HRV biofeedback is safe in pregnancy and will not harm you or the fetus. In fact, studies have shown that HRV biofeedback can help pregnant people experience less stress, fear of childbirth, and anxiety.
Over the course of your pregnancy, the changes to your body can make some parts of biofeedback breathing challenging: as pregnancy progresses, you might notice that it is harder to breathe deeply. Remember the goal in HRV-B is to breathe slowly and gently, not too deeply. As long as your body is seated upright, any modifications (yoga blocks, cushions, pillows) are welcome.