Parenthood can shape a woman’s career by delaying progression and weighing on earnings. A report, published in January by the US National Bureau of Economic Research looking at Danish population data, found that having children creates a 20 percent gender gap in pay for women over the long term.

Indeed, the authors suggested that once variables such as skills, level, sector and tenure are adjusted for, having children is the main cause of today’s persistent gender pay gap.

Economists generally tend to attribute this to childcare responsibilities falling disproportionately to women, thereby lowering their productivity, as well as discrimination by employers towards women with children.

Is there, then, an optimum time for a woman to have children to mitigate some of the adverse effects on her career?

The best age bracket to have a first child to mitigate the effect on salary over a woman’s career, according to a Danish research project.

Another study from 2016 using data from Denmark— which as a country has some of the most progressive childcare and social policies —   found that women of all ages take a dramatic short-term hit to their earnings after having their first child. It also showed, however, that women who had their first child at age 31 or older earned more over their careers than women who had children at a younger age — or indeed women who did not have children at all.

The gains were significant for college-educated women who had children between the ages of 31 and 34, who earned on average 13 percent more over the course of their lifetime than college-educated women who did not have children. This is because older mothers typically are more experienced employees and therefore costlier to replace, so employers invest more in retaining them, the report suggests.

By contrast, “the younger a mother is, the more she has to forgo in terms of opportunity to accumulate experience and human capital in the labor market”, the 2016 study said.

Younger mothers might also self-select into occupations seen as more ‘mother-friendly’, such as public sector roles, which tend to pay less, the report states.

“If you want to earn flexibility in the workplace, you need in-demand skills,” she said.

“Women fear losing ground. The more people are able to increase [their] skills, the easier it is to take a career break without losing ground.”

Others note that women who have children later in life may be more in control of their hours and workflow, more financially stable and better able to afford help when they return to work.

There are disadvantages to waiting to have children. If you have come to be seen as more or less indispensable, it might be harder for you emotionally to opt to take the time off.

Any decision must also be weighed against the decline in a woman’s fertility with age, starting in her early 30s and accelerating after 35, while the risk of complications during pregnancy is higher for older mothers.