According to http://www.emergencycontraception.org/publications/PDFs/policy/MOA_ENG_2011.pdf (from the International Federation of Gynecology & Obstetrics):
"Two studies have estimated effectiveness of LNG ECPs by confirming the cycle day by hormonal analysis (other studies used women’s self-reported cycle date). In these studies, no pregnancies occurred in the women who took ECPs before ovulation; while pregnancies occurred only in women who took EC Ps on or after the day of ovulation, providing evidence that ECPs were unable to prevent implantation."
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Date: 2012-07-24 07:19 pm (UTC)"Two studies have estimated effectiveness of LNG ECPs by confirming the cycle day by hormonal analysis (other studies used women’s self-reported cycle date). In these studies, no pregnancies occurred in the women who took ECPs before ovulation; while pregnancies occurred only in women who took EC Ps on or after the day of ovulation, providing evidence that ECPs were unable to prevent implantation."
(edited for formatting)