For women who meet the criteria (listed below), LAM is >98% effective during the first six months postpartum.
- Breastfeeding must be the infant’s only (or almost only) source of nutrition. Feeding formula, pumping instead of nursing, and feeding solids all reduce the effectiveness of LAM.
- The infant must breastfeed at least every four hours during the day and at least every six hours at night.
- The infant must be less than six months old.
- The mother must not have had a period after 56 days post-partum (when determining fertility, bleeding prior to 56 days post-partum can be ignored).
If not combined with barrier contraceptives, spermicides, hormonal contraceptives, or intrauterine devices, lactational amenorrhea method (LAM) may be considered natural family planning by the Roman Catholic Church.
Breastfeeding delays the resumption of normal ovarian cycles by disrupting the pattern of pulsatile release of GnRH from the hypothalamus and hence LH from the pituitary. The plasma concentrations of FSH during lactation are sufficient to induce follicle growth, but the inadequate pulsatile LH signal results in a reduced estradiol production by these follicles. When follicle growth and estradiol secretion does increase to normal, the suckling stimulus prevents the generation of a normal preovulatory LH surge and follicles either fail to rupture, or become atretic or cystic. Only when the suckling stimulus declines sufficiently to allow generation of a normal preovulatory LH surge to occur will ovulation take place with the formation of a corpus luteum of variable normality. Thus suckling delays the resumption of normal ovarian cyclicity by disrupting but not totally inhibiting, the normal pattern of release of GnRH by the hypothalamus. The mechanism of suckling-induced disruption of GnRH release remains unknown.
Return of fertility
Return of menstruation following childbirth varies widely among individuals. A strong relationship has been observed between the amount of suckling and the contraceptive effect, such that the combination of feeding on demand rather than on a schedule and feeding only breast milk rather than supplementing the diet with other foods will greatly extend the period of effective contraception. The closer a woman's behavior is to the Seven Standards of ecological breastfeeding, the later (on average) her cycles will return. Average return of menses for women following all seven criteria is 14 months after childbirth, with some reports being as soon as 2 months while others are as late as 42 months. Couples who desire spacing of 18 to 30 months between children can often achieve this through breastfeeding alone.
Although the first post-partum cycle is sometimes anovulatory (reducing the likelihood of becoming pregnant again before having a post-partum period), subsequent cycles are almost always ovulatory and therefore must be considered fertile. However, some women find that breastfeeding interferes with fertility even after ovulation has resumed. Luteal phases being too short to sustain pregnancy is a common example.
- Trussell, James (May 2011). "Contraceptive failure in the United States". Contraception 83 (5): 397–404. doi:10.1016/j.contraception.2011.01.021. PMC 3638209. PMID 21477680.
Trussell, James (November 2011). "Contraceptive efficacy". In Hatcher, Robert A.; Trussell, James; Nelson, Anita L.; Cates, Willard Jr.; Kowal, Deborah; Policar, Michael S. (eds.). Contraceptive technology (20th revised ed.). New York: Ardent Media. pp. 779–863. ISBN 978-1-59708-004-0. ISSN 0091-9721. OCLC 781956734.
- ReproLine The Reading Room. Lactational Amenorrhea Method, which cites:
Zinaman M, Hughes V, Queenan J, Labbok M, Albertson B (1992). "Acute prolactin and oxytocin responses and milk yield to infant suckling and artificial methods of expression in lactating women.". Pediatrics 89 (3): 437–40. PMID 1741218.
- McNeilly, As (Feb 1994). "Physiological mechanisms underlying lactational amenorrhea.". Ann N Y Acad Sci 18 (709): 145–155. PMID 8154698.
-  Journal of Tropical Pediatrics, Volume 28, Issue 1, p. 1
- Kippley, Sheila. The Seven Standards of Ecological Breastfeeding: The Frequency Factor, 2008.