The practice in the upper classes was to marry very quickly after announcing engagements - so a pregnancy was often passed off as an 'early' baby in all classes. Plus the wealthy might return from an extended honeymoon with a baby born at an unannounced date abroad.
Wedding dresses tended to be 'best dresses', but depending on your social position it was considered dangerous to be 'too fashionable'. So by 1911 designers like Leon Bakst were cutting edge and some rich women would wear these kind of designs, but most would still be in the styles of the Edwardian era. Since the best frock of a rich womn was often white, it gradully became the case later in the 20th century that women wore white dresses to get married.
A 17-year old male would only be considered marriage fodder if there was summat up ie ill - most men were older than their brides.
On the other hand a 17 yr old female was a great catch if from a rich family.