If by "I waited a week" you mean you waited seven days, you should be fine. However, seven days is the bare minimum so if you mean anything less than a full seven days then it depends on when you began to take them relative to your menstrual cycle. These are typical instructions for beginning birth control pills:
- Start your first pack of pills on the first day of your menstrual period. Use a backup method of birth control (such as condoms with
spermicide) until you start the pill. The pill will start working
right away for birth control if you start on the first day of your
- Start your first pack of pills on the Sunday after you start your menstrual period. You should use a backup method of birth control
(condoms with spermicide) until you start the pill. You also will
need to use a backup method of birth control (condoms with
spermicide) during the first 7 days of the pill package.
- Start your pills today. If you start your pills today, you should use a backup method of birth control (condoms with spermicide) during the first 7 days of the pill package. Birth control pills will start to protect you from pregnancy after the first week, if you take them correctly.
Another answer cites a source that gives a very high effectiveness number. WebMD states that "when taken correctly, it is up to 99.9% effective". However, there are a couple of problems with that number:
First, WebMD doesn't provide a source for the number, and given that an ad for Bayer birth control pills appears directly next to it, I would have to assume that the number came from Bayer's advertising department rather than an objective scientific source.
Second, the language WebMD uses is very important here. They state that it is "up to 99.9% effective" when "taken correctly." Well, that's quite a bit like the gas mileage numbers automobile manufacturers state for their cars. They obtain those figures using professional drivers under strict laboratory conditions and, while accurate, virtually no consumer will ever achieve those same results because they are real people driving on real roads, not professionals driving on closed tracks. Likewise, in the real world women do not achieve 99.9% effectiveness with birth control pills. According to the US Centers for Disease Control, the effectiveness of birth control pills actually observed in the real world is 91%. Other authoritative sources such as the US Department of Health & Human Services, Planned Parenthood, et al. cite the same 91% figure. The 99.9% figure cited by WebMD is in fact obtained from perfect compliance during clinical trials. (And yet it's still incorrect -- the actual figure is 99.7%.) As you might imagine, patient instructions and supervision of compliance during clinical trials is much stricter than it is in the real world of medicine. Perfect compliance in the real world is actually a rare thing. So you should assume going forward that you have about a 1-in-10 chance of becoming pregnant over the course of a year if you use birth control pills as your sole source of protection.
If you would like to consider alternatives to the pill in the future, the CDC has published a very simple comparison of the effectiveness of various options.
Other Facts I Need to Know
Some drugs reduce the effect of the pill and can cause an increased chance of pregnancy. It's probably a good idea to consult your doctor or the pharmacist who fills your prescription when you're given a new prescription.