Here is a more comprehensive outline of ashtanga postures to avoid during each trimester. Inversions, laying on your back and extreme twists are just some of the asanas that are not recommended: http://www.ashtanga.com/html/pregnancy.html
Sadly, I haven't practiced regularly since my first baby was born. But now, in my 7th month with the second baby, headstand for a few minutes is the only thing that relieves back pain. And it does a fabulous job at it.
swollen ankles is a late pregnancy issue, therefore vitparita kurani doesn't really apply, since it cannot be done in the third trimester
it is better to do a side-lying pose with one leg elevated at a time
I do not advise Vitparita Kurani in third trimester and only during the second trimester if a woman is still sleeping on her back at night.
In mid to late pregnancy, women often feel as if there is a weight on their chest when they lay on their back, due to compression of the arteries. It is not recommended to compromise circulation in this way.
In addition, reclining postures (even big couches and lazy-boys) can negatively impact the position of the baby, leading to a more difficult birth.