Conjoined twin, one-month-old Rebeca Martinez, lies in a hospital crib. Rebeca Martinez was born in mid-December at a hospital in Santo Domingo with the head of an undeveloped twin attached to the top of her skull, facing upward. The infant is otherwise healthy but her brain cannot develop normally unless the undeveloped head is removed, said Dr. Santiago Hazim, medical director at the CURE International Center for Orthopedic Specialties, where the surgery is tentatively set for February 6 or 7 in the Dominican Republic. The baby girl was born with one of the world's rarest birth defects, caused when a conjoined twin fails to develop in the womb.