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Until recently, common teaching was that when serum β human chorionic gonadotropin (β-HCG) is >1000–2000 mIU/mL — the “discriminatory zone” — a gestational sac should be seen on transvaginal ultrasound in normal intrauterine pregnancy. To test this, researchers at a Boston hospital analyzed data for 202 patients during an 11-year period who had elevated β-HCG on the same day as transvaginal ultrasound showing no intrauterine fluid collection and then had subsequent ultrasound evidence of a live intrauterine embryo or fetus.
On the day of initial scan, β-HCG levels were <1000 mIU/mL in 80% of patients, 1000–1499 in 9%, 1500–1999 in 6%, and >2000 mIU/mL in 5%. None of the 9 patients with β-HCG levels >2000 mIU/mL had fibroids or other anatomic…