What do you base this on? There are some studies that suggest that Birth control pills (which also prevent implantation) often increase the ability to get pregnant once stopped.
The most common reason they have problems is due to scaring of the fallopian tubes due to Chlamydia that is often not diagnosed in women. (A very good reason not to screw around until married!)
IMHO the morning after pill removes any reason to keep abortion legal. To use abortion as a form of birth control is disgusting. The ability to prevent implantation via regular birth control pills has been practiced from back in the 1960s. This is not new, except the public is learning about it.