There's a great many reasons why a woman's weight may fluctuate. Some might say it's a hectic work schedule preventing them from getting to the gym.
Others may suggest it's the temptation of the office chocolate machine.
But research suggests there's only one real reason - their men.
A study into the ups and downs of the female weight-cycle showed happiness - or the lack of it - in a relationship is the biggest factor.
The report found early courtship usually brings with it a strict diet regime resulting in the loss of around 5lb.
After women enter Stage 2, which is the comfort zone, when they feel secure enough to relax a little, can put on a few pounds.
Wedding plans mean another sprint to shed a few pounds before the arrival of a baby reverses that weight loss, followed by a reinvention - which again sparks a diet drive.
The five stages of weight fluctuation, which can vary over around two stone, emerged in a study of 3,000 women by UK weight management firm.
'Our emotions and relationships have an enormous effect on our health and above all our weight,' said Jane McCadden, of Slendex.