Ok, so different reasons but I shoot too - albeit as part of the cadet forces.
I haven't shot as I've been too sick with Hyperemesis and I'm not sure I could now (we target shoot lying down). Before I got sick I asked around for advice and looked a lot online (I was due to do a week long shooting camp).
Basically it said if you're uncomfortable, stop. Not just lying down but shooting position generally, also, be aware that depending on rifle recoil you may find it harder to cope (relaxin effect on ligaments can mean it hurts more, or feels awkward). Also, centre of gravity moves and adjusting your position to compensate can make for an awkward position which can lead to sprains and strains.
Be aware that you wear ear defenders for a reason (not something as many farmers do.. but then you aren't normally shooting 20 rounds rapidly from a semi automatic!). Obviously, baby doesn't get that protection. No one has researched shooting specifically but there are studies that show mothers that work in a noisy environment can have
babies born with hearing loss.
As for the lead etc. in rounds/cartridges and gunpowder etc. Basic hygiene, wash hands well after handling rounds/empty cases. Or even wear latex gloves. Consider a face mask if in an area with lots of smoke build up, or even better, try to avoid that situation.
I'd decided I would do the shooting (would have only been about 9 weeks) but would stop if I was uncomfortable etc. and take precautions to avoid excessive exposure to lead. It's probably a common sense approach, like many sports in pregnancy, if it hurts, stop!
This is an old forum but it has links to research. www.babycenter.com/400_is-it-safe-to-go-shooting-while-pregnant_6791488_537.bc