A miscarriage is any pregnancy that ends spontaneously before the fetus can survive. A miscarriage is medically referred to as a spontaneous abortion. The World Health Organization defines this unsurvivable state as an embryo or fetus weighing 500 grams or less, which typically corresponds to a fetal age (gestational age) of 20 to 22 weeks or less. Miscarriage occurs in about 15% to 20% of all recognized pregnancies, and usually occurs before the 13th week of pregnancy.
With the development of highly sensitive assays for hCG levels that can detect an early pregnancy even prior to the expected next period (menstruation), researchers have been able to show that around 60% to 70% of all pregnancies (recognized and unrecognized) are lost. Because the loss occurs so early, many miscarriages occur without the woman ever having known she was pregnant.
Of those miscarriages that occur before the eighth week, 30% have no fetus associated with the sac or placenta. This condition is called blighted ovum, and many women are surprised to learn that there was never an embryo inside the sac.
As described above, some miscarriages occur before women recognize that they are pregnant. About 15% of fertilized eggs are lost before the egg even has a chance to implant (embed itself) in the wall of the uterus. A woman would not generally identify this type of miscarriage.
Another 15% of conceptions are lost before eight weeks' gestation. Once fetal heart function is detected in a given pregnancy, the chance of miscarriage is less than 5%.
A woman who may be showing the signs of a possible miscarriage (such asvaginal bleeding) may have her pregnancy referred to as a "threatened abortion."
Cramping and vaginal bleeding are the most common symptoms noticed with spontaneous abortion. The cramping and bleeding may be very mild, moderate, or severe. There is no particular pattern as to how long the symptoms will last.
Vaginal bleeding during early pregnancy is often referred to as a "threatened abortion". The term threatened abortion is used since miscarriage does not always follow vaginal bleeding in early pregnancy, even after repeated episodes or large amounts of bleeding.
Studies have shown that 90% to 96% of pregnancies with demonstrated fetal cardiac activity that result in vaginal bleeding at 7 to 11 weeks of gestation will result in an ongoing pregnancy.
The cause of a miscarriage cannot always be determined. The most common known causes of miscarriage in the first third of pregnancy (1st trimester) are chromosomal abnormalities, collagen vascular disease (such as lupus), diabetes, other hormonal problems, infection, and congenital (present at birth) abnormalities of the uterus. Chromosomal abnormalities of the fetus are the most common cause of early miscarriages, including blighted ovum (see above). Each of the causes will be described below.
Chromosomes are microscopic components of every cell in the body that carry all of the genetic material that determines hair color, eye color, and our overall appearance and makeup. These chromosomes duplicate themselves and divide many times during the process of development, and there are numerous points along the way where a problem can occur.
Certain genetic abnormalities are known to be more prevalent in couples that experience repeated pregnancy losses. These genetic traits can be screened for by blood tests prior to trying to conceive.
Half of the fetal tissue from 1st trimester miscarriages contain abnormal chromosomes. This number drops to 20% with 2nd trimester miscarriages. In other words, abnormal chromosomes are more common with 1st trimester than with 2nd trimester miscarriages. First trimester miscarriages are so very common that unless they occur more than once, they are not considered "abnormal" per se.
They do not prompt further evaluation unless they occur more than once. In contrast, 2nd trimester miscarriages are more unusual, and therefore may trigger evaluation even after a first occurrence. It is therefore clear that causes of miscarriages seem to vary according to trimester.
Chromosomal abnormalities also become more common with aging, and women over age 35 have a higher rate of miscarriage than younger women. Advancing maternal age is the most significant risk factor for early miscarriage in otherwise healthy women.
Collagen vascular diseases
Collagen vascular diseases are illnesses in which a person's own immune system attacks their own organs. These diseases can be potentially very serious, either during or between pregnancies. In these diseases, a woman makes antibodies to her own body's tissues.
Examples of collagen vascular diseases associated with an increased risk of miscarriage are systemic lupus erythematosus, and antiphospholipid antibody syndrome. Blood tests can confirm the presence of abnormal antibodies and are used in the diagnose of these conditions.
Diabetes generally can be well managed during pregnancy, if a woman and her health care practitioner work closely together. However, if the diabetes is insufficiently controlled, not only is the risk of miscarriages higher, but the baby can have major birth defects. Other problems can also occur in relation to diabetes during pregnancy. Good control of blood sugars during pregnancy is very important.
Hormonal factors may be associated with an increased risk of miscarriage, including Cushing's Syndrome, thyroid disease, and polycystic ovary syndrome (PCOS). It also has been suggested that inadequate function of the corpus luteum in the ovary (which produced progesterone necessary for maintenance of the very early stages of pregnancy) may lead to miscarriage. Termed "luteal phase defect", this is a controversial issue, since several studies have not supported the theory of luteal phase defect as a cause of pregnancy loss.
Maternal infection with a large number of different organisms has been associated with an increased risk of miscarriage. Fetal or placental infection by the offending organism then leads to pregnancy loss. Examples of infections that have been associated with miscarriage include infections by Listeria monocytogenes, Toxoplasma gondii, parvovirus B19, rubella, herpes simplex, cytomegalovirus, and lymphocytic choriomeningitis virus.
Abnormal structural anatomy
Abnormal anatomy of the uterus can also cause miscarriages. In some women there can be a tissue bridge (uterine septum), that acts like a partial wall dividing the uterine cavity into sections. The septum usually has a very poor blood supply, and is not well suited for placental attachment and growth. Therefore, an embryo implanting on the septum would be at increased risk of miscarriage.
Other structural abnormalities can result from benign growths in the uterus called fibroids. Fibroid tumors (leiomyomata) are benign growths of muscle cells in the uterus. While most fibroid tumors do not cause miscarriages, (in fact, they are a rare cause of infertility), some can interfere with the embryo implantation and the embryo's blood supply, thereby causing miscarriage.
Invasive surgical procedures in the uterus, such as amniocentesis andchorionic villus sampling, also slightly increase the risk of miscarriage.
What treatment can a woman expect when she has had a miscarriage?
The central goal of the doctor in this situation will be to try to figure out whether the woman has passed all of the tissue from the fetus and placenta. If she has passed all the tissue, she may only require observation by medical personnel.
On the other hand, a woman who has not passed all of the tissue (incomplete abortion) will usually need suction dilation and curettage (D&C) of the uterus to remove any retained products of the pregnancy. This procedure is done with local anesthesia, and sometimes antibiotics may be prescribed for the woman to prevent infection.
When should a woman receive evaluation for underlying causes of pregnancy loss?
Currently, most practitioners will not initiate an extensive medical evaluation for a single pregnancy loss, since the chance of having a normal pregnancy subsequent to even two consecutive miscarriages is 80% to 90%. For women with recurrent pregnancy loss, an evaluation will focus on the pattern and history of the prior miscarriages. Three consecutive miscarriages would suggest a woman should receive further evaluation.
Thus, the following tests are considered for women with three consecutive miscarriages.
Blood testing can be ordered to identify chromosomal abnormalities in the couple that could be transmitted to the fetus. The couple can each appear completely normal but still carry chromosomal defects, which, when combined, can be lethal to the embryo. This type of testing is called karyotyping, and it is performed on both members of the couple. Ahysterosalpingogram (HSG) can identify anatomical abnormalities within the uterus.
Antinuclear antibody, anticardiolipin antibody, VDRL, RPR, and lupus anticoagulant are some of the blood tests used to diagnose autoimmune diseases that can cause recurrent miscarriage.
As described above, some of these illnesses will already by apparent to the woman and her doctor, but not all cases. Other antibody tests may be performed as well.