Until Monday's announcement, the duchess had shown no signs of being with child.
She was photographed just last week bounding across a field clad in black high-heeled boots as she played field hockey with students at her former school.
Still, speculation has swirled about when she and William would start a family from almost the moment they were wed on April 29, 2011, in a lavish ceremony at Westminster Abbey.
The attractive young couple is immensely popular -- with William's easy common touch reminding many of his mother, the late Princess Diana -- and their child is expected to play an important role in British national life for decades to come.
