Correct me if I am wrong, but I believe infanticide was one of the major contributing factors in the fall of the Roman Empire.
Nah, they just ran out of people to conquer and enslave. Not to defend abortion, but infanticide has been common throughout history.
It may have been, but it was a practice not unique to Rome:
http://en.wikipedia.org/wiki/Infanticide
It has been argued but not proven that more-prosperous societies have lower birth rates, and people saying that point to Mexico's falling birth rate. Middle-classness, it is said, cuts off the poor person's drive to have many children -- which is, after all, partly an economic impulse.
In 18th-century and early 19th-century America, when pregnancies abounded, large families ensured many hands to do the farm labor and help ease later life for the parents. It was not uncommon for a traveler to be met, at the door of a back-country cabin, by a woman with nine children. "Poor man's Social Security" was invented eons ago. Prosperity eclipses that drive.
Infanticide and the Tophet were resorted to by people who feared they had too many children to feed -- or who feared that too many heirs would cause dissension and a relapse back into poverty for the family, when the family "nut" was divided up below the critical mass needed for investment income. That, it is said, is why the Phoenicians (and following them, the Carthaginians) began to "sacrifice" children to their gods, even at the expense of blackening their national name forever.