One of the lower back exercises I have always done is called "Supermans" - you lie down on your stomach and at the same time lift all of your 4 limbs - then repeat. Basically your body is supported by your hip bones and stomach... Is this okay to do during the first couple weeks of pregnancy?
Re: Is lying on your stomach safe?
Checking with your doctor isn't a bad idea but you'll know when to quit
With DS, I did regular yoga class during my 1st tri. As my bump grew, poses like bow or child's pose would put too much pressure on my lower stomach and I naturally knew when to modify the exercises.
my doc said you won't hurt the baby so it is more of a comfort/judgment call...
i guess lying on your back is more dangerous b/c you can hinder blood flow w/baby weight on arteries.