Each era has a "look" which women of the time adopt. During the Great War this is the "look" which women wanted to achieve.
It can help in dating portraits if we recognise their aim.

People have the faces nature provides, it is only make-up, hairstyle and clothing which can change.

The Big Bow was the universal look chosen by girls in their teens.
Women in their early twenties, probably engaged in war work chose a look more practical than their pre-war sisters. The hairstyle here is an early form of bob worn by former "flappers".It is the flapper style with the plait removed. Her hair is swept back and has none of the cut edges at either front or sides, we expect in a true "bob".

Engagement Photograph in evening dress

Date about 1917 Aged about 20 years old  • Estimate: born 1997 ±5 years