There are a few different doses of birth control pills that are prescribed these days, but all of them prevent against pregnancy. There is no “half-way” dosage.
There are some new pills like “LoLoestrin Fe” that have a very low does of estrogen, but they still protect against pregnancy. Those super-low doses, however, are more likely to have breakthrough bleeding and need to be taken within the same 3 hours every day, so unless a patient specifically asks for them, I’m not likely to prescribe them.
You’ll see on there that the 1st column is progesterone levels and the 2nd column is estrogen levels.
Lo Loestrin Fr gets down to 10mg of estrogen in each pill, but the average birth control pill is closer to 35mg per pill. No matter what, though, all birth control pills are effective against pregnancy, otherwise they wouldn’t be birth control.