About 5% including women who do not take it as directed. If taken as you should, it's about 1%
I had put out a post on this forum for another user who had asked about BCPs but was worried about spotting and pregnancy. I am going to paste my response in here in hopes that some portion of it answer you question. My answer does have the reliability percentage also.
It is quite normal for spotting to occur while using regular BCP's as it is even more so with pills like LoEstrogen. LoEstrogen is a lowered amount of Estrogen as you can probably guess by the name. Spotting in not uncommon when taking the pill, especially when you are new to taking it, within the first 6 months up to a year, and for some even the whole time they take it.
However, also missing pills and or taking them at not the same time everyday, (for some this means exactly the same time each day) can cause more spotting and a decrease in efficacy with the pill. At best, there is a 92-99.7% efficacy when taking the pill, without errors but that effectiveness reduces in a compounded manner when a woman either takes the pills at different times, forgets a pill or uses antibiotics which can/will counter-effect the pill all together.
If you are spotting, it suggests that your level of ongoing estrogen has been interpreted, but doesn't necessarily always mean that your effectiveness has been reduced. But, since you have mentioned that you took it at a different time, I would suggest you use a back up method of protection, for the rest of your cycle. You can also ask your doctor or public health nurse if taking an additional pill might help. It used to be advised if a pill is missed to take it as soon as possible or take an extra pill. I am not sure if this is still in practise or even recommended, so it is best to check with your health practitioner before doing anything that is not indicated with your BCP.
There is a chart which seems to indicate what I have suggested to do if you miss pills:
Read also this:
If using the BCP is your only method of protection (and you can not use condoms or foam or spermicides - as some suggestions) than you should definitely abstain for the rest of the cycle if you want to ensure you do not accidentally ovulate and then conceive. Your riskiest point for conception would fall on the date and about 6 days after missing that pill or taking it late. What happens is your body assumes that you have ended your pills, (it can be explained in this way - the BCP is designed to fool your body into thinking it's pregnant, and therefor ovulation doesn't occur) and that since it is no longer getting those hormones, (a decrease of estrogen/progesterone) your body which built up the lining during this time will now shed that lining since the hormones are not elevated enough to keep it. When you miss a pill/take it late, mid cycle, your body may release an egg (ovulation) and thus conception can take place.
Here is another great site that might help to alleviate your stress or concerns:
If you have bleeding between periods, try to take your pills at the same time every day. If you have spotting (light bleeding between periods) for several cycles, call the doctor's office for advice.
I hope that this helps you, and the sites I provided help to explain the functions of your BCP and what you should do if you miss one in future. I think for you since you are on LoEs, you need to ensure you are taking them at exactly the same time each and every day. Please do follow up with a secondary method if you are concerned.
I hope that this helped you, but the odds in essence are not as effective as normally explained. When the pill is taken as directed, and maintained properly all through out the cycle, the best percentage is 97%. Not to provide you with additional worries, but some women are just not protected well enough on the pill. When my best friend was 21 and then 27, she got pregnant both times while on the pill. She is adamant that she never missed any pills, and that she conceived both of her daughters while on the pill. It does happen, and is more common than people think. Birth control doesn't necessarily fail, but when used incorrectly, the protection fails. If you are really worried, you could additionally use other forms of contraception. You can never guarentee that you wont be one of the people who fall into the 3-8% category that do get pregnant while on the pill.
I hope this helps you.
In addition to anything I have mentioned, please keep in mind, that while I or others may have extensive knowledge in many areas, you should always seek professional medical advice from your own physician, as it pertains to medical conditions or concerns.
Good luck, and if you have any other questions that I can help you with, please feel free to message me directly.
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Your best bet is to look at the packet insert, and see the odds of the particular kind of pill you take. They probably have a FAQ section on their website also, and if so there might even be more tailored answers depending on how reliably the person takes their pill. (In other words, a person taking it every 24 hours exactly probably has certain odds, and a person taking it every 20-30 hours and maybe missing a pill every now and then has slightly worse odds).