When reaching your 3rd trimesters it is recommended to reduce time lying flat on your back including when exercising.
The weight of the uterus puts pressure on the major vein called the vena cava, which can reduce blood flow to the heart then blood flow to the brain and uterus. Symptoms such as feeling dizzy, short of breath or a bit sick can happen.
It does not mean you can not lie flat on your back at all (unless specifically advised by your doctor) but keep the time short ie. do one exercise on your back then change to doing an exercise in another position such as side lying, 4 point kneeling, kneeling and standing.
Alternatively you can prop yourself up leaning on a sofa or having pillows, cushions or another support under your upper back raising your heart above your hips. See below video.
BALANCE OFTEN AFFECTED – When in the stages of pregnancy it is advisable to use support when doing standing exercises as your centre of gravity will have changed and it becomes easier to topple over.
MAKE THE GYM BALL YOUR FRIEND – A useful piece of equipment in pregnancy to make certain positions more comfortable and give support.
Below are videos to give some ideas.