Usually, you can, but as they said above it must be tagged at check-in. Airlines vary, BA often refuse to check anything heavier than a MacLaren Tecno at the gate becuase of 'health and safety regulations' - apparently the 3 wheelers are too havey for baggage handlers to carry down the steps!!!! Even when they do take it at the gate, they will often deliver it to the luggage carousel at the other end rather than bringing it to the door of the plane. If you are flying in Club they are often much more helpful, but not if the cabin is full of people flying with children (e.g. when half of HK flies back in July or at Christmas). Cathay is usually much better in this regard (gate check and deliver to plane).
It is worth checking with the airline in advance, because it can be a very long walk at the other end with children, no buggy and hand baggage for an overnight flight.
