Amniotic sac reference interval in early pregnancy between 7 and 10 weeks’ gestation

ABSTRACT Objective To establish a normal reference interval for amniotic sac measurements between 7 and 10 weeks of gestation and its relative size in relation to the gestational sac and the embryo. Method This was a prospective, cross‐sectional study of consecutive women presenting to UCLH Early Pregnancy Unit between August 2022 to June 2023. We included live, normally sited, singleton pregnancies with a normal 20‐week anomaly scan. We collected 120 cases per gestational week totaling 360 cases. We performed an inter and intra‐observer variability assessment in the measurement of mean ASD in 30 patients. Regression analyses were used to establish reference intervals for GSD to CRL, ASD to CRL, GSD to ASD and GSD:ASD ratio to CRL. The fitted regression line was calculated, along with a 90% prediction interval and the R 2 value. Results There was good interobserver agreement (difference 0.007mm ± 1.105 (95%CI ‐2.160 to 2.174)) and good intra‐observer agreement between Observer A (0.007 ± 1.105 (‐2.160 to 2.174)) and Observer B (‐0.014 ± 0.919 (‐1.814 to 1.786)) in the measurement of mean ASD in 30 patients. Regression analyses showed a highly statistically significant association between each pair of values (all p‐values <0.001). There were significant quadratic associations between mean GSD and CRL (R 2 = 56%) and mean GSD and ASD (R 2 = 60), significant cubic association between ASD and CRL (R 2 = 90%) and significant quadratic association between GSD to ASD ratio and CRL (R 2 = 68%). The regression equations were used to quantify the values of ASD and GSD to ASD ratios for a range of CRL values and gestational age in days. Conclusion Our study has produced comprehensive reference intervals for amniotic sac size in early pregnancy which could be used in routine clinical practice. This article is protected by copyright. All rights reserved.