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Here is what I was able to find researching the internet.
Birth control pills also contain estrogens and progestins but different kinds and different doses. All estrogens have similar side effects. However, the risks of particular side effects may differ since birth control pills are used by younger women with a different health status compared to postmenopausal women.
As with taking any estrogens, the use of birth control pills is also associated with increased risks of several serious conditions including heart attack, blood clots, stroke, liver tumors, and gallbladder disease. These risks are higher in women with underlying risk factors of high blood pressure, high cholesterol, diabetes, and cigarette smokers. http://www.monheit.com/prempro/faqs.shtml
Prempro is in the FDA pregnancy category X. This means that these medications are known cause birth defects in an unborn baby. Do not take Prempro if you are pregnant or if you could become pregnant during treatment.http://www.drugs.com/prempro.html
I hope that helps.
I found a definite answer to your question:
Because a woman may continue to ovulate during perimenopause, it is still possible for her to become pregnant. A woman who wants to avoid pregnancy should use birth control until a blood test shows that menopause is present. Or, she should use birth control until she does not have a period for 12 months in a row. After that, she may switch to the lower doses of hormones in HRT. The hormone doses in HRT are not high enough to prevent pregnancy.Source:http://atoz.iqhealth.com/HealthAnswers/encyclopedia/HTMLfiles/3033.html