Assalamu alaikum Ursimmy,
I know the question is not addressed towards me, but i hope you don't mind i answered.
Yours is an interesting question. I was asked of it many times, mostly by people who wanted to present Islam as an outdated religion, meaning its laws are not needed anymore, since for example for iddat we have pregnancy tests.
I would like you to bear in mind that for each and every law Allah gave us, there is always an infinite wisdom behind it, and the main purpose is the best for us. Who can know what's the best for us more than the One who created us? He knows how He created us so He knows what we need and what works best for us.
With regards to iddat, its purpose is not solely to see if a woman is pregnant or not. Even in ancient times there were methods to determine a woman's pregnancy before her belly could be seen. Its purpose is also the reconciliation of the couple. You see, as the three months go by, as the loss of each other is inevitable, the couple can realize they love each other and can't live without each other. It's the human nature to realize what he/she lost only after the loss. If that is the case, you have three months to recover your loss
On the other side, a pregnant woman's iddat is until she gives birth. The happiness and love a kid brings to a couple's life can make them reconsider their divorce and be even happier than before, now that they have a kid.
Only in the last century (if i'm not mistaken) can you find in the western countries the laws of divorce that also require husband and wife to live under the same roof for some months before the divorce is finalized, so that the couple can see if they can still work things out. In west such a custom is relatively new, but in the Quran it was ordained 1500 years ago
no matter how many laws people try, in the end they always come to realize that His laws are the best way of life, and the best way of resolving matters.
I hope i was of help,
Your sister Seraphina