There had to be a mistake, Melanie Riffo thought, staring in disbelief at the result of her pregnancy test: Positive. She had been taking her birth control pills without fail, Ms. Riffo said. She and her boyfriend were careful. He’d even been told by doctors that a childhood ailment could have left him infertile. “I couldn’t make sense of it,” said Ms. Riffo, 20, of the test she took in September. “We had been taking all the precautions.” Across Chile, scores of women like Ms. Riffo say they became pregnant last year after taking one of the 276,890 packets of oral contraceptives that were provided by the public health care system — and later quietly recalled over defects that made them potentially ineffective.