Thanks for the query.
due to influence of hormones vascular resistance decreases leading to low blood pressure which is actually beneficial during pregnancy.
The ankle swelling you are could be physiological edema, which is also common finding with increasing gestation.
Usually this will subside with taking rest.
You can follow some general measures to decrease edema like:
Keep one pillow below the feet while sleeping
Sleep in left lateral position
Decrease salt intake etc.