Background Spontaneous preterm birth (sPTB) has a detrimental impact on perinatal outcome, as well as on the resources of health systems in high-income countries. Thus, the objective of the current study was to determine the incidence of modifiable risk factors in pregnancy and their impact on the rate of sPTB. Methods All nulliparous pregnant women, in singleton pregnancies, with free medical and obstetric history, were eligible to participate in this study. The primary outcome of interest was the incidence of specific modifiable risk factors for sPTB. The correlations between these risk factors and sPTB were also investigated. Results Overall, 2027 women were eligible for the study and agreed to participate. The incidence of sPTB was 4.9%; 25.5% (n = 518) of the participants were in extreme maternal age (<20 or >35 years), 34.5% (n = 701) had an abnormal body mass index (BMI) (<18.5 or ≥25 kg/m2), 4.4% (n = 89) reported use of assisted reproductive technology (ART) and 10.9% (n = 220) reported themselves as smokers in pregnancy. In the multivariate analysis, sPTB was significantly correlated with ART conception [odds ratio (OR): 2.494, 95% confidence interval (CI): 1.196-5.199]. Conclusion Approximately one in 20 primiparous pregnant women in the study had a sPTB. The study population included a high percentage of women of advanced maternal age, with abnormal BMI and smokers, but these characteristics did not affect the incidence of sPTB. On the other hand, conception following ART increased two-fold the risk of sPTB.