Most research related to a baby's birth weight focuses on how a baby's size may impact his or her long-term health but a baby's size also has implications for the mother's' health.
A recent study found that having a larger than average baby can increase a woman's risk of developing breast cancer.
In a study led by Radek Bukowski, MD, a professor of obstetrics and gynecology in the Division of Maternal-Fetal Medicine at the University of Texas Medical Branch at Galveston, researchers pulled data from two different groups of women.
The first included 410 women enrolled in the Framingham Offspring Birth History Study who were studied from 1991 to 2008.
They considered the women's own birth weight as well as the birth weights of their babies and looked at the rate of breast cancer in this group, which was 7.6 percent.
Researchers also took into account with this group other known risk factors for breast cancer, including age, race/ethnicity, weight, a family history of breast cancer and whether a a woman had diabetes or used hormone replacement therapy.
They found that women whose babies' birth weights were in the top fifth percentile - 8.25 pounds or more - were about two and a half times more likely to develop breast cancer than women whose babies were under 8.25 pounds.
This doubled risk of breast cancer remained even when the researchers took into account the mother's birth weight and other breast cancer risk factors.
The other group studied by the researchers came from a trial called the First and Second Trimester Evaluation of Risk for Aneuploidy. It involved 24,000 women at 15 U.S. clinics, from 1999 through 2003.
The researchers looked at data on specific hormones in these women that are known to impact a baby's birth weight and the woman's risk for breast cancer. These hormones include estriol (E3), anti-estrogen alpha-fetoprotein (AFP) and pregnancy-associated plasma protein-A (PAPP-A).
The researchers found that all three of these hormones increaed in concentration as the birth weight of a baby increased.
Women with the largest babies (upper fifth percentile) had a 25 percent greater risk of having a high ratio of E3 to AFP and a high concentration of PAPP-A. This means that one out of every four women with babies over 8.25 pounds would have a a high E3/AFP ratio and PAPP-A concentration.
The significance of higher hormone concentrations relates to past research that found a link between the amount of hormones a woman produces and her potential for developing breast cancer.
"This means that they have high levels of estrogen, low levels of anti-estrogen and the presence of free insulin-like growth factors associated with breast cancer development and progression," Dr. Bukowski said. "Recent animal studies have suggested that breast stem cells, which are involved in the origins of breast cancer, may increase or decrease their number in response to hormone exposures, including ones during pregnancy."
He said increased levels of hormones can create a "pro-carcinogenic environment," which means an environment within the body that makes it more susceptible to cancer developing.
"They [breast stem cells] retain a 'memory' of prior hormone exposure, which could explain the increased risk of breast cancer seen following pregnancy, especially in women with a large birth weight infant," he said. "The hormones create a long term effect that may lead to breast cancer later."
The biggest limitation to the first study showing the correlation between large babies and breast cancer was from a relatively small population of women for this kind of observational study (410). The larger group study, in which a link was found between a baby's size and the woman's hormone concentrations, studied different women than the first study.
Dr. Bukowski pointed out that a woman cannot change the hormones her body produces, but there are other actions she can take.
There are several behaviors related to childbirth that have been shown to reduce a woman's risk of developing breast cancer, especially including breastfeeding.
In addition, having more than one child, exercising and eating healthy and balanced diets both while pregnant and in general have all been shown to reduce a woman's likelihood of developing breast cancer.
The study was published July 17 in PLoS ONE. There was no external funding for this study's analysis of data, and the authors declared no conflicts of interest.
The Framingham Heart Study was funded and conducted by the National Heart, Lung, and Blood Institute in collaboration with Boston University, and the FASTER trial was funded by the National Institutes of Health and National Institute of Child Health and Human Development.