A recent study has shown that the ''morning after'' contraceptive pills work by interfering with ovulation thereby preventing fertilization and not by preventing implantation of a fertilized egg.
Reproductive physiologists demonstrated the phenomenon in a controlled clinical trial published in the December 2004 . When the pills, also known as emergency contraception (EC) were given before ovulation, they were able to inhibit or postpone ovulation; pregnancy rates among those taking these pills were lower than among those taking a placebo. However, when EC was given subsequent to ovulation, evidenced by hormonal monitoring researchers say they found was no difference in pregnancy rates between the two groups and it was observed that the rate at which ovulation was impaired in their study was identical to the estimated efficacy of EC. Thus indicating that EC is does not work as an abortifacient.