As far as I am aware laying on your back for extended periods of time from around the third month of pregnancy can cause the baby or your uterus to press down on major blood vessels and lower your blood pressure to dangerous levels and decrease blood flow to the uterus.

As far as the other question is concerned. That is an old wives' tale. Consider all of those elite athletes who quite safely play sports such as netball and basketball, or ride horses, or who swim and spend a lot of time with their arms extended with no ill effects.

But hey...play on it if it gets you out of the nasty chores. Works for me!

Lisa