A comparison of transabdominal and transvaginal ultrasonography for determination of gestational age and clinical outcomes in women undergoing early medical abortion.

BACKGROUND: We sought to establish the accuracy of abdominal ultrasonography in determining gestational age and identifying the presence of a gestational sac and embryonic pole before and after medical abortion.

METHODS: We included all 120 women enrolled in a study of simultaneous oral mifepristone and buccal misoprostol for abortion through 63 days' gestation. Vaginal and abdominal ultrasound examinations were performed before and 24 h after medication administration. Visualization of a gestational sac and embryonic pole and presence or absence of cardiac activity were recorded. Sensitivity and specificity were calculated with the results from vaginal ultrasonography as the gold standard. The effect of body mass index (BMI) on ultrasound findings was also assessed.

RESULTS: Before treatment, the sensitivities of abdominal ultrasonography were 100% (95% CI 97-100) and 68% (95% CI 58-77) for presence of a gestational sac and an embryonic pole, respectively. Overall, abdominal imaging underestimated mean gestational age by 1.6 days (95% CI 1.0-2.2). After treatment, abdominal ultrasonography missed three of 34 retained gestational sacs (sensitivity 91%, 95% CI 76-98%). Fourteen women had gestational cardiac activity by vaginal ultrasound at follow-up. Abdominal imaging identified the gestational sac in all cases, but cardiac motion was only visible in 10 (71%, 95% CI 42-92%). For every 10-point increase in BMI, the odds ratio for missing an embryonic pole at baseline was 2.8 (95% CI 1.5-5.0).

CONCLUSIONS: Abdominal ultrasonography is sensitive for diagnosing the presence or absence of a gestational sac, but less sensitive at detecting an embryonic pole. This may lead to a small underestimation of gestational age and missing a continuing pregnancy at follow-up when one exists.