Mr Thielemans was speaking during an election debate on the occasion of the local elections. His spokesman pointed out that limiting the number of children wasn't part of any policy plan adding that it is not a matter for the local authorities.
The spokesman continued: "It's a fact: in Brussels there are many families with seven or eight children. They apply for social flats, but flats to accommodate such numbers do not exist."
The Brussels Region today records the highest birth rate in Belgium. In 2011 the population of Brussels stood at 1.1 million people. By 2060 this will rise to 1.5 million.
The Well-being barometer shows that the increase in the birth rate is fuelling poverty. Child care and schools too are feeling the pressure.
The Mayor of the City of Brussels did not shy away from naming names: "You encounter this problem among Muslim families, but also Jewish ones and even in Christian households."
Politicians from other parties have given Mr Thielemans's initiative the thumbs down: Belgian Interior Minister Joëlle Milquet, who heads the Christian democrat list in the City, can hardly believe the mayor made the pronouncement, while Francophone liberal Alain Courtois says it's a bad idea.