Objective To examine the correlation between fetal rectal ampulla diameter and gestational age， and to establish reference value ranges for low-risk fetuses between 18 and 40 weeks of gestation in Yibin region. Methods A total of 1，103 low-risk singleton pregnant women between 18 and 40 weeks of gestation were recruited from five hospitals in Yibin City （the Second People′s Hospital， the First People′s Hospital， the Fifth People′s Hospital， the Maternal and Child Health Hospital of Yibin City， and the Maternal and Child Health Hospital of Cuiping District） for routine level Ⅰ， Ⅱ， and Ⅲ prenatal ultrasound screening from October 2022 to March 2024. Fetal rectal ampulla diameters， including anteroposterior diameter， transverse diameter， and area， were measured using prenatal ultrasound. The normality of these measurements was assessed using the Shapiro-Wilk test. Scatter plots depicting the relationship between fetal rectal ampulla diameter parameters and gestational age were generated using the "Overlap Scatter Plot" function in SPSS. Percentiles were calculated using the "Explore" function in SPSS， with reference value ranges described by P5， P10， P50， P90， and P95. Results The visualization rate of the fetal rectal ampulla diameter was 55% at 18 ~ 20 weeks of gestation， 100% at 21 ~ 37 weeks， and 96% at greater than 37 weeks. The fetal rectal ampulla diameter exhibited a significant positive correlation with gestational age （r = 0.925 ~ 0.949， P < 0.01）. Conclusions Prenatal ultrasound measurement of fetal rectal ampulla diameter demonstrates a robust correlation with gestational age. The reference intervals for the rectal ampulla diameter of low-risk fetuses between 18 and 40 weeks of gestation， established in this study， may offer valuable theoretical guidance for prenatal diagnosis of fetal rectal and anal abnormalities in Yibin region.