One study suggests that women with children using IVF are more likely to develop breast cancer.
The medicines used to stimulate the ovaries can increase the risk of tumors by up to 65 percent.
Experts called the results "wake-up calls" and said patients should be warned about the danger.
The researchers analyzed data on 625,712 Danish women who were followed up for up to 21 years.
Some received assisted reproductive treatment, such as IVF, others did not.
In the assisted group, the probability of developing breast cancer was 10 percent higher than in the other.
But the risk far increased among women who had their first child through IVF at the age of 40 or older.
They were 65 percent more likely to get the disease than those naturally pregnant at the same age.
In IVF women are usually given powerful medications that encourage the ovaries to breed more eggs.
However, this increases the level of the hormone estrogen, which drives some breast cancers.
The doctors at the University of Copenhagen said the altered hormones could be to blame.
Prof. Geeta Nargund of St. George's Hospital in London added: "This is a finding of great importance.
"This is a wake-up call about the use of high-dose stimulation in IVF, especially in women over 40 years.
"It highlights the need to advise women on their potential health risk in the future.
"The use of drugs and dosages should be kept to a minimum." Dr. Jane Stewart, chair of the British Fertility Society, said:
"Previous studies have been reassuring for breast cancer, but the discussion is apparently incomplete." Eight out of 1,000 treated women had breast cancer, compared with six out of 1,000 in the other group.
Dr. Roy Farquharson of Liverpool Women's Hospital said the findings should not cause concern.
He added, "The results should trigger further studies to confirm an increased risk." ART treatment. "
In the UK, breast cancer is the most common form of the disease, with 55,000 cases annually.
It is well known that older women and those who are childless or infertile are at greater risk.
The results were presented at the annual meeting of the European Society for Human Reproduction and Embryology in Vienna, Austria.