One in four mothers between ages 25 and 44 exits the workforce -- during the key years for career advancement and earnings.

The average woman works 29 years of her life, while the average man works 38.

An estimated two-thirds of mothers work fewer than 40 hours a week and only about 8 percent work more than 50 hours a week. Women who do work full time earn an average of $35,000, while full-time male workers average $45,000.

Why do all these statistics matter? Because Social Security benefits -- including SSDI benefits -- are calculated using your lifetime earnings, and there's a large gap between the lifetime earnings of men and women. The gap is even larger between the average man and the average mother.

Plus, since Social Security provides dependent benefits to spouses, divorced spouses and widows, single mothers who have never been married aren't eligible for some benefits at all.

Your Eligibility and Benefit Level Depends on Your Work History and Earnings -- And Motherhood Doesn't Count

In order to be eligible for Social Security disability or retirement benefits at all, you must have earned "Social Security credits" -- you have to have worked for a certain amount of time in a job that counts toward Social Security.

Once you meet this basic eligibility requirements, the amount of your check is calculated based the on your lifetime average earnings covered by Social Security. Your highest-paid 35 work years are used in this calculation.

Being a homemaker or stay-at-home mom does not count toward Social Security -- it doesn't provide Social Security credits, and it doesn't count toward the qualified earnings used to calculate your retirement or disability benefits.

Because women are much more likely than men to drop out of the workforce for a period of time, the average retirement benefit they receive from Social Security is lower. Since the average man works for 38 years and Social Security only considers your top-earning 35 years, men get to drop their 3 lowest-paid years from the calculation. In contrast, since women work an average of 29 years, their benefit is calculated using 6 years where their income was zero.

Although the story is more complex for women with disabilities, the same basic principles apply. If women work fewer years, fewer hours during those years, and are paid less for their work, their benefits are going to be lower. If they take time out for motherhood, it will count against them. And, if they don't marry, they won't have access to benefits for spouses, divorcees and widows.

"The biggest injustice for women in the Social Security system is that it simply doesn't consider their work as mothers as work," observes Ann Crittenden, author of The Price of Motherhood. "Raising a child, who will grow up to support the Social Security system in the future, does not count as a contribution to the system. You earn a zero for every year you spend raising your own child."

These benefit disparities have been noticed. Considering that women also tend to live longer than men, it's not hard to understand why the poverty rate among elderly women is nearly twice the rate among elderly men.

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