Final answer:
The 21-day pack of oral contraceptives contains 21 hormone pills, followed by a seven-day break for menstruation, whereas the 28-day pack includes 7 additional inactive pills to keep a daily routine. Extended-cycle pills reduce the frequency of menstruation to three to four times a year. All types work by inhibiting ovulation and thickening cervical mucus.
Step-by-step explanation:
The main difference between the 21-day pack and the 28-day pack of oral contraceptives is the number of active versus inactive pills. In a 21-day pack, all 21 pills contain hormones and are actively working to prevent ovulation for three weeks. After these pills are taken, there is typically a seven-day break during which menstruation occurs. In contrast, a 28-day pack contains the same 21 active pills, but also includes an additional 7 inactive (placebo) pills, allowing the individual to maintain a daily pill-taking routine even during the menstruation week.
Extended-cycle pills are another form, where individuals only experience menstruation three to four times a year. This is possible due to 12 consecutive weeks of active hormonal pills followed by one week of inactive pills.
Oral contraceptive pills, whether taken on a traditional or extended-cycle schedule, contain synthetic hormones which work effectively as birth control by inhibiting ovulation and thickening cervical mucus. The failure rate of these pills is quite low when used perfectly and still relatively low with typical use despite being slightly higher.