International Women's Day is a great opportunity to take stock and celebrate what has been achieved in improving women's rights and their access to decision-making roles as well as to focus our attention on what still needs to be done.

Women's representation in decision-making roles is no longer discussed in terms of equality and fairness but in terms of the benefits that their presence brings to the upper echelons of organisations: more innovation and better decision-making, a better reflection of a company's inclusive values and better representation of the customer base. There is also evidence that organisations that embrace gender diversity report better results.

Despite these benefits, the numbers of senior women in business, politics and the media are low and frequently lamented: the European average for women on company boards is about 10% and just under 16% in the US. Numbers in Asia range from just under 9% in Hong Kong and Thailand to just under 2% in South Korea and less than 1% in Japan.

International Women's Day this year finds the UK with a continued focus on increasing the number of women on corporate boards. Last year's publication of Lord Davies' report and his recommendations to organisations to set voluntary targets to increase the representation of women on boards to 25% by 2015 has set the tone of the debate. Earlier this year, the debate intensified further when the Prime Minister mentioned the possible introduction of mandatory quotas after attending the Northern Future Forum Summit in Sweden.

So what is keeping women from moving beyond middle management in equal numbers to men? The obvious answer is that women just do not want to progress to senior roles and are not as ambitious as their male counterparts. The evidence, however, suggests that young women's levels of ambition are on a par with those of men. Senior women also report similar levels of ambition to their male counterparts. The evidence about women in the 'in-between layer' is more mixed, however. We know that during a woman's thirties and once she has reached the mid-management level, she is more likely to drop out of the pipeline than at any other time.

Perhaps this is not surprising as these have been referred to as the 'make-or-break' years, when women hit two of the most difficult challenges of their careers; motherhood and the ascent from middle to senior management. With those two challenges comes a set of powerful stereotypes that has hardly budged over the years: Women are still largely seen as primary care givers to a young family while men are still seen as a more natural and safe choice when it comes to appointing senior leaders.

Although controversial, I believe that voluntary targets or mandatory quotas are an important incentive for organisations to make women's increased representation in senior decision-making roles a priority. In order to build a sustainable pipeline of future female leaders, however, we need to go broader than quotas that focus on increasing women at one particular level. Data from Norway, for example, shows that despite a mandatory target of 40% of board roles being filled by women, there are still very few women at senior executive level; there has been no spillover effect [1] to the level below the very top. Quotas or targets on their own do not do enough to eliminate the underlying barriers and obstacles that have held women back from moving to senior executive and boardroom positions up to now. 

Our efforts to increase the number of women in senior decision-making roles must extend across the leadership pipeline, and in particular pay attention to the difficult make-or-break years. We must strive to help women and organisations retain and further develop high-performing middle managers who are the talent pool for future senior management roles. In order to provide effective support to women and organisations we need to explore barriers and possible solutions across three different levels: (a) the individual woman herself, including her leadership and career management behaviors; (b) her immediate work environment and her access to important career resources such as sponsors and critical job assignments, finally and (c) the wider organisational culture.

We can do much more to help women stay the course during these challenging years and if we want to build a sustainable pipeline, we need to stop women from exiting at this point.