It is unclear whether or not sexual intercourse (SI) in mid-pregnancy increases the risk for spontaneous labor onset (LO), secondary to prostaglandins, infection, or male-superior position during SI. We prospectively interviewed 83 women within 24 hrs of spontaneous LO at 22-35 weeks gestation using a battery of in-depth and directed questions related to life-style habits and sexual activity for 7 days prior to LO (“event day”). The controls were 238 women matched for gestational age attending the OB clinic; their “event day” was the interview day. Case-crossover, and case-control methods of analyses were used.Results: The maternal age, gestational age, and ethnicity were similar: 24 yrs and 31 wks (median, cases) and 21 yrs and 28 wks (controls); and 59% black and 22% Hispanic (cases), and 65% black and 31% Hispanic(controls). 7% of controls and 11% of cases reported SI within 24 hrs of the event (P=NS). The SI frequency ranged between 6-21% for the remaining 6 previous days, with no clear trend. Within group comparison (day 1 vs days 5 and 7), and between groups comparison (day 1), yielded odds ratios for LO with SI from 1.1 to 4.2, but the 95% CI included 1. In two subgroups (27-30 wks, and 31-35 wks) SI frequency 24 h prior to event was slightly higher in cases, but the difference was not significant: 12.2-13.6% cases, and 3.1-6.4% controls, P=NS. Condom use (0-7%), orgasm (55-67%), and male superior position during SI (56-65%) were similar. The reported cocaine abuse frequency was 1%(controls) and 4% (cases). In 10 cases LO occurred 1-27 hrs (median 4 hrs) following SI. Conclusions: In this SILO study, there was no clear association between SI and LO. Because the frequency of cocaine abuse(hisotry) was low, cocaine and SI influence on LO, if any, will be explored in this ongoing study.