Miscarriages are pregnancies that stop developing. Development stops at different times and for different reasons. The great majority of miscarriages are early occurring before 10 weeks due to a genetically/chromosomally abnormal conceptus – the result of an abnormal egg or sperm fertilization. Most often it is an abnormal egg because the egg is much older than sperm. Women are born with a certain number of eggs, they never make any more eggs their entire life. Eggs accumulate genetic/chromosomal errors as a function of time. Sperm live approximately 90 days, die off and are continually being replaced, so that the great majority of sperm are genetically normal. When you look at miscarriage rates they reflect mother’s age. If the egg is grossly abnormal, fertilization does not occur. If the egg is somewhat less abnormal, fertilization may occur and development may proceed for a short time, but almost always ends prior to ten weeks. Babies are born with genetic/chromosomal abnormalities, these errors are the least severe that can still be compatible with life. Development usually ends early because all the genetic material is important for development. Most women whether they know it or not have had 2-4 losses some time in their reproductive life. Often when a woman is a week or so late for her cycle and then bleeds she has had a miscarriage. Miscarriages beyond 10 weeks are still most commonly due to a genetically/chromosomally abnormal pregnancy, but other factors start to become more common, i.e. anatomic and clotting problems. Knowing the time at which development stopped is helpful in determining the cause for loss. Anatomic and clotting problems are correctable. If losses are felt due to a genetically/chromosomally abnormal conceptus, in vitro fertilization with pre-embryo genetic testing can be performed to try and identify a normal embryo for transfer.
Kevin L. Winslow, M.D.
Director for Florida Institute for Reproductive Medicine