Why wait? The obvious reason is the one that all couples are given when a pregnancy is first known: don't tell anyone until after the first trimester, when most miscarriages occur.
Jessica Hay said the following in an interview with New Idea magazine: “They’re planning to make an announcement in December.” Hay was described as a close friend of Middleton's.
Kate Middleton is 30; Prince William is also 30.
Of course, with William being the firstborn of Prince Charles and the late Princess Diana, expectations and desires for he and Kate to "produce" an heir are high.
While Hay is described as a close friend of Kate's we'll see how things work out after she leaked this information, assuming it is true. Hay attended the couple’s wedding reception and shared a school dormitory with Kate at 14.
Hay also added, in the interview with New Idea, that Kate has recently gained a little weight and that “William and Kate are focused on starting a family.”
As far as the baby's gender goes, Hay said Kate is secretly hoping for a boy first and then a girl. “It seems very traditional but of course they’ll be happy with two boys or two girls, they don’t want more (than two children). They’ve discussed it endlessly and don’t want to be older parents.”