Fetal Doppler Ultrasound

Anxiety in your situation is completely normal, but you have to know that worrying so much is not good for your or for your baby.
I had some complications as well and there were times when the heartbeat was difficult for my doctor to find depending on the way the baby was turned in my early pregnancy.
I'm not sure that trying to check at home would be the best thing because if it's a situation where the baby isn't turned right or the heart beat sounds faint, you could send yourself into a spin.
Surely you have a good doctor who is working very closely with you and is aware of your special needs and is monitoring everything very closely.
You are receiving regular ultrasounds.
I would just follow up with your doctor as you're doing and not let the worry take over to the point that you can't just relax and take care of yourself and visualize a healthy baby growing inside you.
Over worrying is not a good thing, that's all I know.

In the days between your ultrasounds, feel at peace and think positive thoughts and give your baby a zen place to grow.

Best wishes.

My advice is to call your OB anytime you don't feel right, and to calm down. Your stress is more harmful to the baby than anything else right now (assuming you're living a healthy lifestyle and taking folic acid). You should get an ultrasound at your 12 week appointment, that will hopefully give you peace of mind.

Have you seen your OB yet? 12 weeks is very early. I'm not sure I had had an ultra sound yet at 12 weeks.

I tend to think it might be a better idea to talk to your OB about your concerns. He/she can discuss them with you and hopefully help relieve some of your fears. Not sure using a doppler would really help.

Your anxiety is understandable...it is sometimes difficult to locate the fetal heart tones so early in a pregnancy and that would indeed cause more anxiety .
Another reality at 12 weeks is that you cannot do a whole lot if you cannot locate a fetal heart... your doctor can see movement with ultrasound which is the most diagnostic way of making sure everything is going along okay. It sounds like you have been through a lot already and perhaps sitting down with a professional and discussing your anxieties and getting some tools to deal with them will make you feel better.
Why don't you discuss the doppler and your anxiety with your doctor and get his/her opinion...that would give you a better perspective.
Good luck ...I hope all goes well for you. L. Lavine www.parentingintheloop.com