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 8% to 20% of all recognized pregnancies, and usually occurs before the 13th week of pregnancy.
With the development of highly sensitive assays for human chorionic gonadotropin levels that can detect an early pregnancy even prior to the expected next period (menstruation), researchers have been able to show that around half 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, a portion has 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.
Causes of miscarriage
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. 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. 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 erythematous, 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 professional 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,
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.