Risk prediction of placenta previa based on the distance from the lower edge of the gestational sac to the internal cervical os in early pregnancy.

Objectives: To explore the relationship between the distance from the lower edge of the gestational sac to the internal cervical os in early pregnancy and placenta previa.

Material and methods: A prospective cohort study of women who underwent pregnancy examination in Weifang People's Hospital or Sunshine Union Hospital from January 2020 to June 2021. The distance from the lower edge of the gestational sac to the internal cervical os was measured at 5-6 weeks' gestation. There were 86 women with distance < 2.5 cm, and 105 women with distance ≥ 2.5 cm were randomly selected. There were 92 cases of scarred uterus and 99 cases of non-scarred uterus among the 191 women. They were divided into six groups according to the distance: (1) < 1.0 cm; (2) 1.0 cm to < 1.5 cm; (3) 1.5 cm to < 2.0cm; (4) 2.0 cm to < 2.5 cm; (5) 2.5 cm to < 3.0 cm; (6) ≥ 3.0 cm. All included women were followed-up during pregnancy and pregnancy outcome, and the likelihood ratio of different distances in early pregnancy was calculated and risk stratification was performed, and ROC curve was constructed.

Results: There were 15 women in the included studies who were lost to follow-up, 47 had a scarred uterus with placenta previa and 29 had a non-scarred uterus with placenta previa after delivery at 28 weeks or later. The distance from the lower edge of the gestational sac to the internal cervical os in early pregnancy of the scarred uterus < 1.5 cm, and the likelihood ratio was ∞; and the distance ≥ 3.0 cm, the likelihood ratio was 0. The distance from the lower edge of the non-scarred gestational sac to the internal cervical os < 1.0 cm, and the likelihood ratio was ∞; and the distance ≥ 3.0 cm, the likelihood ratio was 0. The ROC curve showed that when the area AUC under the curve was 87%, the optimal diagnostic cut-off value was 2.4 cm.

Conclusions: When the distance from the lower edge of the gestational sac to the internal cervical os was < 1.5 cm and the distance between the non-scarred uterus was < 1.0 cm, it eventually developed into placenta previa; the distance from the lower edge of the gestational sac to the internal cervical os in the first trimester of pregnancy between the scarred uterus and the non-scarred uterus was ≥ 3.0 cm, and it would hardly develop into placenta previa. When the distance from the lower edge of the gestational sac to the internal cervical os in early pregnancy was ≤ 2.4 cm, it could be used as a predictor of placenta previa.