Noah Millman has an interesting analysis (no permalinks; scroll down to the first entry under April 10) on demographic changes in Iran:

According to the CIA World Factbook, Iran's fertility rate is right at replacement: 2.01 children per woman. This is, to say the least, not typical of the region. Here are some comparisons to neighboring countries:

Iraq: 4.63
Saudi Arabia: 6.21
Kuwait: 3.14
Jordan: 3.15
Turkey: 2.07
Pakistan: 4.25
Afghanistan: 5.72
Turkmenistan: 3.54
Armenia: 1.53
Azerbaijan: 2.29

The only country in the vicinity with a lower fertility rate is Christian and post-Soviet Armenia. Iran, a fundamentalist Muslim theocracy, has roughly the same fertility rate as secular Turkey (or, for that matter, the USA).

When did this transition to lower fertility happen? It can't have happened that long ago, because 31% of the population is under 14 (versus 21% in the US and 24% in China). Could we just be dealing with the after-effects of economic development under the Shah?