FDA assigns class C to metoprolol for use in pregnancy. Class C refers to the class of drugs that should be used only when the benefit outweighs the risk in pregnancy.
To directly answer your question- there are no controlled human studies to tell you the effect/ adverse effect of metoprolol in pregnancy.
Although metoprolol readily crosses the placenta, there are no signs of beta blockade (low heart rate, blood pressure) reported in the fetus.
I recommend that you discuss the plans of conception with your doctor, so that he can closely monitor your condition.
clearly, you do require a great control medication for supraventricular tachycardia, as uncontrolled heart rate might pose a greater risk to your pregnancy.
Hope that helps