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Women who give birth to multiple babies after IVF treatment are 44 percent more likely to develop breast cancer than those giving birth to singletons or who remain childless, a new study suggests.

Dutch scientists explained that the elevated cancer risk might be explained by a maternal trait related to higher implantation potential and to breast cancer itself.

While the latest study results were derived from a large nationwide cohort study, researchers emphasize that the findings should be replicated in future studies before earlier breast cancer screening is recommended to the general population.

Researchers presented the study July 9 at the annual meeting of European Society of Human Reproduction and Embryology.

Lead researcher Dr. Els Groeneveld from the VU University Medical Center of Amsterdam, the Netherland and her team analyzed data from the Omega study, a arge Dutch nationwide cohort of 19,861 women who received IVF or ICSI treatment between 1983 and 1995. Between 1997 and 1999, all patients were asked to fill out a questionnaire about their treatment and its outcome.

The 12,589 women who completed the questionnaire were cross-referred to the Netherlands Cancer Registry. Study results shoed that over a median 16.7 years of follow-up, 13 percent or 1,688 women in the study gave birth to multiples, 48 percent of 6027 women delivered singletons and 39 percent or 4,874 women remained childless.

Researchers said among the study participants, there were 317 confirmed diagnoses or breast cancer.  Fifty-seven of the patients had given birth to multiples, 155 to singletons and 105 remained childless.

When these results were analyzed statistically, the study revealed that mothers of multiples had a 44 percent higher breast cancer risk than the mothers of singletons when adjusted for year of IVF treatment, number of IVF cycles, height and age at first birth. Childless women were at no increased risk of breast cancer, according to researcher.

Interestingly, only multiple pregnancies conceived after the complete implantation of all transferred embryos were associated with an increased breast cancer risk, whereas multiple pregnancies conceived after incomplete implantation were not significantly associated with breast cancer. Researchers said this finding supports the hypothesis that there is a link between high embryo implantation potential and breast cancer risk.

"It has been generally assumed that increased levels of estrogen and progesterone in multiple pregnancies stimulate cellular proliferation in the breast, which increases accumulation of somatic mutations during cell division and leads to the development of breast cancer. Thus, breast cancer could be seen as a consequence of the multiple pregnancy itself. However, we also hypothesize that an additional maternal trait might be associated with an increased breast cancer risk in these women," Groeneveld said in a news release.

Groeneveld and her team say that such a trait could be maternal serum concentrations of vascular endothelial growth factor (VEGF), a known protein involved in breast cancer progression and which was recently linked to better embryo implantation potential.

"In our study women who developed a multiple pregnancy from all transferred embryos represent women with high embryo implantation potential, possibly through increased levels of VEGF," Groeneveld explained.

"Future studies will reveal whether the significant association between improved embryo implantation potential and breast cancer risk shown in our study is actually based on increased levels of VEGF," she added.

"The main message of our study is that the increased breast cancer risk is not only a consequence of the multiple pregnancy itself because of high hormone levels, but that the risk of breast cancer may already be elevated prior to conception in women who have the potential -- the 'maternal trait' -- to develop a multiple pregnancy from all transferred embryos," Groeneveld concluded.