Coins were created for the 2013 birth and christening of Prince George, the first child of the Duke and Duchess of Cambridge, and his first birthday.
Limited numbers of the various coins were made, with the 5-pound first birthday coin costing 80 pounds and a "gold proof kilo" christening coin costing 50,000 pounds.
Queen Elizabeth II has approved the decision to create a coin for the second child's birth, which is due in April, the BBC reported.
Prince William, 32, and Kate, 33, have so far not confirmed any date for the birth of their second child except to confirm that he or she is due in April.
She said: "I'm due mid-April, to the end of April. Not long to go now.