The city is D.C., according to the Washington Post.
The reason? The high cost of childcare. According to the article, childcare "can be as expensive as housing in some parts of the country."
Here's the story, which breaks down cost of living for singles and families and also contains a neat interactive graphic.
Childcare costs exceeded our housing costs when we first moved to DC. Not sure how that compares to other parts of the country though…