Newer progestational contraceptives primarily act by:
FOGSI is Federation of Obstetrics and Gynaecological Societies of India and the highest governing body in Obs and Gynae in India.
Progesterone only pills act mainly by causing thickening of cervical mucus.
Progesterone only pills
Mechanism of action of POP's is mainly on cervical mucus -

"All Estrogen free pills except the newer desogesterel pill primarily rely on changes in cervical mucus as they do not inhibit ovulation consistently."
The cervical mucus effect peaks within 3-4.hours after taking the tablet and lasts for about 22 hours. Hence the next tablet must be taken within 27 hours of the preceeding tablet or else the contraceptive benefit of the cervical mucus effect will decrease and finally subside.
Unlike COC's that almost always prevent ovulation, traditional POP's (progesterone only pills) inhibit ovulation in 40-50% of cycles. "A randomized double blind study, performed over 13 cycles showed that 75 lig desogestrel daily was sufficient to inhibit ovulation in 97% of cycles. Hence for newer POP's containing desogestrel the primary mode of action is inhibition of ovulation". - FOGSI Focus Jan 06 issue on Modern pill, Chapter Estrogen free pill p 41
Thus, for newer progestational agents main mechanism of action is by inhibition of ovulation.

Remember

Traditional POP's are also known as Low-dose progestogen only pills.                                                    
Main progesterone used are:  
 Norethisterone 350 mcg
Norgestrel 75 mcg
Levonorgesterel 30 mcg
Their main mechanism of action (as discussed earlier) is thickening of cervical mucus.Q
	They also:
Render endometrium unsuitable for implantation
Accelerate tubal motility
Disturb corpus luteal function.
Oviductal motility
Uterine endometrium
Cervix
Inhibiting ovulation