Well... yes, actually. BUH DUM CHING!!!!
As the Black Plague killed off millions and millions of people, those remaining alive had the task of repopulation. Thus to be fertile was to be beautiful. To project her fertility, a lady would bend back and stick our her abdomen, mimicking how it looks to be pregnant. This body posture has been called the gothic slouch.
Even as the plague finished doing what it does best, the fertile look continued to be popular.
Here is the famous Arnolfini Wedding Portrait from the 15th century. The woman in the portrait isn't pregnant, but she is holding up some of the extra fabric in her dress to simulate a pregnant belly, symbolizing that she is fertile and will soon give her new husband many sons who will carry on the grand Arnolfini pimp-hat-wearing tradition.
In an age where life expectancy was extremely short and most of your children would die before adulthood, the best thing a woman could be was very fertile. Marriages weren't necessarily about finding some one you love or vaguely get along with, although that was great if that happened. Marriage was most importantly about procreation. The family name had to be carried on, and that meant at least one son who lived to adulthood and had children of his own. That also meant surviving multiple pregnancies (even just having the child could very easily kill you), and your chances of survival would be greater if there was more fat around your lower body. By sticking out your abdomen, you give the illusion of a plump lower body which will help you survive many childbirths.
Compare this to today, where any sign of belly fat is abhorred. With the improvement of medical technology, childbirth is not nearly as dangerous and most children live into adulthood. Therefore a woman does not need to show that she has enough belly fat to survive childbirth, because these days she doesn't need that. And because weight doesn't have as much effect over the ability to survive multiple childbirths, signs of fertility are not tied up in how much belly fat a woman has. Furthermore, many couples choose not to have any children at all, society having developed to a point where having many children is no longer an absolute requirement (mothers desiring grandchildren, however, are another matter entirely). Marriage has become mainly about finding some one you love. So now the ideal for a woman is to stand up straight and suck it in. Unless you're Paris Hilton. She doesn't have bad posture, she's just trying to bring back the gothic slouch!