A 34-year-old woman comes to the clinic because of left ...

An ultrasound shows a left-sided adnexal mass. Beta-human chorionic gonadotropin levels are positive, but low for gestational age. Her blood type is O, Rh-negative. A laparoscopy is performed and an ectopic pregnancy is resected. She recovers from the procedure and is scheduled to be discharged in 24 hours. The most appropriate next step in ... ................
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