It actually harks back to when we were dinosaurs. Our origins stem from biogenesis, and when the dinosaurs appeared, they all looked the same.
Sex confusion.
The more beautiful dinosaurs discovered that they had eggs inside them, but didn't have any proper way of showing the other dinosaurs this fact. So the beautiful dinosaurs decided to show their stomachs when at their most fertile.
And that's why women bare midriffs at specific times.