Factors that may contribute to a decrease in effectiveness:
Missing more than one active pill in a packet,
Delay in starting the next packet of active pills (i.e., extending the pill-free, inactive pill or placebo pill period beyond 7 days),
Intestinal malabsorption of active pills due to vomiting or diarrhea,
Drug-drug interactions among COCPs and other medications of the user that decrease contraceptive estrogen and/or progestogen levels.In any of these instances, a backup contraceptive method should be used until hormone active pills have been consistently taken for 7 consecutive days or drug-drug interactions or underlying illnesses have been discontinued or resolved. According to CDC guidelines, a pill is considered "late" if a user takes the pill after the users normal medication time, but no longer than 24 hours after this normal time. If 24 hours or more have passed since the time the user was supposed to take the pill, then the pill is considered "missed." CDC guidelines discuss potential next steps for users who missed their pill or took it late. Role of placebo pills
The role of the placebo pills is two-fold: to allow the user to continue the routine of taking a pill every day and to simulate the average menstrual cycle. By continuing to take a pill every day, users remain in the daily habit even during the week without hormones.
A study done in British rivers supported the hypothesis that the incidence and the severity of intersex wild fish populations were significantly correlated with the concentrations of the E1, E2, and EE2 in the rivers.A review of activated sludge plant performance found estrogen removal rates varied considerably but averaged 78% for estrone, 91% for estradiol, and 76% for ethinylestradiol (estriol effluent concentrations are between those of estrone and estradiol, but estriol is a much less potent endocrine disruptor to fish).Several studies have suggested that reducing human population growth through increased access to contraception, including birth control pills, can be an effective strategy for climate change mitigation as well as adaptation. According to Thomas Wire, contraception is the greenest technology because of its cost-effectiveness in combating global warming — each $7 spent on contraceptives would reduce global carbon emissions by 1 tonne over four decades, while achieving the same result with low-carbon technologies would require $32. See also
Estradiol-containing oral contraceptive
Hormone replacement therapy (HRT)
List of estrogens available in the United States
List of progestogens available in the United States
Progestogen-only injectable contraceptive

References
Further reading
Black A, Guilbert E, Costescu D, Dunn S, Fisher W, Kives S,  et al. (April 2017). "No. 329-Canadian Contraception Consensus Part 4 of 4 Chapter 9: Combined Hormonal Contraception". Journal of Obstetrics and Gynaecology Canada. 39 (4): 229–268.e5. doi:10.1016/j.jogc.2016.10.005. PMID 28413042. External links
The Birth Control Pill—CBC Digital Archives
The Birth of the Pill—slide show by Life magazine