Two heifers on 10 acres is certainly a very low stocking rate. Of course, sensible stocking rates for a property will depend on the terrain, the type and age of cattle on it and the quality of the pasture (determined by grass types present and soil fertility).
If your cattle have consistent access to more pasture than they need, they will be at risk of becoming overweight, even whilst they are pregnant. Maternal nutrition during pregnancy has been shown to have an effect on fetal size from as early as day 39 of pregnancy. Additionally, maternal over nutrition to the point that the cow gains condition in the last trimester may result in fat deposition in the birth canal, increasing the risk of dystocia (difficult birth).
Body condition in cattle is assessed using a 9 point scale. Condition over the back, ribs and tail head are examined to give an overall score. Ideally, cattle should calve at a condition score of 5-5.5. Significantly lower or higher body condition scores at calving can increase the risk of birthing and metabolic problems in the mother. If you are not familiar with condition scoring cattle you will need help to do this. Some management changes may be necessary, depending on the current condition scores of the heifers and the stage of pregnancy they are at.
Prior to calving, some people will vaccinate cows with a 5 in 1 clostridial vaccine. The purpose of this is to confer immunity to the calf for the first two months of life through ingestion of antibodies in the colostrum. Ideally this vaccination will be given 1 month prior to calving. If the cows are not vaccinated prior to calving it is sensible to make sure the calves receive their 5 in 1 vaccinations promptly. The 5 in 1 vaccine is recommended for use in calves ages from 6 weeks of age.
I hope this helps! The information provided here is general, and I would recommend that you contact your veterinarian for further advice specific to your animals and property.
Dr Emma Hill BVSc , Veterinary Surgeon