Study Progress
Study Progress
Prior to starting the main study, pilot studies involving around 6,000 women were conducted between 1994 and 1996. These preliminary studies showed that the questionnaire was acceptable to women and that receiving the questionnaire did not affect whether or not they went to screening.The pilot studies also showed that around one third of women attending the National Health Service Breast Screening Programme were using hormone replacement therapy, and that if a large scale study of hormone replacement therapy and women’s health were to go ahead, the vast majority of those attending screening would take part.
The Million Women Study was officially launched in 1997 and now involves 66 National Health Service Breast Screening Centres nationwide. Thanks to staff working on the study and the enthusiasm of women throughout the UK, the study finished recruiting in 2001 and 1.3 million women are taking part in the study.
All Million Women Study publications can be found on the Publications page.
Some early findings
Initial analysis of responses received to the recruitment questionnaire showed that among women participating in the study:
- 1 in 2 women had taken the oral contraceptive pill
- 1 in 2 women had tried HRT
- 1 in 3 women was currently using HRT
- 1 in 4 has had a hysterectomy
- 1 in 11 has a close female relative with breast cancer
- 1 in 70 have had breast cancer in the past
More detailed analyses based on the replies of over 1 million women showed that whether or not a woman was currently using HRT did not depend much or at all on where she lived, when she entered the study or on socio-economic or “lifestyle” factors (such as number if children, smoking or exercise), but was strongly influenced by her age and by her medical history. For example, among women who had had a hysterectomy, 48% were current users, but among women with a history of breast cancer only 6% used HRT. These findings will be taken into account in future analyses so that we can be sure we are looking at the effects of HRT and not the effects of factors influencing HRT use.
Analyses have also shown that current users of hormone replacement therapy are more likely to be recalled for further assessment after mammography.
HRT and breast cancer (2003)
Follow-up of over 1 million women in the Million Women Study confirmed findings from other recent studies that women currently using HRT are more likely to develop breast cancer than those who are not using HRT. Past users are not at increased risk. The Million Women Study was able to show that this effect is substantially greater for combined (oestrogen-progestagen) HRT than for oestrogen-only HRT; and that the effects were similar for all specific types and doses of oestrogen and progestagen, for oral, transdermal and implanted HRT, and for continuous and sequential patterns of use. Current users of oestrogen-progestagen HRT were at 2 fold increased risk of developing breast cancer, and current users of oestrogen-only HRT at 1.3 fold risk. Use of HRT by women aged 50-64 in the UK in the decade from 1993-2003 resulted in an estimated 20,000 extra breast cancers. See Publications for full results.
HRT and endometrial (womb) cancer (2005)
It is well known that post-menopausal women who have not had a hysterectomy are at increased risk of cancer of the endometrium (the lining of the womb) if they take oestrogen-only HRT. Follow up of over 700 000 women in the Million Women Study confirmed this and showed that the risk of endometrial cancer is also increased in women who take tibolone; but is not altered, or may even be reduced, in women taking combined oestrogen-progestagen HRT. These effects depend also on a woman’s body mass index (BMI, a measure of obesity) such that adverse effects of tibolone and oestrogen-only HRT are greatest in thinner women, and the beneficial effects of combined HRT are greatest in fatter women.
For full results see Publications page.
HRT and ovarian cancer
New results from the Million Women Study (April 2007) show a small increase in risk of ovarian cancer in women taking HRT (see Publications). Such an increased risk had been suspected from some previous studies, and has now been confirmed with the larger numbers available in the MWS. The findings come from analyses on 948,576 post-menopausal women in the study, followed up for about 5 years. Women currently taking HRT were at higher risk of developing and of dying from ovarian cancer than women not using HRT. Past users were not at increased risk. The risk in current users was increased about 1.2 fold; for every 1000 women using HRT, 2.6 developed ovarian cancer over 5 years, compared with 2.2 in those not taking HRT. The risk was the same for oestrogen-only, combined oestrogen-progestagen and other types of HRT (including tibolone) and did not vary by specific type of oestrogen or progestagen, or between oral and transdermal (patch) administration.
These results are equivalent to one extra case of ovarian cancer for every 2500 women taking HRT, and one extra death from ovarian cancer per 3300 women taking HRT, over 5 years. The results need to be looked at in the context of the other risks and benefits of HRT. In particular, we can now estimate the overall effect of HRT use on three common cancers in women: breast cancer, endometrial (womb) cancer and ovarian cancer. Together, these cancers account for about 4 in 10 cancers in women in the UK. In women aged 50-69, about 19 of these cancers will develop over 5 years in every 1000 women not taking HRT. In women taking HRT we estimate the number of cancers to be increased to about 31. The overall increased risk is higher in women using combined oestrogen-progestagen HRT (an increase from 19 to 34 cancers over 5 years) than in women using oestrogen-only HRT (an increase from 19 to 26 cancers over 5 years) because most of the overall increase is due to an increase in breast cancer, and users of combined HRT have a higher risk of breast cancer than users of oestrogen-only HRT.
In general, these new findings on the risk of ovarian cancer in HRT users will not affect current advice on HRT prescribing, which remains to use HRT for as short a time as possible to treat menopausal symptoms.
Public Health Implications: impact of the Million Women Study
Results from the Million Women Study, together with those from other studies such as the Women’s Health Initiative trial from the USA, have influenced national policy, including recent recommendations on the prescribing and use of hormone replacement therapy from the Royal College of Obstetricians and Gynaecologists and from the Commission on Human Medicines
