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Published July 22, 2012, 11:30 PM

Hot Topic: Women with big firstborns may have higher breast cancer risk

Women whose firstborn infants have a high birth weight may have more than double the chances of having breast cancer decades later in life, a new study suggests.

By: Source: NBCnews.com’s “Vitals” blog, INFORUM

Women whose firstborn infants have a high birth weight may have more than double the chances of having breast cancer decades later in life, a new study suggests.

Researchers found that women with big babies – those weighing in the top fifth of babies on a growth chart, or more than 8.25 pounds – had a risk of breast cancer that was 2.5 times higher than that of women with smaller infants.

“We were surprised at how strong this effect was,” said study researcher Dr. Radek Bukowski, a professor of obstetrics and gynecology at the University of Texas Medical Branch in Galveston. “We were not expecting a large baby to be that strong a predictor of breast cancer.”

The results held true even after researchers took into account factors known to affect a woman’s risk of breast cancer, such as her age when she gave birth, her age at her first period and at menopause, body weight and maternal history of breast cancer.

The researchers said they suspect that giving birth to a large baby creates a hormonal environment during pregnancy that seems to favor the development of breast cancer in women almost 40 years later.

The findings are published online Tuesday in the journal PLoS One.

In the study, researchers looked at two groups of women. They found that 31 participants, or almost 8 percent, were diagnosed with breast cancer, usually in their late 50s or early 60s.

Researchers also evaluated data collected from nearly 24,000 pregnant women participating in a study known as the FASTER trial (First and Second Trimester Evaluation of Risk for Aneuploidy). In this trial, information was gathered regarding the women’s health histories and their babies’ birth weights, along with the women’s levels of three pregnancy hormones: estrogen, anti-estrogen and insulin-like growth factors.

Results showed that women with large firstborn babies had higher levels of estrogen and insulin-like growth factors during pregnancy, and they had lower anti-estrogen levels, compared with women whose babies were smaller. These hormones are thought to be involved in breast cancer development and progression.

“What happens during pregnancy has long-term consequences for the health of a woman,” Bukowski said.

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