Lauri de Brito first experienced the joy of motherhood when her first daughter was born 11 years ago.
She and her husband wanted to have more children, and they are now the parents of two daughters and a son.
But the six years between her first and second daughters were filled with unimaginable frustration, loss and sadness.
"In those six years, I had about eight miscarriages. The longest my pregnancies went was 11 weeks," she said. She went through multiple cycles of in vitro fertilization, but still couldn't get pregnant.
After egg donation also failed, she eventually used a surrogate, who gave birth to her second daughter and son, now ages 5 and 3.
De Brito is one of many women nationwide who suffer from secondary infertility -- the inability to conceive or have a full-term pregnancy after having had children without trouble before.
The estimates of how many women have secondary infertility varies widely, but somewhere around 30 percent of infertility in women is secondary infertility, according to experts. It can be emotionally devastating for women -- and their partners -- who go through it.
Their frustration with not being able to have more children is often mixed with feelings of guilt, despair and obsession.
"People asked me why I couldn't be happy with the child I have. I wondered the same thing myself. I had this perfect child, and I felt I was ruining the first years of her life. I wondered why I couldn't be happy," said de Brito.
Kristin Peoples, age 42, has been through the same emotional lows as de Brito. She now has two children, who are 15 and 4, and it took her seven years to become pregnant with her second child.
"It felt like failure. It was hard to put your mind around why I was able to do it once and not again," she said.
Guilt and shame are extremely common among women – and men, but to a much lesser extent – who experience secondary infertility.
"I think there's also guilt on the part of couples because they may deprive their children of a sibling, and they don't their family as a family unless they have more than one child," said Linda Applegarth, director of psychological services at the Ronald Perelman and Claudia Cohen Center for Reproductive Medicine and Infertility at New York Presbyterian Hospital-Weill Cornell Medical Center.
For many women, their desire for more children is overwhelming.
"I spent more than $250,000. Physically, I was a basket case and emotionally, I was in a really bad way," said de Brito. "Nothing ever came up in my testing, which hurt, because if they would have found something wrong, they could fix it."
The desire was so great for Kristin Peoples that she underwent ovulation induction for almost two years.
"I did artificial insemination and I had to monitor my period during the day, and I had to take an injection in the thigh on certain days during the month, and then we would have to have sex during the same time frame," she said.
It even got in the way of family vacations.
"I had to take medicine with me. If I didn't plan just right, I had to count the days and do the injection," she said.
Eventually, she and her husband gave up trying. A few months later, she got pregnant. She says that won't happen again – at least not intentionally.
"It was so frustrating that we decided he's the last one," she said.
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."