Women who go through in vitro fertilization (IVF) early in life are at a higher risk of developing breast cancer compared to those who don't undergo the treatment, according to an Australian study.
But the findings, based on a study of more than 21,000 women and published in the journal Fertility and Sterility, cannot determine whether IVF contributed to the cancers or whether something else could explain the risk.
"I don't think it's a huge increased risk that you should worry or panic (about)," said Louise Stewart, the study's lead author and a researcher at the University of Western Australia in Crawley.
She added, however, that the findings did show a link between the two and doctors should keep that in mind.
For the study, Stewart and her colleagues collected information on 21,025 women between the ages of 20 and 40 who went through fertility treatment at the hospitals of Western Australia between 1983 and 2002.
They were able to piece together enough data to follow the women for some 16 years to see if they developed breast cancer.
Roughly 1.7 percent of the 13,644 women who only used fertility drugs without IVF ended up developing breast cancer by the end of the study. That figure was about two percent for women who used fertility drugs and underwent IVF - a difference that researchers said wasn't statistically significant.
When they divided the women into different age groups, though, that changed.
Women who started taking fertility drugs around their 24th birthday and went through IVF had a 56 percent greater chance of eventually developing breast cancer compared to those in the same age group who only went through fertility treatments without IVF.
But there was no increased risk for women who started fertility treatments when they were about 40 years old, regardless of whether they had IVF or not.
Stewart told Reuters Health that a possible reason that younger women see an increased risk of breast cancer is that they are exposed to higher levels of circulating estrogen during their cycles of IVF treatment.
"The development of breast cancer is linked to estrogen exposure and the longer one is exposed, the greater the risk," said Linda Giudice, president-elect of the American Society of Reproductive Medicine, in a statement.
"In an IVF cycle there is a short, but significant elevation in circulating estrogen, and whether this is linked to the observations found in the study is not clear at this time."
Stewart added that another explanation could be that younger women who undergo IVF may be different in some significant way from those who only have other types of fertility treatment.
"If, for example, younger women who had IVF were more likely to have a specific cause of infertility, and this was related to an increased risk of breast cancer, then it would appear that IVF was related to breast cancer when in fact it was the type of infertility that was more common in women who had IVF," she said.
She emphasized that this is just speculation, adding that the data used in the study didn't include information about the women's causes of infertility - and that the current study's results need to be confirmed by future research.
Hundreds of thousands of climate activists have descended on Paris ahead of the UN Climate Conference and are preparing to defy a ban on street protests joining thousands of protestors around the world.
'This is all bad news for the planet,' head of the World Meteorological Organization says
UN agency calls for urgent climate deal as hundreds of thousands fall victims to weather disasters
Fifty grams of processed meat eaten daily increases risk of colorectal cancer by 18 percent, say experts
COP21 conference aims at securing a pact on greenhouse gases that would limit global warming to two degrees Celsius over pre-industrial times.
Earth is on track for average warming of 2.7 degrees Celsius by 2100, exceeding UN's anticipation
The London Project to Cure Blindness was established a decade ago to try to reverse vision loss in patients with age-related macular degeneration
Improvement has been seen around the globe, even in the key tropical rainforests of South America and Africa
Christiana Figueres says there is 1.2 million euros deficit to cover upcoming sessions
Some of the world's major cities, most Pacific island nations are at risk of disappearing due to rising seas
Yemen forces allied to the Houthi have launched a scud missile toward southern Saudi Arabia
Three dozens of fires may cause an ecological catastrophe to Lake Baikal in Russia
National and international slow responses to epidemies show how far the world is from achieving global health security
Call for action in madrassas and mosques around the world is intended to emphasize the role Islam can play in combating climate change
New reports claim parent company Alphabet is attempting to fight malaria and other diseases transmitted via insects
Scientists are predicting that Earth could be heading towards an extraordinary event which would see compasses point south and the sun rise in the West.