Purpose: This study examines the difference in the oral microbiome during pregnancy and the postpartum period of a Chinese population, with the focus on P. gingivalis, P. intermedia and P. nigrescens and their shift during pregnancy, in order to understand the host-microbe relationship in maintaining homeostasis during pregnancy. Materials and Methods: This cross-sectional study was carried out in a total of 117 women who underwent prenatal or regular examinations at four public hospitals, including 84 pregnant and 33 postpartum women. Women in the postpartum group were examined within 0.5-1 year after delivery, while the pregnant group was divided into early pregnancy (0-13 weeks), middle pregnancy (14–27 weeks), and late pregnancy (28–39 weeks) according to gestational age. Sociodemographic parameters were self-reported by recruited women. The study required evaluations of probing depth (PD), bleeding index (BI), clinical attachment loss (CAL), and plaque index (PlI). Unstimulated whole saliva samples were collected for the detection of P. gingivalis, P. intermedia, and P. nigrescens. Bacterial populations were evaluated using 16S rRNA-based polymerase chain reaction. Results:P. nigrescens exhibited higher prevalence in the pregnant group compared to the postpartum group (45.67% vs 12.10%, p 0.01). P. nigrescens was more frequently detected in late than in early pregnancy (57.7% vs 48.3%, p 0.05) and middle pregnancy (57.7% vs 31.0%, p 0.01). Initially high prevalence of P. gingivalis in early pregnancy wanes in middle and late pregnancy (69.0%, 44.8%, 38.5%). However, the prevalence of P. gingivalis in the postpartum group (81.8%) exceeds all of the pregnant groups (p 0.01). The change in the prevalence of P. intermedia among different groups was not statistically significant. The percentages of bleeding on probing (BOP%) sites and PD ≥4 mm sites in the postpartum group were statistically significantly higher when compared with each of the pregnant groups (p 0.01). During pregnancy, women experienced elevated PlI, BI, PD, and BOP% (p 0.05). The proportions of subjects in the pregnant group who agreed with the statements 'Gingival bleeding is normal', 'Can't brush teeth within 1st month postpartum', 'It's unnecessary to see a dentist if not uncomfortable' were 39.3%, 28.6%, and 35.7%, respectively. Conclusions:P. nigrescens is more related than P. gingivalis to pregnancy status. The periodontal status of Chinese women progressively deteriorates during pregnancy and persists into the postpartum period, which may result from lack of dental care knowledge. Keywords: dental care behaviours, periodontal health, periodontal pathogens, periodontal status, pregnancy