The effectiveness timing of birth control pills depends on the type and when you start taking them:

• For combination pills (containing estrogen and progestin), if the first pill is taken within the first 5 days of the menstrual cycle, it is effective immediately. If started at any other time, it takes 7 days to become effective.
• For progestin-only pills ("mini-pills"), if taken within the first 5 days of the menstrual cycle, they can work immediately. Otherwise, they take about 2 days to become effective.

If the pill is started after childbirth or pregnancy loss, timing for effectiveness can vary but often the pill works immediately if started within a few days. During the initial period before effectiveness, it is recommended to use a backup method like condoms. In summary, birth control pills can be effective immediately if started within 5 days of the period starting; otherwise, the effectiveness starts after 2 to 7 days depending on pill type.