Why is it not "plus des femmes"?
"des" is an indefinite plural article:
In our case, "des femmes" is preceded by "plus de" (determiner), so "de" (from "plus de") and "des" (from "des femmes") get contracted into "de":
How can you tell the difference in conversation?
The difference is in the pronunciation.
You can practice your listening skills with these links:
There, you can click on the "speaker" button and listen to the differences of pronunciation.
Should I say,in the structure word + de + des + another_word ,just like plus de des femmes in this case,the de + des part will be contracted into only one word de ?
Right, "plus de des" is grammatically incorrect.
You have to say:
I learned from @Sitesurf :"plus de", "autant de", "moins de" = all are constructed without an article.