Between 50 and 70 percent of first trimester miscarriages are thought to be random events caused by chromosomal abnormalities in the fertilized egg.
In other cases, a miscarriage is caused by problems that occur during the delicate process of early development — for example, when an egg doesn't implant properly in the uterus or an embryo has structural defects that don't allow it to continue developing. Since most healthcare practitioners won't do a full-scale workup after a single miscarriage, it's usually impossible to tell why the pregnancy was lost. And even when a detailed evaluation is performed — say after you've had two or three consecutive miscarriages — the cause still remains unknown in about half of cases.When the fertilized egg has chromosomal problems, you may end up with what's sometimes called a blighted ovum
(now usually referred to in medical circles as an early pregnancy failure). In this case, the fertilized egg implants in the uterus and the placenta and gestational sac begin to develop, but the resulting embryo either stops developing very early or doesn't form at all. Because the placenta begins to secrete hormones, you'll get a positive pregnancy test and may have early pregnancy symptoms, but an ultrasound will show an empty gestational sac. In other cases, the embryo does develop for a little while but has abnormalities that make survival impossible, and development stops before the heart starts beating.
Once your baby has a heartbeat — usually visible on ultrasound at around 6 weeks — your odds of having a miscarriage drop significantly.
