What can blood sac in uterus without pregnancy be?

See Below:: A gestational sac with no obvious pregnancy on ultrasound can be a normal finding in early pregnancy. It can also be a blighted ovum. This is when the fertilized egg implants in the uterus and develops a sac but no embryo. It is something you can see by ultrasound prior to a miscarriage. Sometimes it is a pseudo-sac and can be seen with pregnancy outside the uterus (ectopic pregnancy).