Why is it that most birth control pills have a cycle of 21 active pills and 7 placebos, while others have 24/4? is one system more preferable?

BC pill variation: Today's oral contraceptives are very low in estrogen, and the bleeding is mild , reflective of the low growth of the uterine lining resulting from low estrogen. There is no medical need to prolong the interval from active pills to active pills in a "new" package, Spontaneous ovulation and pregnancy are also reduced with the short "off time".

Birth control pills: The 24/4 pills are designed for shorter, lighter periods than the 21/7 pills. Either is effective for birth control. Patient preference is the main factor in which one to take.