Is It Safe To Take Metoprolol For Tachycardia During Pregnancy?

I have tachycardia and take 1/2 a pill twice daily of 25mg of metoprolol tartrate. I am now 20 weeks pregnant and have been listed as high risk because of the tachycardia. I haven t had any side effects from the tachycardia and I have great blood pressure. I want a natural birth but every place I ask I m told they can t take me because I m high risk. What are the risks involved?