The duke and duchess met German Chancellor Angela Merkel and toured the Brandenburg Gate, a landmark that represents the country's unification.
"We will just have to have more babies", she said to laughs.
The couple's children, Prince George (who turns four tomorrow) and Princess Charlotte, will make one more appearance when the family bids farewell to Germany and boards their plane back to the United Kingdom.
Princess Charlotte stumbled and fell on a visit with her brother and parents to a plane factory in Germany.
It is one of a series of European visits for the family this year that are widely seen as an attempt to use the royals as part of a Brexit "soft diplomacy" charm offensive.
PM Modi walks up to opposition in Parliament, Greets Sonia Gandhi, Others
Lok Sabha Speaker Sumitra Mahajan on Sunday said all parties have assured that the monsoon session will proceed without any ruckus or uproar.
After their first stop at the German Cancer Research Institute, the royal couple met with the mayor of the town, which is twinned with Cambridge in the UK.
However, another talking point has, of course, been the Duchess of Cambridge's stunning outfits.
But her best accessory of all was her husband, Prince William, who looked pleasantly dapper in a navy tux and burgundy tie.
For day two of the five-day royal tour of Poland and Germany, Kate has been spotted in a floral Erdem two piece. They never thought that one day they would return to the place where they were prisoners in 1940s. The two then went to the local museum, where they saw the exhibit of the piles of shoes of Holocaust victims.