And now, a large study following the fate of the ‘DES daughters’, as they became known, has dropped a new bombshell on the long-discredited drug. Women exposed to the drug while in the womb face two to three times the risk of breast cancer as those not exposed to hormones prenatally (Cancer Epidemiol Biomarkers Prev, 2006; 15: 1509–14).

This new disclosure is particularly unsettling as it identifies the age with the highest risk as the over 50s. Most of the DES daughters, exposed to the drug in the 1950s and 1960s, are now in their 40s or 50s.

At least 100,000 women in the UK were exposed to the drug while in the womb.

Between 1938 and 1971, up to 10 million American women took DES, believing it to be a safe and effective way to prevent miscarriage. It was only in the 1970s that studies revealed that the drug was a time bomb. The mothers themselves were at risk of developing breast cancer, while their daughters who’d been exposed to the drugs in utero were at high risk of developing a rare cancer of the cervix or womb. Even ‘DES sons’ had a risk of developing genital abnormalities and non-cancerous growths on their testes.

Although the risk of the DES daughters developing clear cell carci-noma is now undisputed, researchers suspected that exposure to DES in the womb also predisposed the women to breast cancer once they’d reached maturity. DES daughters appeared to have a higher incidence of breast cancer, but no one knew the extent of the risk.

Since the 1970s, researchers at Slone Epidemiology Center at Boston University have been attempting to work out the risk of exposure. They compared the incidence of breast cancer among women exposed to DES while in the womb with a comparable group of women who were not exposed to the hormone prenatally.

They recruited 4817 mostly white women, mostly born in the 1950s, who had been exposed to DES in utero and compared them with 2073 women born around the same time, but who had not been exposed to the drug. All of the women were sent questionnaires about their health. Any breast cancers that developed were confirmed by pathology reports. The researchers also controlled for other risk factors for breast cancer.

When the results were tallied, the researchers discovered 102 cases of breast cancer, 76 of which occurred in DES-exposed women and 26 among women not exposed. Those who’d been exposed to DES had a 91 per cent greater risk of breast cancer after age 40 as those unexposed, while women over 50 had a threefold greater risk.

Julie Palmer, a professor of epidem-iology at Boston University’s School of Public Health, who led the study, believes that the extra dose of hormone to which these women were exposed stimulated the development of more breast stem cells than usual, and that this increased number of such cells increases breast-cancer risk.

DES is not an oestrogen or even a steroidal compound. Nevertheless, in 1938, it was discovered that this chemical had all the hallmarks of an oestrogen. Because its molecules lock into the same cell receptors as does oestrogen, the body is duped into thinking that this activity has been stimulated by its own stores of natural oestrogen (Grant E. Sexual Chemistry: Understanding Our Hormones, The Pill and HRT. London: Cedar Press, 1994). It was produced by drug giant Eli Lilly and a number of smaller drug firms, although Lilly discontinued manu-facturing the drug several years ago.