Ten months ago, Vanessa Thiemann lay in bed unable to sleep.

The 42-year-old single mother of two had a sinus infection, and the pain was making her restless. She tried getting comfortable on her left side, then her right, but she ended up staring at the ceiling in complete darkness, her left hand coming to rest on her chest.

It was at that moment her fingers brushed a tiny knob under her skin.

"I felt a rock-hard lump next to my nipple," Thiemann recalls. "I just knew at that moment I had cancer."

Thiemann did have cancer. And while her breast surgeon assured her the tumor was isolated to her right breast, and she knew she had no family history of the disease, Thiemann opted for a double mastectomy.

"It was important to me to remove both breasts," she says. "I knew that if I kept my other breast, I would obsess over it, and I'd always worry that cancer would develop there, too."

Researchers are paying close attention to the increasing rate of double mastectomies in the United States.

The number of women with early stage breast cancer who went on to remove both breasts (even though only one breast had cancer) increased by more than 150 percent between 1998 and 2003, according to a study presented by Dr. Kelly Hunt at the annual conference for the American Society of Clinical Oncology.

Since that meeting, Hunt has continued crunching the numbers. In unpublished reports released to CNN, 8 percent of patients sought prophylactic removal of their unaffected breast at MD Anderson Cancer Center in Houston in 2010. The percentage increased to 12.6 percent in 2011, and rose again to 14.1 percent in 2012, according to Hunt, who is chief of breast surgery at MD Anderson.

Data from New York's Memorial Sloan-Kettering Cancer Center echo these numbers. The rate for women choosing to remove both breasts when only one has cancer jumped from 6.7 percent in 1997 to 24 percent in 2005, according to a 2011 study in the Journal of Clinical Oncology. And from 2005 to 2009, the latest year data is available, the proportion of women undergoing mastectomies overall also rose, according to Kathy Cronin, a statistician with the National Cancer Institute.

Experts say several intersecting factors could be fueling this trend.

Access to genetic screening

Since testing became available in 1996, nearly 1 million people have been screened for BRCA1 and BRCA2, the genetic mutations associated with increasing a woman's lifetime risk of developing breast cancer as much as 87 percent, according to Myriad Genetics, the diagnostic company that helped isolate the two genes and later developed a test to detect them.

For women who test positive, removing both breasts is seen as a viable, cancer-preventing option -- especially in the United States.

In a global study conducted by Dr. Steven Narod, senior scientist at Women's College Research Institute in Toronto, the United States had the highest rate of prophylactic mastectomy in BRCA1 and BRCA2 mutation carriers. The U.S. rate was 36.3 percent, far outpacing the number of preventive surgeries performed by the majority of other countries Narod studied, including France and Canada.

One reason for the spike may be that American women fear cancer more than women in other regions. About 70 percent of women in the United States who have both breasts removed after a cancer diagnosis don't have a proven medical reason for undergoing the procedure, according to a 2012 study conducted by the University of Michigan Comprehensive Cancer Center.

"The dilemma we're facing is more and more women are choosing to remove both breasts," said Dr. Michael Sabel, associate professor of surgery at the University of Michigan Medical School, in a statement announcing the findings. "We're greatly overestimating the risk of women with breast cancer developing another breast cancer."

However, authors of the study indicate that a double mastectomy may make sense for women with a strong family history of breast or ovarian cancer, or for women who've tested positive for genetic mutations in the BRCA genes.

Dr. Susan Domchek, executive director of the Basser Research Center for BRCA at the University of Pennsylvania, said the escalating rate is linked to easy access of information.

"The data we've been collecting is evolving quickly, and these women are living with this information in real time," she said in an email. "Since 1994 enormous progress had occurred: (T)he genes have been cloned, clinically available tests for gene mutations have been developed, and the implications of having BRCA1/2 mutations are better understood. Women as young as 25 are using this data to make informed choices."