While both de Brito and Peoples say their doctors never found reasons for their infertility, there are generally some fairly common causes.
"The major cause is age," said Dr. James Grifo, program director at the fertility center at New York University. "The older you get, the harder it is to get pregnant, and if you delay it, it makes it even harder."
"It can also be caused by a low sperm count," said Dr. Alan Penzias, associate professor of obstetrics and gynecology at Harvard Medical School.
Other known causes include endometriosis, pelvic inflammatory disease or other conditions that cause scarring and blocking of the fallopian tubes, said Penzias.
There's some speculation about a genetic component, but it's unclear what that may be.
"People with a history of early menopause may be at a higher risk for themselves having early menopause and decreased ovarian function," said Penzias.
Doctors use the same treatments for secondary infertility as they would for patients who have trouble conceiving a first child.
"Generally, you start with a work-up, then fertility drugs," said Grifo. "If those don't work, we'll try artificial insemination and then in vitro fertilization, which is the most effective and last treatment option."
"The prognosis is generally quite good," said Grifo. "Once you've proven you can have a pregnancy, your chances are good, except in older women."
Doctors diagnose secondary infertility using a couple of standard criteria.
"For women younger than 35, if there are no other problems, they're having normal menstrual function, haven't used contraception for a year and still can't conceive, that's when it's considered secondary infertility. In women older than 35, it happens if you have no other problems, have normal menstrual function, have gone 6 months without contraception and haven't conceived," said Penzias.
While there's no way to predict whether a woman will experience secondary infertility, Penzias advises women to take prenatal vitamins, excercise routinely and eat right.
For Kristin Peoples, her quest for more children is over, but the story goes on for Lauri de Brito. At age 49, she's trying for a fourth child through another surrogate, her desire still burning strong.
"It doesn't go away. Unless you're given a reason why you can't have another child, it doesn't go away."