Women aged in their 20s and 30s are risking poverty in retirement by cutting contributions as they pay for childcare, a think tank says.
The Fawcett Society, which campaigns for equality, said many young women were relying on their partner for financial security in retirement.
It said many women in this age group were taking on the full cost of childcare.
The government said free childcare was being doubled.
The Fawcett Society said that student debt, childcare costs, and a break from work were preventing women from saving for later life.
Sam Smethers, chief executive, said: "The gender pay gap becomes a pensions gap in retirement. In particular, women are taking a big hit on their pensions when they have children, but are not aware of the impact this will have on them in the long-term.
"Women are putting everyone else's needs before their own, especially when it comes to who pays for childcare. Their baby becomes her childcare bill."
A Department for Work and Pensions spokesman said: "We want all women to have a financially secure retirement, which is why we introduced wide ranging reforms to make pension saving easier and clearer, including through the introduction of the new state pension and automatic enrolment.
"We are also committed to supporting mothers by making childcare more affordable and more accessible, and we are doubling free childcare for nearly 400,000 working parents of three and four-year-olds from 15 to 30 hours."
The Pensions Policy Institute recently warned that the new state pension, introduced this month, will leave more than 11 million workers now aged their 20s and 30s worse off than they would have been under the previous system.
A BBC News investigation in 2010 revealed how pensions had fallen down the priority list for many young women.