The definition of miscarriage is loss of a pregnancy loss before the twentieth week. In proper medical language this is also known as spontaneous abortion. There are four different categories of miscarriage:
Threatened abortion –which means that bleeding is occurring but there has been no dilation of the cervix; if the bleeding stops, this type of pregnancy may continue to term.
Missed abortion--which means that the fetus is no longer living, but remains within the uterus. Often there are minimal or no symptoms associated with this type of pregnancy loss.
Incomplete or inevitable abortion—which means that actual expulsion of the pregnancy products is occurring or will occur because the cervix has dilated.
Complete abortion—which means that all of the pregnancy products have been expelled and the uterus is empty; there is usually no need for surgery (d&c) following this type of miscarriage.
What are the common causes of miscarriage?
Causes of miscarriage can be broken up into two categories, genetic and environmental. By far the most common cause is genetic with 70-85% of miscarriages resulting when embryos that were genetically abnormal (too few or too many genes) failed to develop beyond the first trimester. Genetically abnormal embryos result from mistakes in cell division occurring in the egg and sperm cells which must first divide in half and then fuse together to form an entirely new person. Mistakes are extremely common with up to 75% of pregnancies ending in miscarriage—many before they are even noticed. As the parents age (especially beyond 35 for women and 55 for men, the number of these mistakes increases).
Environmental causes are far less common, but are important, because would-be parents can do something about them. An abnormal uterine cavity, such as one containing a septum (dividing membrane) or a fibroid (benign tumors) can cause miscarriages when the embryo fails to receive an adequate blood supply, or doesn’t have sufficient room to grow. Diabetes and thyroid disease, obesity and progesterone deficiency are health conditions which increase the risk of miscarriage, as are infection, blood clotting disorders, or abnormal antibodies such as may occur in some women with Lupus. Exposure to cigarette smoke, alcohol, recreational drugs and caffeine have been shown to increase miscarriage risk in some studies, although the data are mixed, especially in the case of caffeine.
Ideally, health is optimized before conception. Getting blood sugars or thyroid levels under control is very important. Maintaining a healthy BMI, and stopping bad habits such as smoking can also improve pregnancy outcomes. In many cases, the causes of miscarriage aren’t discovered or even investigated until one or more miscarriage has already occurred. Once diagnosed though, many of these conditions can be remedied. For example, often a septum or fibroid can be removed from the uterus. Progesterone supplementation is relatively simple. Blood clotting disorders can be treated with low dose aspirin or blood thinners. Nothing can be done about age, however, recent advances in egg freezing technology may make it very popular in the future for women on the career path.
A pregnancy which lacks an embryo is referred to as an anembryonic gestation or blighted ovum. The pregnancy test will be positive, and all the signs and symptoms of pregnancy may be present, but because there is no embryo or fetal heart beat, a blighted ovum always results in a miscarriage.
Most blighted ova are genetically abnormal. Accidents in cell division prior to or at the time of conception produce these abnormal pregnancies which consist of an empty fluid filled gestational sac without a fetus.
A blighted ovum can be suspected if pregnancy hormone levels (beta HCG) are rising too slowly. In a normal healthy pregnancy this value should increase by at least 60% every 48 hours. Low progesterone levels can also be a sign of a blighted ovum. Studies suggest that in a normal healthy pregnancy the progesterone level is almost always greater than 9 ng/ml. The definitive diagnosis is made by ultrasound. If a gestational sac has reached 17 mm or greater in size, and there is still no evidence of a fetal pole or heart rate, a blighted ovum is diagnosed. Similarly, an embryo should be present any time beyond 43 days gestational age. If no embryo is present by then, none will develop.
A recent research paper published a method in which abnormal pregnancies could be detected by sampling cells from the maternal cervix, so in the future, blighted ova may be detected by a procedure similar to a pap smear.
Implantation bleeding is the release of a small amount of blood as the embryo implants within the wall of the uterus. Since implantation occurs 10 days post conception, implantation bleeding is usually noticed at just about the time of or slightly before the missed period. In most cases implantation bleeding involves a small amount of blood in a single instance. Implantation bleeding does not occur in the majority of pregnancies, although if it does happen, it is not a sign or symptom of a problem.
Sometimes the timing of the bleeding is the only difference between implantation bleeding and a threatened miscarriage. Bleeding occuring beyond 14 days post conception probably does not represent implantation bleeding. Other differences may be the quantity of blood, or the duration of bleeding. Implantation bleeding does not happen more than once. All bleeding in pregnancy should be reported to one’s doctor. Women who are RH negative should receive rhogam.
Typically bleeding associated with a miscarriage is fairly heavy. Bleeding that is as heavy as or heavier than a normal menstrual period is probably not related to implantation. Again timing is important. If the pregnancy is advanced beyond the 5th or 6th week, bleeding much more likely represents a threatened miscarriage. Clotting, cramping, or multiple episodes of bleeding are very unlikely to be implantation bleeding.
A normal fetal heart rate is between 120 and 160 beats per minute. In very early pregnancy however, a heart rate slower than this can be normal. At least one reference suggested that a normal heart rate for a 5-6 week pregnancy was between 90-115 bpm, with that number increasing for every gestational week.
It is generally accepted that a fetal heart rate of less than 90bpm is associated with an increased risk of miscarriage. In at least one well done study, the miscarriage rate was increased by 25% in pregnancies with a slow heart rate versus those with normal heart rates. This study also found an increased risk of birth defects in the pregnancies with slow heart rates that did continue. (5% in the slow heart rate group vs. 2% in the group with normal heart rates). Another study showed no difference in the miscarriage rate in pregnancies with a slow heart rate at 5-6 weeks if the heart rate had increased into the normal range by 6-7 weeks. In this study, 90-95% of pregnancies which continued into the 7th week resulted in live born babies. Most authors recommend at least one repeat ultrasound study if the first ultrasound reveals a slow heart rate.
Fetal demise is the term for any fetus whose heart has stopped beating. Fetal demise can occur at any gestational age. As is the case with a blighted ovum, a fetal demise can not result in a live born baby.