As explained earlier in this chapter, increased thickening of the endometrium can take place when oestrogen alone (unopposed therapy) is used in HRT. Unopposed oestrogen leads to a dose- and duration-dependent increase in endometrial problems. Hyperplasia is common and can lead to cancer. In women who have not had a hysterectomy, unopposed oestrogen carries a substantial risk.

There is now convincing evidence that when a progestogen is added to oestrogen therapy the endometrium is protected from these changes. This regimen replicates what the body was doing all through the woman's fertile years: oestrogen was present in the proliferative first half of the cycle, and then the ovary added progesterone during the second half to protect the endometrium from excessive build-up and brought about a shedding (menstruation) at the correct time.

A study in the USA has shown that the occurrence of endometrial cancer in women who use opposed therapy (i.e. with a progestogen added) is significantly lower even than that found in women who are using no HRT (49 women in 100,000 as opposed to 245.5 women in 100,000).

Opposed oestrogen therapy should be used wherever oestrogen is being administered, whether orally, transdermally, as an implant or even as a vaginal cream if it is used for an extended period of time, in order to protect against endometrial hyperplasia (overgrowth of the uterine lining). The exception to this is after a hysterectomy, when oestrogen alone can be used.

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Women’s health