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

Objective To establish a normal reference interval for amniotic sac diameter (ASD) between 7 + 0 and 9 + 6 weeks' gestation and its relative size in relation to gestational sac diameter (GSD) and the embryo crown–rump length (CRL). Methods This was a prospective, cross‐sectional study of consecutive women presenting to the Early Pregnancy Unit, University College Hospital, London, UK, between August 2022 and June 2023. We included live, normally sited, singleton pregnancies with a normal 20‐week anomaly scan. We collected 120 cases per gestational week, from 7 + 0 to 9 + 6 weeks' gestation, totaling 360 cases. We performed an inter‐ and intraobserver variability assessment in the measurement of mean ASD in 30 patients. Regression analyses were used to establish reference intervals for GSD and CRL, ASD and CRL, GSD and ASD, and GSD/ASD ratio and CRL. A fitted regression line was calculated, along with a 90% prediction interval and R 2 value. Results There was good interobserver agreement (mean ± SD difference, 0.007 ± 1.105 mm (95% limits of agreement (LoA), −2.160 to 2.174 mm)) and good intraobserver agreement for Observer A (mean ± SD difference, −0.080 ± 0.741 mm (95% LoA, −1.532 to 1.372 mm)) and Observer B (mean ± SD difference, −0.014 ± 0.919 mm (95% LoA, −1.814 to 1.786 mm)) in the measurement of mean ASD. Regression analyses showed a statistically significant association between each pair of values ( P < 0.001 for all). There was a significant quadratic association between mean GSD and CRL ( R 2 = 56%), mean GSD and ASD ( R 2 = 60%) and GSD/ASD ratio and CRL ( R 2 = 68%), and a significant cubic association between mean ASD and CRL ( R 2 = 90%). The regression equations were used to quantify the values of ASD and GSD/ASD ratios for a range of CRL values and gestational ages. Conclusion Our study has produced comprehensive reference intervals for amniotic sac size in early pregnancy, which could be used in routine clinical practice.