The lactational amenorrhea method involves the use of a woman's natural postpartum infertility which occurs after delivery and may be extended by breastfeeding.[99] This usually requires the presence of no periods, exclusively breastfeeding the infant, and a child younger than six months.[24] The World Health Organization states that if breastfeeding is the infant's only source of nutrition, the failure rate is 2% in the six months following delivery.[100] Six uncontrolled studies of lactational amenorrhea method users found failure rates at 6 months postpartum between 0% and 7.5%.[101][needs update] Failure rates increase to 4–7% at one year and 13% at two years.[102] Feeding formula, pumping instead of nursing, the use of a pacifier, and feeding solids all increase its failure rate.[103] In those who are exclusively breastfeeding, about 10% begin having periods before three months and 20% before six months.[102] In those who are not breastfeeding, fertility may return four weeks after delivery.[102]