My understanding was that the best time for women to have children was between 20-30 years old, not 18-20.
From what I've read, 20-30 is when women have most of their eggs at the best quality, and have the lowest health risk in pregnancy.
Where does the 18-20 come from? Like I guess you're most fertile at 20 but that's the only metric I can see thats best at 20. Hell, a lot of sources even say you have the best chance of natural conception in your late 20s.