Why was my child born without a heart valve?

My blood group is AB+ and my wife's is B(-). My wife gave birth to a girl earlier this year. After birth, her breathing was not normal as her heart was not normal. The doctor told me her heart is bigger in size. The doctor diagnosed that her valve was absent and she had infection in her lungs. He gave 14 days course of antibiotic injection after which she looked good and took milk. In the mean time, she got some fever after which she died. What was the reason for abnormal heart or absence of valve? What precautions do we have to take in future?