Rural studies in Nigeria, Senegal, Rwand, Bangaldesh and Jave showed a potponement of menstruation by more than a year in breast feeding women compared with those who did not breast feed. In Korea, Tawiwan and India, a postponement of between 8 and 12 months has been found. By comparison, women who do no breast feed return to regular menstuation in about 3 months after the birth of the baby.
Nutrition in Mother and Child Health, G.J. Ebrahim 1983
As this resource is slightly dated here is a link to a recent metanalysis on pubmed. You can also access to other articles through the website.
Contraceptive efficacy of lactational amenorrhoea.
Kennedy KI, Visness CM.
Family Health International, Research Triangle Park, North Carolina 27709.
Pregnancy is rare among breastfeeding women with lactational amenorrhoea. The lactational amenorrhoea method (LAM) is the informed use of breastfeeding as a contraceptive method by a woman who is still amenorrhoeic, and who is not feeding her baby with supplements, for up to 6 months after delivery. Under these three conditions, LAM users are thought to have 98% protection from pregnancy. It can be difficult, however, to determine when supplementation of the baby's diet begins. We have analysed data from nine studies of the recovery of fertility in breastfeeding women to assess the effectiveness of lactational amenorrhoea alone, irrespective of whether supplements have been introduced, as a fertility regulation method post partum. Cumulative probabilities of ovulation during lactational amenorrhoea were 30.9 and 67.3 per 100 women at 6 and 12 months, respectively, compared with 27.2 at 6 months when all three criteria of the LAM were met. Cumulative pregnancy rates during lactational amenorrhoea were 2.9 and 5.9 per 100 women at 6 and 12 months, compared with 0.7 at 6 months for the LAM. The probability of pregnancy during lactational amenorrhoea calculated from these studies is similar to that of other modern contraceptive methods, and it seems reasonable for a woman to rely on lactational amenorrhoea without regard to whether she is fully or partly breastfeeding. So that amenorrhoea and fertility suppression can be maintained, counselling about good breastfeeding and weaning practices remains important.
PIP: Researchers analyzed data on 346 women from prospective studies conducted in 8 different countries which examined return of ovulation in breast feeding mothers to determine the contraceptive effectiveness of lactational amenorrhea. The countries included Mexico, Thailand, Egypt, Pakistan, Philippines, Canada, Australia, and England. As a result of the Bellagio conference on breast feeding, health researchers have promoted the lactational amenorrhea method (LAM) as a family planning method for 6 months postpartum. Lactational amenorrhea provided significant protection from pregnancy. The researchers learned that the effect of LAM during amenorrhea and before food supplementation was a life table pregnancy rate of 0.7/100 women at 6 months postpartum. Indeed when they considered food supplementation the rates for 6 months and 12 months postpartum stood at 2.9 and 5.9 respectively. These rates showed that LAM provided either greater than or equal protection against pregnancy during the 1st 12 months postpartum in the US than typical modern contraceptive method use. The researchers did observe, however, that LAM had high discontinuation rates. For example, by 12 months, menstruation had returned to 87.5% of all the women. The researchers recommended that health workers continue to promote breast feeding practices that encourage increased suckling since reduced suckling precipitates the return of ovulation. In fact, they hypothesized that improved breast feeding practices which increase the duration of lactational amenorrhea may actually result in greater protection. Since the findings revealed that LAM had low pregnancy rates and high discontinuation rates, LAM can be used to time when to begin other contraceptive methods.
PMID: 1346183 [PubMed - indexed for MEDLINE]
Another article from 1993