Congratulations on the definate, positive, positive!
Were you ever checked for incompetent cervix after your 2nd miscarriage? It's a condition where your cervix does not stay closed like it should when you are pregnant, and it causes repeated miscarriage- once the baby gets to a size where it's putting some pressure on the cervix, it just starts to open. It's easily fixed up- they just out in a stitch (minimal risk to baby) then they take the stitch out when the baby is ready to be born.
Ask your doctor about it.
All the best to you and your family.