Your mom is widowed or her ex has died. Once a breadwinner dies, so does the 50% limit on spousal benefits. Your mom can claim up to 100% of the retirement benefit of a deceased spouse -- or a deceased ex. Furthermore, she can receive benefits at a younger age -- 60, or 50 if she's disabled -- and she can even remarry, as long as she does so after age 60.
To qualify for a survivor's benefit, she typically must have been married to the covered worker for at least nine months before his death. If they were divorced, the marriage must have lasted at least 10 years.
The nine-month minimum for current spouses has some exceptions. If the breadwinner died in an accident or in the line of duty as a member of a uniformed service, or if the couple was previously married to each other and the past marriage lasted nine months, the minimum is waived, and the survivor can apply for benefits.
As with other Social Security benefits, claiming a survivor's benefit before she reaches full retirement age will result in a smaller check, but the minimum is typically 71-1/2% of the dead spouse's benefit. If the person who died was receiving reduced benefits, the survivor's check is based on that reduced amount.
Another difference between spousal and survivor benefits is more flexibility in switching between benefits. If your mom receives benefits as a surviving spouse or surviving divorced spouse, she can switch to her own retirement benefit as early as age 62. Or she can wait to switch until full retirement age to her own, unreduced benefit.
If your mother does remarry after 60, she may qualify for benefits both as a widow and as a spouse. She can choose which benefit to take, but she can't take them both. Social Security will calculate both benefits, and she can choose the larger one.
Clearly, Social Security has a lot of moving parts. It's easy to get confused and make a mistake that could result in a smaller benefit than what she's entitled to. What's important is educating yourself and running the numbers, using the Social Security benefits estimator and tools like AARP's retirement calculator, to understand all your options.
"At least then people won't do stuff because they didn't know any better," Peterson said.
If you suspect your mom should be getting a bigger check, you or she can contact the Social Security Administration at 800-772-1213 to get the review process started.
Liz Weston is the Web's most-read personal-finance writer. She is the author of several books, most recently "The 10 Commandments of Money: Survive and Thrive in the New Economy" (find it on Bing). Weston's award-winning columns appear every Monday and Thursday, exclusively on MSN Money. Join the conversation and send in your financial questions on Liz Weston's Facebook fan page.


