous preterm birth (PTB) is a major risk factor for PTB in the current pregnancy. Our goal is to evaluate placental histology for clues related to the recurrent nature of PTB. Study design: Four hundred fifty-seven mother/infant dyads delivering between 23 and 32 weeks were first classified as having a spontaneous (S) or indicated (I) PTB, and then sorted into the following mutually exclusive categories by pregnancy history: 1) nulliparous; 2) having no previous PTB; 3) having any previous IPTB; or 4) having a previous SPTB. The placentas were evaluated for acute inflammation in the free membranes, umbilical cord, and chorionic plate, chronic inflammation in the membranes and decidua basalis, thrombosis in the chorionic plate and umbilical cord, and diffuse decidual leukocytoclastic necrosis (DDLN), a lesion associated with decreased placental perfusion. Results: Women who had a SPTB were far more likely (85.5 vs 14.4 P < .0001) to have a SPTB in the previous pregnancy, while women with an IPTB were significantly more likely to have had a previous IPTB (89.7 vs 10.3 P < .0001). Nulliparas and women with previous term births each had about 64% SPTB and 36% IPTB. Acute inflammation at any site was present in 73.9% of SPTB versus 8.0% of IPTB (P < .0001). Chorionic plate thrombosis was also more common in SPTB than IPTB (16.2 vs 7.6, P = .01). Chronic inflammation at any site was more common in IPTB than SPTB (21.0 vs 12.7%, P = .02), as was DDLN (46.5 vs 16.1, P < .0001). When classified by SPTB and IPTB in the current pregnancy, the histologic results were not further influenced by the previous pregnancy history. Conclusion: SPTB and IPTB are strongly repetitive. Women with SPTB are significantly more likely to have acute inflammation in the free membranes, chorionic plate, and cord, and chorionic plate thrombosis, while women with an IPTB are significantly more likely to have chronic inflammation and especially DDLN. Past obstetric history does not further influence the placental histology. © 2006 Mosby, Inc. All rights reserved.