It is usually a jolt of recognition that persuades someone they are overweight and it is time to start dieting.
That could be struggling to do up a top button on a favourite pair of jeans, having to buy clothes in a larger dress size or noticing your thighs in the mirror one morning.
But the most common trigger for women changing their lifestyle, according to new research, is an unflattering photograph.
Subtle changes in size and shape can be hard to notice, but looking back at holiday snaps forces us to assess ourselves objectively.
The surprise leads to impressive results, however, with women dieting after seeing an unattractive picture losing a staggering 14 pounds on average.
Embarrassing double chin moments are now frequently displayed to more people than we want them to be through Facebook, making many women see photographs as a very public measure of their worth.
Half of the 1,000 women surveyed by eatwater.co.uk admitted to having been shocked into slimming by a photograph that highlighted unwanted bulges.
Other popular reasons for starting a diet included struggling to fit into clothes and catching sight of a reflection in a shop window.
Old photos as well as recent ones were seen as highlighting weight issues, with 42 per cent of women dieting after noticing how many pounds they had piled on on since pictures were taken.
An Eat Water spokesman said: ‘Sometimes it takes something like a photo for people to acknowledge the fact they even have a problem with their weight.
‘It is all too easy to become complacent with our body shape and size, and think we are doing okay, when really we’re carrying round a few extra pounds.
‘A photograph can be like a bolt from the blue – often people are absolutely flabbergasted that they could have let things get so bad.’
For some people, wanting to look good for an event such as holiday or wedding was the main impetus to start slimming.
The most negative triggers included feeling fat in comparison to work colleagues, a dwindling sex life, and being cheated on by a partner.
Some women said they felt too self-conscious when going swimming.
A woman dieting after being dumped can expect to lose 14 pounds, while women shed around 11 pounds to feel more confident in their holiday bikinis.
A desire for a new job or to fit into a favourite pair of jeans leads to an average weightloss of 10 pounds.
But just under half of women admitted that they find it hard to maintain weight.
The average female goes on at least two diets a year, which last around three weeks each time.
Simone Brenner, a leading Dietitian and Nutritionist for Eat Water, said: ‘A major difficulty my clients face when starting on a new diet is adjusting their eating habits so that they consume a smaller quantity of food with a higher nutritional value.
‘Often they find themselves feeling hungry and turning to the very foods they need to avoid in order to achieve their target weight.’