The pills are reasonably effective right away.

Some physicians recommend that women use a back-up method of contraception (such as condoms) during the first month of BCP use. This is based on the observation that BCPs probably do not achieve their 99.9% effectiveness until after the first month of use.

It is also true that the BCPs are pretty effective, even starting with the first BCP. The pill does many things to prevent pregnancy beyond simply suppressing ovulation. Some of these other things (for example, inhibition of uterine contractions) occurs shortly after starting the BCPs.

Many BCP manufacturers suggest that the BCP is effective after 7 days of continuous use. Even before 7 days of BCPs have been taken, considering that phase of the menstrual cycle, pregnancy is not very likely. For these reasons, the BCPs are probably about as effective as using a diaphragm (~85%-95% effective) as soon as they are started. For women seeking a higher level of protection against pregnancy (99.9%), using a backup method of contraception during the first month of BCP use may be considered.

The original thinking on waiting one month before relying on the BCP came from the belief that the way the BCPs worked was by suppressing ovarian function, and that could take as long as several weeks. More recently, physicians appreciated that suppresion of ovarian function was not the only way the BCPs prevented pregnancy…it probably isn’t even the primary way. Because these other mechanisms (stopping uterine contractions, changing cervical mucous so sperm can’t penetrate it, changing (stopping) fallopian tube motility, to name a few) begin to work right away, there is a solid theoretical basis for relying on the BCP as soon as it is started. Clinical testing of this theory has supported these findings, and most birth control pill clinics now start the BCP as soon as the patient is seen, without regard to where she is in her menstrual flow.