Women whose mothers took a pregnancy drug should be vigilant for the signs of breast cancer, experts warned.
The drug DES - diethylstilbestrol - has been linked to a dramatic increase in the risk of breast cancer in the daughters of mothers prescribed the drug.
Research published in the US journal Cancer Epidemiology Biomarkers & Prevention found that women over 40 whose mothers had taken the drug were almost twice as likely to develop breast cancer.
The risk was even higher among those aged over 50 and, across all age groups, the increased risk was 40 per cent.
However, the researchers did say that there was no apparent increased risk for women under the age of 40.
DES was prescribed until about 1975 to protect against miscarriage and combat morning sickness.
It has already been found to have increased the risk of breast cancer in mothers who took it and to cause fertility problems in their daughters.
Lead researcher Professor Julie Palmer said of the study: "This is really unwelcome news because so many women worldwide were prenatally exposed to DES and they are just now approaching the age at which breast cancer becomes more common.