Between 50 and 70 percent of first-trimester miscarriages are thought to be random events caused by chromosomal abnormalities in the fertilized egg. Most often, this means that the egg or sperm had the wrong number of chromosomes, and as a result, the fertilized egg can't develop normally.
Sometimes a miscarriage is caused by problems that occur during the delicate process of early development. This would include an egg that doesn't implant properly in the uterus or an embryo with structural defects that prevent it from developing.
Since most healthcare practitioners won't do a full-scale workup of a healthy woman after a single miscarriage, it's usually impossible to tell why the pregnancy was lost. And even when a detailed evaluation is performed – after you've had two or three consecutive miscarriages, for instance – the cause still remains unknown half the time.
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.
If your baby has a normal heartbeat – usually first visible on ultrasound at around 6 weeks – and you have no symptoms like bleeding or cramping, your odds of having a miscarriage drop significantly and continue to decrease with each passing week.
