Birth Control Pills
Combination (Synthetic progestin plus synthetic estrogen)
Progestin-Only (Synthetic progestin)
Brand and Manufacturer – List of Commercially Available Birth-Control Pills
Product information – Human birth-control pills are available in different dose formulations with various administration regimes, from 21 days with 7 days of placebo to allow menstruation, to 90 days before the placebo week. Treatment can begin in any phase of the cycle, but may not be effective in the first month if treatment begins near the time of ovulation. Survey reports to the WCC database indicate that birth-control pills have only been used in apes and some felids, and that, typically, the human regimen was followed. Recent data from treatment of women indicate that the placebo week is not necessary for uterine health, so continual treatment to prevent sexual swellings, for example, is an option. The limited data in the WCC database regarding the use of these orally active progestin-only contraceptives is primarily for great apes and a few Old World monkeys.
Latency to effectiveness - As with implants and injections, separation or alternative contraception should be used for 1-2 weeks after initiation of treatment. Efficacy may take one month if treatment begins near the time of ovulation.
Estrous cycles during contraceptive treatment - Signs of estrus can occur during the placebo week of both combination birth control pills and progestin-only pills. Synthetic progestins may effect contraception by blocking ovulation, causing thickening of cervical mucus, slowing ovum transport, and/or interfering with fertilization or implantation. However, follicle growth may continue and sometimes be accompanied by estradiol production sufficient to cause estrous behavior. Ovulation may occur even though pregnancy does not ensue. Higher progestin doses may be preferred, so that estrous behavior is prevented, but may not be effective in completely suppressing follicle growth and some estradiol production.
Duration of efficacy and reversibility - Duration of efficacy for combination pills is more than one day, because conception is blocked during the one week of placebo administration in humans. However, minimum dosages have not been established for other species. Daily administration outside the placebo week is probably necessary to ensure contraception. Latency to reversal may be slightly longer than for oral progestins, since the estrogen component more effectively suppresses follicle growth, increasing the time to ovulation. For progestin-only pills, duration of efficacy may not be much more than one day, so pills must be administered daily. Following cessation of treatment, rapid clearance can result in ovulation within a few days, but actual latency to conception will vary by individual.
Use during pregnancy – Degree of risk to the fetus is uncertain, and manufacturers recommend discontinuation of combination birth control pills during pregnancy. Progestin-only pills are not recommended in pregnant animals because of the possibility of prolonged gestation, stillbirth, abortion, etc. in some species, although the effect may depend on dose. Progestins in late pregnancy seem not to interfere with parturition in primates, but this is a taxon-specific phenomenon.
Use during lactation – Combination birth-control pills may interfere with milk production or affect the developing infant, so are not recommended. Progestin-only pills are sometimes prescribed for lactating women and are considered generally safe for nursing infants.
Use in pre-pubertals or juveniles - Lack of data on pre-pubertal treatment and potential long-term effects on fertility contraindicate recommending contraception before puberty. Fertility is less likely to be affected by treatment with combination pills administered after puberty. Future reproduction was not affected in calves of domestic cows on MGA-treated feed (another synthetic progestin), but no studies of pre-pubertal treatment with MGA or Progestin-Only pills have been conducted with other species, so possible long-term effects on fertility are not known.
Consideration for seasonal breeders - Treatment should begin at least one month before the anticipated onset of the breeding season.
• Weight gain is less likely than with progestin-only treatment, but the estrogen component may
cause fluid retention.
• Possible deleterious effects on uterine and mammary tissues vary greatly by species; see cautions for
• Some generic formulations have been found to be less effective, so brand-name products may be
• Some antibiotics may decrease birth-control-pill efficacy. See list of potential drugs that can interfere
Reporting requirements - All institutions using Birth-Control Pills are asked to submit a complete Contraception Center Survey to the AZA Wildlife Contraception Center. It is essential that accurate records of doses and intervals be maintained and results reported to the Wildlife Contraception Center Database to contribute to dosage development.
Please submit surveys to:
Mary Agnew, Program Coordinator
AZA Wildlife Contraception Center
Saint Louis Zoo
1 Government Drive
St. Louis, MO 63110
314-646-4595; fax: 314-646-5534