I am 21 weeks preg found from US that my fetal heart beat is slow 50-60.worried.please help?

i had an ultrasound today and the dr said everything else is normal(anatomy and heart fn too) except that my baby s heart beat rate is slow.the dr said it seems like an atrio ventricular block(cause may be due to some lupus antibodies).they have asked us to come after 4 weeks which they have mentioned as fetal heart block follow up.I am very worried whether my baby will make up to full term and also if she makes up what will be her situation later.has anyone had aslow fetal heart beat which has improved later and normal.Is there a possibility that baby after birth can have a normal rate.please help those who have any knowledge or experience in similar sitn.please PLEase.Thanks