The longer women work in casual or temporary jobs, the less likely they are to have their first child by their mid-thirties, a new study suggest.

Researchers out of Australia's University of Adelaide found that the likelihood of childbirth by 35 is reduced by eight per cent for every year a woman spends in temporary employment.

"Our findings suggest that, regardless of their socioeconomic circumstances, women generally aspire to economic security prior to starting a family," the study authors write.

Researchers say the finding is important because it challenges the widespread notion that highly-educated women are delaying motherhood to focus on their careers. The study found the effect of temporary work on beginning a family was consistent among the study participants, regardless of their socioeconomic status.

Researchers studied data collected from 663 Australian women who took part in the Life Journeys of Young Women Project and who were born between 1973 and 1975 in a large hospital in Adelaide, South Australia. Interviews were conducted with the women from 2007 to 2009, when they were between 32 and 35 years old, to collect information on significant events in their lives such as relationships, childbirth and employment from the age of 15 onward. 

At the time of the interviews, 67 per cent of women had given birth to at least one child. At the time the child was born, the majority of study participants were permanently employed while 11 per cent were temporarily employed.

Researchers found that one year of causal work was associated with an eight per cent reduction in the likelihood of a woman having her first child by age 35, compared to women who had not worked in temporary positions. That means the likelihood of a woman having a first baby by around age 35 was reduced by 23 per cent after three years of temporary employment, and by 35 per cent after five years.

The study also showed that 61 per cent of women who had received a university education had at least one casual job after graduation, and 30 per cent 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," study co-author Dr. Lynne Giles said in a statement. "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."

In their paper, the authors note that governments tend to provide financial support to parents after they have children, but say policies aimed at helping couples start a family should be considered.

"Since all socioeconomic groups are implicated, we suggest that upstream labour market reforms could be considered in order to remove barriers to child-bearing," the authors write.

According to Human Resources and Skills Development Canada, in 2009 the average age of mothers in Canada at the birth of their children was 29.4 years – which is an increase over the last three decades.

Of the mothers who gave birth in 2009, 50 per cent were 30 or older, which is about two and a half times the percentage of 1974.