I haven't researched this recently, but the recommendations seem have
changed over the years I've been having babies.

When I was pregnant with my first in '98, there were strict rules about
keeping your heart rate down below a set low number. It was hard to
have fun, even in the pool, without cheating.

By the time I was pregnant with my 9 mo, the heart rate rule had been
replaced by a perceived exertion rule.

Obviously I have old books! Even when it was the heart rate rule, do you
know if it was thought to be something fundamental about that rate or if
it was a measure of the perceived exertion, that just isn't all that
accurate?

Cheers
Anne