Tri-Phasil is, like all of the combination oral contraceptive pills, effective through three mechanisms of action. The third mechanism, which pharmaceutical companies choose to downplay, can have an abortive effect by acting against the endometrium to prevent implantation of the newly-created child. Documentation from the FDA confirms this.
In addition to the mechanisms that “prevent” a baby, it is also important to know that these products can possibly result in serious cardio-vascular (heart) disease for women. The statistics for heart disease show a marked increase when a woman is taking “the Pill.” Though to my knowledge studies have not been done to analyze the link between maternal death after prolonged hormonal contraceptive use and during or following birth, it seems reasonable to assume that cardiac affects may impact the mother’s health when she finally does give birth.
Combination oral contraceptives primarily act by suppression of gonadotropins. Although the primary mechanism of this action is inhibition of ovulation, other alterations include changes in the cervical mucus (which increase the difficulty of sperm entry into the uterus) and the endometrium (which reduce the likelihood of implantation).
For more information on this subject, see Randy Alcorn's book Does the Birth Control Pill Cause Abortions?