There's a disease going around that only spreads via amniotic fluid. It's a virus, able to rewrite cells and survive and spread through surface contact. Originally, it wasn't too much of an issue. The only real symptoms were an increase in horniness and ovulation. Those infected would gain a deep desire for a round, full belly, a need to get pregnant. Eventually, their water would break somewhere (occasionally in a public setting) and, as they labored, the virus would spread to nearby hosts.
As it's mutated in recent years, some strains have gotten better at spreading. An increase in amniotic fluid during pregnancy has led to much larger bellies, with parents carrying around hugely swollen wombs that look like they should contain more than just one baby. With this strain, labor almost always begins around other suitable hosts. Gallons of fluids gush from between the legs of the parent as their belly squeezes, and if they're unlucky enough to be wearing a dress or skirt, it's all but guaranteed someone near by gets infected.
More recently, it has become far more common to spot pregnant men and women, as rates of pregnancy go up. Basically everyone has seen someone with a low hanging bump suddenly begin laboring, as it became more and more common for someone in public to spread their legs and push out a baby.
Eventually, another strain mutated. This one caused immediate self-impregnation. By the time anyone realized what was happening, the first births had already begun. A warning went out, avoid any massively pregnant person. It was too late. A full womb of any size could be ready to burst, dousing surrounding people and impregnating them almost instantly. For some, their water broke slowly, each contraction causing a little bit more fluid to trickle out. Eve sitting somewhere where a new parent had been in active labor was enough to start a baby growing and rounding out your own womb.
Quarantines were only so effective. Even trying to isolate, inevitably you'd watch as those close to you, your family and your friends, all got knocked up, their wombs filled and their bellies beginning to grow bigger and bigger.
It didn't take much longer for the world to accept gravidiness to be a new normal.
