I wouldn't take second hand information. The Ft. Dodge WNV vaccine had huge crazy rumors going around the first year it was out on the market, but they weren't supported by any evidence (such as necrospy) and often there were many things going on in the horse's environment that were likely to cause abortion.
The Ft. Dodge WNV vaccine has been studied in pregnant mares and breeding stallions with no evidence of abortion or sterility. http://www.thehorse.com/pdf/Vaccinat...ations_pt3.pdf
The other WNV vaccine is not currently labelled or studied in pregnant mares.