Pregnency, the Pill & the Odds

On a show last night someone was quoted as saying the Pill was only 49.9% more effective than nothing. The reasoning being everytime you have sex there is a 50-50 chance you will get pregnent. IF the pill is 99.9% effective thus it is only 49.9% more effective than nothing.
This strikes me as wrong but I exactly can’t say why. I guess the closest thing I can come up with is this is similar to someone saying “the sun will rise in the morning or it won’t, thus the chance of the sun not rising is 50%.” It seems wrong.

Was TV incorrect (alas no!!!)

TV was definitely wrong, because first of all there is not a 50-50 chance of getting pregnent everytime you have sex. There is only a period of time (7 days of each month IIRC) where a woman can get pregnent. I don’t know what the odds are of a woman getting pregnent during that period of time.

Even assuming that it were 50-50 it would still be wrong.

50% chance of not getting pregnent + (50% chance of getting pregnent * 99.9% of having the pill prevent the pregency) = 50% + 49.95% = 99.95%.

So not matter how you look at it the show was wrong.

When one says that Pill is 99.7% effective, it’s calculated based on the odds of getting pregnant within one year, not per time you have sex. BTW, the odds of getting pregnant in one year without using any form of protection are about eight out of ten.

Incidentally, a woman isn’t fertile for seven days out of her cycle. It’s closer to three or four (but don’t count on it & use protection every time to be safe…public service announcement…)

~Stella F.

Comparing percentages is always dangerous, and subject to abuse. If there is 50% chance of event A occurring, and A suddenly becomes 50% more likely to occur, there is now a 75% chance of it occurring, not 100%.

In your example, if we take your figures as accurate, there is a 50% chance one way and a 0.1% chance the other way. They should have said that you were 500 times less likely to become pregnant while on the pill. As noted before, the accuracy of these figures is dubious.