Most twins surviving past 50 days will spontaneously abort at 6-8 months. Premature foals may have serious medical problems and are less likely to survive. The mare should go into the breeding season fit and perhaps gaining weight. Severely underweight mares will have more trouble conceiving than will mares of appropriate weight. Avoid stressing the mare as much as possible.
Stress can cause a drop in progesterone, a hormone which helps maintain pregnancy.
Use caution when exposing your mare to other horses. You should avoid any undue risk of injury or disease transmission. Provide nutritious forage, but don't overfeed. Supplementing with vitamins and minerals is unnecessary in mares being fed a balanced diet. Make sure the mare is current on vaccines and deworming.