Coins were created for the 2013 birth and christening of their first child, Prince George, and his first birthday.
Limited numbers of the various coins were made, with the £5 first birthday coin costing £80 and a "gold proof kilo" christening coin costing £50,000.
The Queen approved the decision to create a coin for the second child's birth, which is due in April.
The Royal Mint said it could not provide any further information - such as the design or cost of the coin - until "nearer the time when we are ready to launch".
A £5 coin will be created to mark the birth of the Duke and Duchess of Cambridge's second child.