Early pregnancy assessment with transvaginal ultrasound scanning.

Objective: To establish normal parameters in early pregnancy through transvaginal ultrasonography so that gestational age can be determined and to correlate the sonographic findings with serum human chorionic gonadotropin (hCG) levels calibrated against the first international reference preparation standard.

Setting: Infertility clinic.

Patients: Thirty-five women with normal intrauterine pregnancy.

Interventions: Serial measurement of the serum hCG level and the diameter of the gestational sac through transvaginal ultrasonography.

Main Results: The gestational sac could not be visualized when the hCG level was less than 1100 IU/L. The average growth rate of the sac was 0.9 mm/d. The threshold values for sac diameter, serum hCG level and gestational age below which the yolk sac was not visible were 3.7 mm, 1900 IU/L and 36 days respectively; the corresponding values above which the yolk sac was always visible were 6.7 mm, 5800 IU/L and 40 days. The threshold values below which cardiac activity was not visible were 8.3 mm, 9200 IU/L and 41 days respectively, and the corresponding values above which cardiac activity was always visible were 14.0 mm, 24,000 IU/L and 46 days. The mean gestational ages and the 95% confidence and prediction intervals were tabulated so that measurement of the gestational sac diameter could be used to estimate gestational age early in normal pregnancy.

Conclusions: Transvaginal ultrasonography enables detection of an intrauterine sac and reliable estimation of gestational age on the basis of sac dimensions before an embryo can be seen.