Does the mini pill (progestin-only pill) stop ovulation?

Mechanism of Action of Progestin-Only Pills (Mini Pills)

Progestin-only pills (mini pills) work primarily by thickening cervical mucus to prevent sperm penetration, not primarily by inhibiting ovulation. 1 This distinguishes them from combined hormonal contraceptives, which reliably prevent ovulation.

Primary Mechanisms of Action

The FDA-approved drug label for norethindrone (a common progestin-only pill) clearly states that these contraceptives work through multiple mechanisms 2:

1. Thickening cervical mucus - This creates a barrier that inhibits sperm penetration
2. Suppressing ovulation - But only in approximately half of users
3. Slowing ovum transport through the fallopian tubes
4. Altering the endometrium - Making it less receptive to implantation

Effectiveness and Ovulation Suppression

• The effectiveness of progestin-only pills varies based on user compliance:

  • Perfect use failure rate: 0.3-0.5%
  • Typical use failure rate: 8% 1, 3

• Unlike combined hormonal contraceptives where 7 days of continuous use reliably prevents ovulation, progestin-only pills have a more variable effect on ovulation 1:

  • Only about 50% of users experience ovulation inhibition 2
  • This is why strict adherence to timing is crucial for effectiveness

Clinical Implications

The limited ovulation suppression has important clinical implications:

• Strict timing requirements: Traditional progestin-only pills must be taken within a 3-hour window each day 3
• Backup contraception needed: If a dose is missed by >3 hours, backup contraception should be used for 2 days 3
• Not first-choice for adolescents: Due to the need for strict adherence to timing 3
• Alternative for those with estrogen contraindications: Despite limited ovulation suppression, they remain an important option for women who cannot use estrogen-containing contraceptives 3

New Developments

Recent research has focused on developing progestin-only pills with more reliable ovulation inhibition:

• A newer drospirenone-only pill (4mg) has shown more consistent ovulation inhibition, even with 24-hour delays in pill intake 4
• This represents an advancement over traditional progestin-only pills, potentially offering greater flexibility and reliability

Common Side Effects

The most common side effect of progestin-only pills is irregular bleeding patterns:

• Breakthrough bleeding
• Spotting
• Prolonged bleeding
• Amenorrhea 5

These menstrual disturbances are the most frequently cited reason for discontinuation, affecting up to 25% of users 5.

Summary

While progestin-only pills can inhibit ovulation in some users, their primary mechanism of action is thickening cervical mucus to prevent sperm penetration. Their effectiveness depends heavily on consistent and timely use, making them less forgiving of missed doses compared to methods that more reliably suppress ovulation.