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How does estrogen dose affect protection against pregnancy?

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Michelle asks:

Does the amount of estrogen in birth control pills (ex. low, high or moderate levels) have any effect on protection against pregnancy? For example, is it better to take a birth control pill with a higher level of estrogen than one of the newer "low dose" ones? Will the protection be the same?

Susie replies:

The dosing mostly matters depending on your body composition. Estrogen is a fat-soluble hormone. Therefore if you have a significant amount and proportion of body fat, some of that estrogen will dissolve into the fatty tissue and not make it to its receptor sites in your sex organs. If you have more fatty tissue, you will need more estrogen to prevent ovulation (that's estrogen's role in birth control pills).

This is why it's important to see a clinician and get an examination before obtaining a prescription for birth control. Your doctor will need your vital stats to figure out which pills will work best.

For most women even if they're a little overweight, they should still be able to take lower-dose pills. If you are moderately overweight, significantly overweight or obese, your doctor should prescribe you something stronger to ensure full effectiveness.

written 23 Jan 2008 . updated 28 Jan 2009

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