Prince William and Catherine lay a wreath at Plac Solidarnosci in Gdansk, Poland. Agencja Gazeta/Renata Dabrowska/via REUTERS

Prince William and his wife Catherine are welcomed by the Polish President Andrzej Duda and his wife Agata Kornhauser-Duda at the Presidential Palace in Warsaw. Agencja Gazeta/Slawomir Kaminski via REUTERS

Prince William, the Duke of Cambridge, his wife Catherine, The Duchess of Cambridge, Prince George and Princess Charlotte arrive at Tegel airport in Berlin, Germany, July 19, 2017. REUTERS/Kay Nietfeld/POOL

Catherine, The Duchess of Cambridge and Agata Kornhauser-Duda wait as Polish president Andrzej Duda and Prince William, the Duke of Cambridge light candles at the Warsaw Rising Museum in Warsaw, Poland July 17, 2017. REUTERS/Kacper Pempel

Prince George arrives at Tegel airport in Berlin, Germany, July 19, 2017. REUTERS/Fabrizio Bensch

Prince William, the Duke of Cambridge and Catherine, The Duchess of Cambridge sign the book of visitors at the museum of former German Nazi concentration camp Stutthof in Sztutowo, Poland July 18, 2017. REUTERS/Kacper Pempel

Prince William, the Duke of Cambridge, his wife Catherine, The Duchess of Cambridge, Mayor of Berlin Michael Mueller and his daughter Nina pose at Brandenburg Gate in Berlin, Germany, July 19, 2017. REUTERS/Fabrizio Bensch

Prince William and Catherine at Plac Solidarnosci in Gdansk, Poland. Agencja Gazeta/Renata Dabrowska/via REUTERS

Prince William, the Duke of Cambridge and his wife Catherine, The Duchess of Cambridge visit Brandenburg Gate in Berlin, Germany, July 19, 2017. REUTERS/Axel Schmidt

Prince William, Catherine, and Lech Walesa visit the European Solidarity Centre in Gdansk, Poland. Agencja Gazeta/Renata Dabrowska/via REUTERS

Prince William and Catherine visit the European Solidarity Centre in Gdansk, Poland. Agencja Gazeta/Renata Dabrowska/via REUTERS

German Chancellor Angela Merkel chats with Prince William, the Duke of Cambridge and his wife Catherine, The Duchess of Cambridge, at the Chancellery in Berlin, Germany July 19, 2017. REUTERS/Axel Schmidt

Catherine, The Duchess of Cambridge greets people, at Brandenburg Gate in Berlin, Germany, July 19, 2017. REUTERS/Fabrizio Bensch

Catherine, The Duchess of Cambridge greets people at the central market square in Gdansk, Poland July 18, 2017. REUTERS/Bogdan Popescu