According to JustMommies.com, a subchorionic bleed is a blood clot that happens during pregnancy when blood collects between the uterus and the placenta. Although as of 2014, no direct cause is known, one suggested theory is that the fertilized egg tears slightly away from the uterus during implantation causing the uterus lining to bleed and blood to become caught between the uterus and the developing placenta.
Ob.Gyn. News states that subchorionic bleeds are the most commonly detected prenatal abnormalities of live births by ultrasound, and there is no scientific finding of a link between a subchorionic bleed and a greater risk of miscarriage in these circumstances. Therefore, women who are far enough along that a heartbeat can be detected have no heightened risk of miscarriage, but women for whom a viable pregnancy has not been established do not have the same peace of mind. As of 2014, enough is not known about subchorionic bleed causation. Wikipedia warns that prior to nine weeks gestation, there is an increased risk of miscarriage.
JustMommies.com describes the symptoms of a subchorionic bleed as intermittent or early trimester vaginal bleeding and cramping. By 20 gestational weeks, most bleeds have disappeared without medical intervention; however, pregnant women are advised to seek medical assistance for all bleeding. They must also avoid engaging in sexual activity or using tampons during periods of bleeding.