Flying in a small plane...

Hello. My wife is pregnant with our first child. She is currently 12
weeks into an apparently uncomplicated pregnancy. She's in good
health, and has no specific problems that we know of. I am a pilot,
and we own our own small plane. I have heard conflicting advice about
flying in small, unpressurized planes while pregnant. She's very
comfortable flying, and we like to travel, and would like to travel
via general aviation both before and (after a suitable age) after
delivery. The aircraft we fly in is not pressurized, and I can
generally fly anywhere between 3000 and 11,000 feet above sea level. I
have a few questions that I was hoping that a knowledgeable person
might help with.

1. What is the maximum altitude that would be 'safe' for for an
expectant mother to fly at?

2. Does this vary with the stage of pregnancy?

3. Is there a time when we should simply stop flying?

4. Are there any resources that anyone know of concernign flyign with
small children (i.e. headsets, inner-ear issues, etc.?

Thanks in advance for any help

Cheers,

Cap