Waiting it out in super-chic. Mothers-to-be show off, rather than hide, the happy fact

THE Princess of Wales appears to be directly responsible for the super-chic new image in European maternity clothes. Since her marriage, the princess has been acknowledged as a leading fashion light in Great Britain, and her maternity wardrobes exerted influence even across The Channel as French designers watched the two royal pregnancies as carefuly as any anxious father-to-be. Today the old theory of loose, shapeless smocks and tents naively intended to disguise the coming event is as outmoded as the Victorian corsets worn in the 19th century, when no woman more than three-months pregnant would dream of appearing in public. Now it's a matter of pride, showing off rather than hiding the happy fact. Many maternity silhouettes even take on belts, which were formerly a No. 1 decree on the list of ``no-nos.''

In France there are over 800,000 births each year, and the mothers are getting progressively younger along with the fashions. Each woman buys an average of three maternity dresses chosen primarily for fit and cut rather than any specific size. Two-piece styles are deemed most comfortable and best for the last two months. New priorities include pretty prints for summer, bright colors, imaginative fabrics, and good detailing, not to mention reasonable prices that will not scorch the checkbook.

Sportswear exerts a strong influence on daywear, with loose middies and tunics teaming with cleverly cut trousers designed with expandable crisscross straps at the front. Many women continue working through at least half their pregnancy, and skirts are considered better for the office, with latest models scaled to the longer mid-calf lengths.

A chain of stores called Prenatal does the largest turnover in France, with six shops in Paris and 294 others scattered throughout the country. There's everything for mom and the baby, including such salient items as 190 tons of diapers sold last year. Prenatal's fashion approach is trendy, with many styles designed by that most versatile of all couturiers, Pierre Cardin, who is into everything from maternity attire to restaurants (Maxim's), plumbing, and candy boxes.

A second influential place for the ladies enceinte is Balloon, although some clients feel that the name has a slightly derogatory connotation. The shop was founded and is owned by a former journalist, Veronique Delachaux, who also does most of the designing. Mme. Delachaux inaugurated Balloon after the birth of her first child in 1971, when she claimed that she could not find a single attractive maternity dress in Paris. Here the emphasis is less on the expandable-type silhouettes than on adaptations of what's really happening in top couture and ready-to-wear collections, adapted to much larger sizes. Some of the fashions are so appealing that many clients go right on wearing them after the baby is born. Several women who are definitely not pregnant also shop at Balloon.

The funniest thing out at the collective ready-to-wear shows at the Porte de Versailles are the numerous firms dealing in maternity clothes and employing nonpregnant mannequins. The girls are provided with plump, round cushions attached to an elastic belt to strap on beneath the dresses. But when the working day is over, it's an instant transformation back to that wasp waistline.