Background: Pregnancy can significantly alter posture and stability, thereby affecting spine curvatures. A positive relationship between the number of full-term pregnancies and the prevalence of low back pain (LBP) has been reported previously. This study aimed to analyze the impact of pregnancy on spine posture and LBP. Methods: Thirty pregnant females who were nulliparous (Group 1, n = 15) or had one or two pregnancies (Group 2, n = 15) were examined using the photogrammetric method in the first, second, and third trimesters of pregnancy. Further, a correlation analysis was conducted among the body mass index (BMI), pain intensity (VAS scale), and spine posture parameters. Results: The parous groups did not differ significantly in the parameters of the spinal posture. The thoracic angle decreased in trimester II compared to trimester I (157.77° vs. 160.55°, p = 0.004), which, according to the measurement methodology used, means that the thoracic kyphosis curvature increased. BMI was associated with the angle of trunk inclination in trimester I in Group 1 (r = 0.54, p = 0.04), as well as with the thoracic angle in trimesters II and III in Group 2 (r = 0.54–0.62, p < 0.05). A statistically significant correlation between pain intensity and spine posture parameters was more frequently observed in Group 2. Conclusions: Parity does not affect spine posture during pregnancy or pain intensity. The intensity of LBP was associated with spine posture changes during pregnancy, but the character of association differs between groups of parity. Alterations in spine posture should be monitored during pregnancy to prevent back pain. Keywords: posture; spine; pregnancy; body mass index