The delivery room was tense as Sandy Byrne struggled and strained to push her son out. Already it had been two painfully difficult hours.

"This tension was building of 'When is this thing going to happen?' " said Byrne, a mother of three who lives in Austin, Texas.

When her son, Jensen, finally was born, the tension lingered, but for a different reason: Jensen was a big, big baby.

"My doula was like, 'Oh my god, he's huge,' " said Byrne. "Everyone (in the delivery room) was trying to be supportive about letting me hold him, but they were anxious. 'We want to weigh him. ... Get him on the scale quick.'"

Jensen, who was born two years ago, tipped the scales at just over 10 pounds.

His hefty birth weight may put his mother at more than twice the risk of breast cancer compared with a woman who had a comparatively smaller baby, according to preliminary data from two studies, published Tuesday in the journal PLoS One.

"Giving birth to a big infant is associated with a two-and-a-half-times increased risk of breast cancer," said lead study author, Dr. Radek Bukowski, who added that that is independent of traditional risk factors for breast cancer.

It is an interesting finding that Bukowski stressed is an early one.

One study included 410 women in the Framingham Offspring Birth History Study. During the follow-up period, from 1991-2008, 31 women (7.6%) had been diagnosed with breast cancer.

Women with bigger babies had a higher risk of developing breast cancer. That link held true even when scientists considered commonly known risk factors such as BMI, use of hormone replacement therapy, family history and age.

Another tantalizing clue for study authors came from a study of 23,824 mothers of singleton infants participating in the FASTER clinical trial between 1999-2003.

Turns out, women who delivered large babies (8.25 pounds or larger) also had what Bukowski calls a "pro-carcinogenic" hormonal profile.

Meaning that "if a woman has a big baby, the profile of hormones is one that favors the future development and progression of breast cancer," said Bukowski, professor of Obstetrics and Gynecology at the University of Texas Medical Branch in Galveston.

The theory behind the so-called "favorable" profile may have something to do with the hormonal environment for a woman carrying a heavy baby.

During pregnancy, a dynamic typically develops between two hormones called estrogen and anti-estrogen. In women who have heavier babies, the ratio of estrogen to anti-estrogen is unusually high.

Another hormone, called pregnancy-associated plasma protein-A (PAPP-A), also soars during pregnancy.

Simply put, a bigger baby means more hormones -- especially estrogen -- in the body, and more estrogen is associated with increased breast cancer risk.

"Higher levels of free estrogen, that is significant," said Dr. Eva Chalas, director of Clinical Cancer Services at Winthrop University Hospital. "It affects how DNA is repaired and does affect risk of cancer."

Chalas added that although the study results suggest a baby's birth weight as an independent risk factor for breast cancer, obesity among women should not be discounted as another -- common -- risk.

"People think having big babies is a good thing, that it means (the baby is) healthy," said Chalas, who was unaffiliated with the current study. "But people should take this study seriously. Bigger babies usually come from bigger mothers.