The appearance of the central uterine cavity was analyzed in 39 cases of ectopic pregnancy. In nearly 20% of cases, a fluid collection surrounded by an echogenic ring was present which could be mistaken for a normal early intrauterine pregnancy. When ectopic pregnancy is suspected clinically, and there is fluid in the cul-de-sac or an adnexal mass on ultrasound, a fetal pole should be evident within the central fluid collection before a definitive diagnosis of intrauterine pregnancy is made.
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Marks et al. (1979) studied this question.