Like a lot of things, maternity fashion has had a bit of a transformation over the years. Things that once were functional or boring or simply a stress purchase necessity - all these things have been made cool by design. Design is the DNA of any manufactured object from an orange juice carton to a spaceship. Designers like Philippe Starck have transformed every day objects like the citrus press or the tin opener and made them almost into works of art.
Clothes are the same - look at any old pic from the 1950s of people in the street, and about 90% of people aren't dressing to look cool, they're just wearing clothes to suit whatever weather it is.
Design changed all that and it's the same with maternity clothes - celebrities like Myleene Klass are often seen in very stylish maternity outfits, and the months of pregnancy are no longer on a lesser fashion level.