Catholics who argue in favor of oral contraceptives often claim that the risk of a break-through ovulation, which then opens the way to possible conception and an abortifacient event, is low. There are many problems with this claim.
1. Risk of break-through ovulation (BTO) varies with type of Oral Contraceptive (OC). Risk range on the high end is 65% per cycle for one OC, and 35% for another.
2. Risk of BTO is usually stated per cycle. The risk per year is the more relevant and realistic value. Many women have short cycles, and therefore more cycles per year, and more risk per year. Then women with longer cycles would have comparatively lower risk. But if you don’t take into account the yearly rate, you lose track of that important factor. The average number of cycles per year is 12.8.
3. After considering the yearly BTO rate, we should then consider how this affects the number of prenatals likely to be lost over a 5, 10, or 15 year period of time. Women usually take OCs for years at a time.
4. Studies give BTO rates for perfect use. They exclude any cycles if the woman skipped a pill for a day or two, or took an antibiotic or antifungal that might interfere with the pill. This presents an unrealistically low BTO rate. Imperfect use is common, and multiplies risk of BTO by 2 or 3 times.
5. Imperfect use is not the result of being irresponsible. It is usually the result of side effects, which are common. The woman skips taking the pill for a day or two, until the side effects subside.
6. BTO rates given in studies offer a 95% Confidence Interval, which is a range of possible rates, going much higher than the stated rate in many cases.
For example, a few studies found zero ovulations in a set of cycles from 10 women. The 95% CI was 0 to 30.8, meaning that there is a 95% level of confidence that the true value lies between zero and a 30.8% chance of BTO. There is no such thing as 0% break-through ovulation; it is a range of possible values, which is never 100% certain.
7. The newer third generation OCs have been found to have higher risks of some serious medical disorders (venous thromboembolism) than the previous (second) generation progestins, along with lower BTO rates. So the lower BTO rates come at a cost.
8. Some women will have higher BTO rates than studies show, and you cannot tell if you are among those women. No individual woman can tell what her particular BTO rate is, without medical tests on a monthly basis. Risk of BTO for one woman versus another can be several times higher.
9. The use of even the OCs with the lowest risk of break-through ovulation, across the millions of women who use OCs worldwide (about 105 million), will still result in millions of prenatal losses.
How is all that low risk?
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