An 18-day-old Los Angeles boy became the 12th baby to undergo human heart transplant surgery at Loma Linda University Medical Center, hospital officials said. The infant, known only as Baby Renny, was in critical but stable condition after undergoing surgery Saturday night. Baby Renny was born with hypoplastic left-heart syndrome, a fatal underdevelopment of the left side of the heart. The surgery was performed by Dr. Leonard Bailey, Loma Linda's chief of pediatric cardiac surgery. The patient's parents, Renny, 40, a tool and die maker, and Millie, 31, a housewife, expressed their gratitude to the donor family, whose identity was not released.