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Can someone explain the science behind the mini-pill? #619/10

I was wondering if someone could explain a little about the science behind the mini-pill. My girlfriend is switching from the combined pill to the mini-pill because of side-effects she was having. It makes me a little nervous because I've read that the mini-pill doesn't stop ovulation. I know the statistics similar for both pills and she is obsessively good about taking them at the same time to the minute, but it scares me that the egg and the sperm could still meet somehow.


How does it have such a high success rate if an egg is still present? Should we use a condom or some other method in addition?

Thank you for this service.

If the period of time between taking pills is more than 24 hours and you have sex, use of a condom would be wise.  Some women using mini-pills use condoms all the time when they have intercourse, but this would be up to you.


The following information about the mini-pill is taken from the 19th edition of

Contraceptive Technology:


PROGESTIN-ONLY PILLS by Elizabeth Raymond, MD, MPH


POPs may prevent pregnancy by the following mechanisms: 

·         Ovulation is inhibited in a variable proportion of cycles.

·         Cervical mucus is thickened and decreased in amount, which may prevent sperm penetration.

·         the activity of the cilia in the fallopian tube is reduced, which may prevent the sperm and the egg           from meeting.

·         The endometrium is altered, which may inhibit implantation of a fertilized egg.

The specific mechanism affects a particular cycle may vary between women and, in any single woman, between cycles.  POPs containing desogestrel 75 mg may inhibit ovulation more consistently than POPs containing levonorgestrel 30 mg.


POPs can be a highly effective contraceptive method when taken properly by motivated users. In studies reviewed in Chapter 27 on Contraceptive Efficacy, and in another comprehensive review, the proportion of women becoming pregnant in the first year of use was between 1% and 13%. Pearl indices in these studies ranged up to 3 pregnancies per 100 woman-years. However, effectiveness among typical users outside of trials who are not receiving close counseling and monitoring may be lower than these figures suggest. Several studies showed that the absolute pregnancy risk among women using POPs is strongly influenced by age, coital frequency, lactation, possibly body weight, and other characteristics of the user. 

Scant data are available comparing efficacy of different POP products. One randomized trial found no significant difference in efficacy between POPs containing norethindrone and POPs containing levonorgestrel. Pregnancy rates in both POP groups in this trial were greater than 9% at one year, higher than in many other studies. Another small randomized trial found fewer pregnancies among women using POPs containing desogestrel than among women using POPs containing levonorgestrel, although the difference was not statistically significant.

POPs are widely believed to be less effective than COCs. This belief probably stems largely (and reasonably) from the fact that the dose of hormone is lower in POPs than in COCs. However, data directly comparing actual pregnancy rates in users of the two methods are limited. One randomized trial comparing two COC and two POP preparations found that the pregnancy rate was significantly lower in one group of COC users than the rates in the other three groups. However, losses from the study for reasons other than pregnancy were very high in all the groups, which seriously compromised the comparisons.  In typical use, factors such as compliance may influence pregnancy rates more than relatively minor differences in inherent method efficacy.

Guidelines on use of POPs emphasize that for maximum efficacy, the pills must be taken within several hours of the same time every day. This recommendation is based primarily on data about serum progestin levels, which peak shortly after pill ingestion and then decline to nearly undetectable levels 24 hours later. In this respect, POPs differ from COCs, which produce higher and longer-lasting serum progestin levels. No clinical data are available that correlate pregnancy rates with timeliness in taking POPs.  One study suggests that POPs containing desogestrel 75 mg may inhibit ovulation reliably even when pills are occasionally taken 12 hours late.

·         Progestin-only pills contain a progestin hormone and no estrogen. A woman using progestin-only pills takes one tablet every day.

·         Progestin-only pills are highly effective if taken as directed, although they are possibly less effective in typical use than combined oral contraceptive pills.

·         Progestin-only pills are safe for almost all women, including many women with contraindications to using combined oral contraceptive pills.

·         The most common complaint of progestin-only pill users is irregular bleeding.

·         Progestin-only pills containing norgestrel or levonorgestrel can be used for emergency contraception.

Progestin-only pills (POPs), sometimes called “mini-pills,” contain a progestin hormone. One pill is taken every day with no hormone-free days. POPs containing one of two progestins (norethindrone 0.35 mg or norgestrel 0.075 mg) are currently available in the United States. POPs containing desogestrel or other progestins are available in other countries. The amount of progestin in POPs is lower than the amount in combined oral contraceptive pills (COCs) containing the same compounds.

This answer came almost entirely from the 19th edition of Contraceptive Technology, a book that describes how all the contraceptives work, the advantages and disadvantages of each and much more!  The chapters are by nationally recognized experts.


Click here to see how to order this important reference book and other important books for clinicians, counselors and women and men wanting the latest contraceptive information.




 Key Words:  mini-pills, science, switching, combined pills, side-effects, ovulation, egg, sperm, success rate, condom, progestin-only pills, Contraceptive Technology, Dr. Elizabeth Raymond, mechanism, inhibited, cycles, cervical mucus, thickened, fallopian tube, fertilized egg, effectiveness, estrogen, irregular bleeding



Raymond EG. Progestin-only pills IN Hatcher RA, Trussell J, Nelson AL, Cates Jr. W. et al Contraceptive Technology 19th edition, pages 181, 182 and 183: Ardent Media Inc. 2008

Posted 7-14-2010, Updated 7-18-2010, Updated 7-24-2010, Updated 7-30-2010



Robert A. Hatcher MD, MPH
Emeritus Professor of Gynecology and Obstetrics
Emory University School of Medicine
Atlanta, GA

The directors and owners of this website and any publications and information concerning health matters offered here advise a person with a particular problem to consult a primary-care clinician or a specialist in obstetrics, gynecology, or urology (depending on the problem or the contraceptive) as well as the product package insert and other references before diagnosing, managing, or treating the problem.
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