Temp work reduces a woman's chances of starting a family, a study has shown.
Women in temporary jobs, like many thousands of office secretaries in the UK, are not as likely as others to have their first child by the age of 35, researchers found.
The longer a woman spent in casual employment, the less chance she had of becoming a mother by her mid-30s. Every year spent working as a temp cut the expectation of giving birth by around 8%.
Compared with women not in temporary jobs, the likelihood of having a first baby by 35 was reduced by 23% after three years' temping, and 35% after five years.
By her mid-30s, a woman is already past her reproductive prime. After 35, fertility declines rapidly as her supply of viable eggs runs out.
Around one third of couples in which the woman is over 35 experience fertility problems, rising to two-thirds for women over the age of 40.
A leading specialist in reproductive biology recently urged women not to wait until the "clock strikes 12" to have a baby.
Speaking at the British Science Festival in September, Professor Mary Herbert, from the University of Newcastle, said: "I would be worried about my daughter if she hadn't had a child by 35."
Researchers conducting the new study collected data from 663 Australian women as part of the Life Journeys of Young Women Project. When the women were aged between 32 and 35, they were questioned about significant events that had shaped their lives, including relationships, employment, and motherhood.
Co-author Dr Lynne Giles, from the University of Adelaide, Australia, said: "Our results showed that 61% of women who had received a university education had at least one casual job after achieving their first qualification, and 30% of these jobs were managerial or professional. This highlights the fact that temporary employment is no longer the sole domain of low-skilled, poorly paid people.
"Our results also show that having children at an older age and childlessness are not just a matter of individual women's choices. They reflect the broader structural arrangements in society. These over-arching determinants deserve more attention and study so that we can better understand the barriers to family formation."
At the time of the interviews, 442 of the women, or 67%, had given birth to at least one child. Most were permanently employed, while 11% were still working in temporary jobs.
Around two-thirds of the group had experience of casual work, while 225 had spent no time in temporary employment. A third had a university qualification, and three-quarters were living with a partner.
The impact of temp work on the risk of childlessness was not altered by a woman's socioeconomic status or education, or her partner's education or parents' birthplace.
Outlining the results in the journal Human Reproduction, the researchers wrote: "Our findings suggest that, regardless of their socioeconomic circumstances, women generally aspire to economic security prior to starting a family.
"This finding is important because it challenges the pervasive media representations of delayed childbirth as a phenomenon arising from highly educated women choosing to delay motherhood to focus on their careers."
They added: "Current policy responses generally provide financial and other support to parents after they have children; there remains a need to develop complementary policies to facilitate the ability of couples to commit to family formation.
"Since all socioeconomic groups are implicated, we suggest that upstream labour market reforms could be considered in order to remove barriers to child-bearing."
Study leader Professor Vivienne Moore, also from the University of Adelaide, expected the findings to apply to other Western countries with similar workplace cultures.
"The argument that women's employment conditions have an influence on the timing of family formation would seem to be relevant, especially for Western countries with neo-liberal outlook," she said.