Women whose mothers used an anti-miscarriage drug in pregnancy have double the risk of breast cancer than others their age, a US study suggests.
The study looked at breast cancer risk
Diethylstilbestrol (DES) was given to up to 300,000 UK women between the 1940s and the early 1970s.
It has already been linked to a higher risk of reproductive system cancers.
UK experts said the Boston University study did find DES increased breast cancer risk, but said exposure did not mean women would develop the disease.
DES, a synthetic oestrogen, was developed in 1938 as a treatment for women with low levels of natural oestrogen during pregnancy, who doctors believed were at an increased risk of abortions and premature births.
However, in 1953, a clinical study found DES did nothing to reduce the risk of miscarriage.
But it was used until 1971, when it was found the daughters of women who took the drug were at an increased risk of rare cancers of the vagina and the cervix .
Further research also linked DES to an increased the risk of breast cancer in mothers.
'Breast checks needed'
In this latest study, published in the Cancer Epidemiology, Biomarkers and Prevention journal, a team from the Boston University School of Public Medicine, monitored 4,817 women over 40 known to have been exposed to DES while in their mothers' wombs and compared them with 2,073 unexposed women of the same age.
There had been 102 cases of breast cancer across both groups.
Other factors, such as the age at which the women gave birth and how many children they had had, which can influence breast cancer risk, were taken into account.
It was found that women exposed to DES had 1.9 times the risk of developing breast cancer, compared with unexposed women.
The scientists say the explanation might be that the oestrogen in the drug increases the number of breast tissue stem cells available at birth - cells which could become cancerous.
Professor Julie Palmer, who led the research, said: "This is really unwelcome news because so many women worldwide were pre-natally exposed to DES, and these women are just now approaching the age at which breast cancer becomes more common."
She urged women who were exposed to DES to ensure they have regular breast checks, and to be careful about using hormone replacement therapies.
"It might be wise for exposed women to avoid such supplements.
"Use of hormone supplements is, in itself, an independent breast cancer risk factor, and women may choose not to compound their already increased risk."
Ed Yong, at Cancer Research UK, said: "This study suggests that daughters of mothers who took the drug DES during their pregnancies may have a higher risk of breast cancer as they get older, but this does not mean that they will definitely develop the disease.
"Early detection of breast cancer is important and all women should be aware of any unusual changes to their breasts and report them promptly.
"Women should be sure to attend their breast screening appointments when invited."
Dr Sarah Rawlings, of Breakthrough Breast Cancer, added: "The link between women who took DES during pregnancy and their daughters potentially having a higher risk of breast cancer is very concerning.
"If women are worried about this information, we would advise them to speak to their doctor who will be able to assess their risk."