Just been reading in the news that the couple did NOT make the baby announcement to family at the wedding, close family including Jack and Eugenie already knew. For some it was the first opportunity to congratulate them in person. That is all. No announcement was made to the guests.
They probably felt they wouldn't be able to keep it under wraps much longer and announcing the pregnancy in Oz would give them a breather and a chance for everyone in the UK to get used to the idea.