It is worse for Iran than the 1.8 children per women suggests. Only 50% are Persians, who are not reproducing. The Arabs, Turkmen, Azerbajanis, Kurds are reproducing. In about 15 years, the Persians will be a significant minority.
Why? Because the smart and well educated women refuse subjection to the current religious troglodytes. Taxes are high, so the wealthy can not afford the harems with tens of children.
I don’t think that they aren’t having children for political reasons. I suspect it is the same problem as the West - they have become “material girls”. Hedonism is their religion, not Islam, and children burden the lifestyle.