As mentioned a vet would be your best option. We had a mare once that was carrying twins and we didnt know it, and she aborted both of them when she was 8 weeks out. One was born alive and the other was dead. The vet was out at the time of the birth and had to put the little guy down because his lungs werent fully developed and was suffocating.
Sorry OT, but from our experience she lost both at once not just one, but a vet can find out for ya.