How common are miscarriages and why do they happen?
Many women suffer miscarriages -- but how often does pregnancy loss really happen? And what are the most common reasons for such a loss? Family Physician Jane Forester has some information about this sad event -- one that's not as uncommon as you might think.
Jane Forester, Family Physician
Your question
How often do miscarriages really occur, and what are the most common reasons for pregnancy loss? - Joni in Alpharetta, Georgia
The expert answers
There are two statistics to answer your question regarding how often miscarriages occur. As far as we are able to determine, it can be approximated that in the first 8 weeks of pregnancy, the miscarriage rate is about 10-15 per cent of a clinically recognizable pregnancy. Often a woman may not know that she is pregnant, and after being slightly late for her period, get a heavier than usual menstrual cycle. This is often not recognized as a miscarriage but instead just a late, heavy period. That's why in the earliest part of pregnancy it is difficult to quantify actual percentages of miscarriages.
The second statistic is that if a live, appropriately grown fetus is present at 8 weeks gestation, the fetal loss rate (miscarriage) over the next 20 weeks (up to 28 weeks) is about 3 percent.
Even now, there is still much confusion about the various causes of miscarriage. Some documented causes of miscarriage include infections such as rubella and syphillis; environmental factors such as exposure to extreme radiation; maternal smoking and alcohol consumption. Women who smoke 20 cigarettes daily and consume more than seven standard alcoholic drinks per week have a fourfold increase in their risk of miscarriage.
There are medical disorders which may be connected to an increased risk of miscarriage. Three which are supposedly linked are diabetes, hypothyroidism and systemic lupus erythematous. In addition, women who are over 40 years old have a 10 percent chance of miscarriage, in comparison to a 2 percent chance in someone less than 30 years old. The uterus of the woman being misshapen or the cervix being incompetent may also increase the chances of miscarriage. Finally, the most common cause of miscarriage is a significant genetic abnormality of the fetus. In miscarriages in the first trimester, approximately two thirds have significant chromosomal anomalies, with about half of these being Down's Syndrome. This is not an inherited trait from the mother or the father; it is usually one single nonrecurring event.
Sometimes when a pregnancy occurs, there is an empty yolk sac seen on ultrasound -- this is called an "anembryonic pregnancy" or a "blighted ovum." Interestingly, this type of pregnancy normally will miscarry itself, as well as documented pregnancies (by ultrasound) which are grossly genetically or morphologically abnormal. This would lead us to believe that somehow nature has a way of identifying some of its major, nonsurvivable mistakes and causes them to be miscarried.
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