After the embryo attains a CRL of 5 mm, the probability of subsequent loss falls to 7.2%. The loss rate drops rapidly thereafter to 3.3% for embryos with a CRL of 6 mm to 10 mm and to 0.5% for embryos with a CRL of 10 mm or more (i.e., 5 weeks postovulation or 7 weeks after the last menstrual period). That 3.3%? That's us.
Only 1.9% of pregnancies have small gestational sacs in relation to crown-rump length.
According to that same study, 80% of pregnancies fail when the difference between MSD (mean sac diameter) and crown-rump length (CRL) is less than 5 mm.
Another study with a smaller sample, however, found a more ominous failure rate: 94%.
Pregnant women have a lot of magical talismans they like to invoke against the potential for loss. One of them is the mythical 5% "Once you've seen the heartbeat, your chances for miscarriage drop to 5%." There are a lot of things wrong with that statement, primarily the assumption that any statistic pertains to any individual. I am most painfully aware that even if you assume that magic number is accurate, somebody still has to be in that 5%.