Researchers suggest women may want to think about delaying motherhood if they want to lower their career income losses.
Study coauthor Man Yee (Mallory) Leung, PhD, a postdoctoral research associate at Washington University School of Medicine, and colleagues publish their findings in the journal PLOS One.
According to the Centers for Disease Control and Prevention (CDC), the proportion of women in the US who have their first child after the age of 30 has increased significantly, rising from 5% in 1975 to 25% in 2010.
Previous studies have suggested that mothers in full-time, paid employment earn less than women who never have children - a finding that Leung and colleagues say is widely referred to as the "motherhood gap."
However, Leung notes that their study is the first to assess how the age at which a woman has a first child impacts total lifetime career earnings.
To reach their findings, the team analyzed 1996-2009 data of almost 1.6 million Danish women aged 25-60. The researchers hail Denmark as a "gold mine" for researchers, because the country collects socioeconomic and health data for the entire population.
The researchers assessed the women's birth information - including the age at which they had their first child - employment status, income and other household information.
They used the data to calculate the average annual salaries for each woman, using this average to estimate the short- and long-term lifetime career income losses for women who had their first child before the age of 25 and for each 3-year age range thereafter, up to the ages of 40 and older.
2-2.5 years' annual salary lost for women who have a first child before 25
Compared with working women who had their first child over the age of 30, those who became a first-time mother under the age of 30 had lower lifetime incomes, regardless of whether they had a college degree.
Women who had their first child at age 31 or older were found to earn more over their entire careers than those who had no children, while women who had their first child after the age of 37 benefitted from an additional 0.5 years' salary on their lifetime earnings.
Women in employment who had their first child before the age of 25 were found to have the greatest loss of career income; college-educated women who had children before this age lost an average of 2 years' annual salary, while those with no college degree lost an average of 2.5 years' annual salary.
Women without a college degree who had their first child after the age of 28 experienced a short-term loss in career income, but gradually, their lifetime earnings caught up with those of women without children.
Compared with college-educated women who first gave birth aged 28-31, non-college-educated women who first gave birth in the same age range experienced a lower loss of short-term income, at losses equal to 65% and 53% of average salary, respectively.
The longer both of these groups delayed having children, the lower the loss of short-term income.
'There is a clear incentive for delaying'
Commenting on their results, study co-author Raul Santaeulalia-Llopis, an assistant professor of economics in Arts & Sciences at Washington University, says:
"Children do not kill careers, but the earlier children arrive the more their mother's income suffers. There is a clear incentive for delaying."
The researchers note that in Denmark, new mothers receive up to 18 months of paid maternity leave, while in the US, new mothers normally receive 12 weeks of unpaid leave. As such, they say the loss of career income may be significantly higher for women in the US who have a child before the age of 30.
"The fact that highly productive women who have children earlier enter a lower income path is not only a loss for them, but for the entire society," says Santaeulalia-Llopis.
"If children are shutting down women's career growth and these pervasive effects vanish after the mid-30s, then we should start taking seriously the case for employer-covered fertility treatments. But we need to dig deeper to establish causation and assess costs and benefits."
In February, Medical News Today reported on a study suggesting that having a baby at the age of 40 or older may increase a woman's risk for heart attack and stroke.