Miscarriage in the first trimester is usually due to fetal anomaly and is nature’s decision to prevent an abnormal pregnancy. The risk of another abortion increases slightly the next time, but if precautions and tests are done before planning the next one, the outcome is good in over 80-90% of cases. You need to have some blood tests like blood group, husband and wife, urine test, blood sugars, and VDRL and torch test (optional). I don't have too many details of your previous pregnancy. The arcuate uterus too can be a cause of abortions occasionally. Most pregnancies however, go to beyond 36 weeks even in arcuate pregnancy, if it is not associated with a cervical incompetence as well.
Plan the next pregnancy after at least 6 months, while taking Tab Folic acid for three months around the planned conception, once all the tests are normal. At around the third month, get repeated ultrasound scans to make sure there is no early cervical dilatation (mouth of the uterus), in which case a stitch can be put at about the fourth month of pregnancy, if the cervix does dilate. Due to arcuate uterus, some times, there may be an abnormal position like a transverse position of the baby in later pregnancy, or an oblique one, which is not a big problem, because if that happens, a cesarean delivery is possible. If the position is normal, with a head down one, normal delivery can take place. That is determined only in the 37-38th week.
During the pregnancy however, she needs to take enough rest, avoid carrying weights and avoid sex especially in the first four months.