Miscarriage: Causes, Symptoms and Prevention

What is a miscarriage?

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 as vaginal bleeding) may have her pregnancy referred to as a “threatened abortion.”

What causes a miscarriage, and what are the tests for the different causes?

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.


Chromosomal abnormalities

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 from1st 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

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.


Other causes

Invasive surgical procedures in the uterus, such as amniocentesis and chorionic villus sampling, also slightly increase the risk of miscarriage.


What does NOT cause miscarriage?

It must be emphasized that exercise, working, and sexual intercourse do not increase the risk of pregnancy loss in routine (uncomplicated) pregnancies. However, in the unusual circumstance where a woman is felt by her physician to be at higher risk of spontaneous abortion, she may be advised to stop working and refrain from having sexual intercourse. Women with past history of premature delivery and other specific obstetrical conditions might fall under this category.


Are there lifestyle factors associated with miscarriage?

Smoking more than 10 cigarettes per day is associated with an increased risk of pregnancy loss, and some studies have even shown that the risk of miscarriage increases with paternal smoking. Other factors, such as alcohol use, fever, use of nonsteroidal anti-inflammatory drugs around the time of embryo implantation, and caffeine use have all been suggested to increase the risk of miscarriage, although more studies are needed to fully clarify any potential risks associated with these factors. Of course, alcohol is a known teratogen (a chemical that can damage the developing fetus), so pregnant women are advised to abstain from drinking alcoholic beverages.


What are the symptoms of a miscarriage?

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.


What will the doctor look for during an examination with suspected miscarriage?

A woman’s cervix might have some bloody discharge, but nothing else unusual will be characteristic of threatened abortion. Some women will have mild uterine tenderness during the manual examination of the uterus. The doctor may look to see if the cervix is dilated and will check to see if the uterus is enlarged to an extent appropriate for gestational age of the pregnancy.


How is threatened abortion evaluated?

Pelvic ultrasound is used to visualize fetal heartbeat and to determine whether a pregnancy is still viable. The ultrasound examination can also distinguish between intrauterine and ectopic pregnancies. The doctor may also order blood levels of serial human chorionic gonadotrophin (HCG) to help determine the viability of a pregnancy if the ultrasound examination is not conclusive. During the evaluation, the woman may be advised to rest and avoid sexual intercourse (activity).


What are common terms a woman might hear during evaluation for miscarriage?

  • “Miscarriage” (spontaneous abortion) is termination of pregnancy before the fetus is viable (able to survive).
  • “Complete abortion” describes spontaneous (not intentionally induced by medication or procedures) passage of all fetal and placental tissue. This is common prior to 12 weeks’ gestation.
  • “Incomplete abortion” is when some, but not all, the fetal and placental tissue is expelled.
  • “Products of conception” refers to the combination of fetal and placental tissue.
  • “Threatened abortion” is when a miscarriage does not actually occur, but there is vaginal bleeding from the uterus. The cervix will not be dilated and does not show signs of imminent passage of fetal and placental tissue.
  • “Missed abortion” describes a fetal death in the uterus prior to viability, but the products of conception are not passed.
  • A “septic (infectious) abortion” is caused by bacterial infection and accompanied by fever, chills, pain, and a pus-containing discharge.


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. A hysterosalpingogram (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.

Can something be done to prevent future miscarriages?

The treatment of recurrent miscarriage depends on what is believed to be the underlying cause. This often is not as simple as it sounds. Careful evaluation may turn up several potential factors which alone or together may be responsible for the pregnancy losses. If a chromosomal problem is found in one or both persons, then counseling as to future risks is the only option for the couple, since there is currently no method to correct genetic problems.

If a structural problem is encountered with the uterus, surgical correction could be contemplated. It should be emphasized that just because a structural abnormality is found, it does not necessarily mean that it caused the miscarriage. Removal of a fibroid or uterine septum does not guarantee a future successful pregnancy, since the fibroid or uterine septum may not have been the cause of miscarriage in the first place.

Adequate control of diabetes and thyroid disease is critical in trying to prevent recurrent pregnancy loss in women with those conditions. For women with immunologic problems, such as such as systemic lupus erythematosus and antiphospholipid antibody syndrome, certain medications are being studied that may be useful in achieving successful pregnancy outcomes. Blood thinners such as aspirin and heparin can, in some cases, prevent further pregnancy loss.

The use of progesterone to increase the blood levels of this hormone is sometimes used for patients with recurrent pregnancy loss, although large-scale controlled studies that confirm the utility of progesterone supplementation have not been carried out. However, many physicians report success with progesterone therapy. Progesterone may be given as vaginal suppositories, or in tablet or gel form.

In dealing with recurrent pregnancy loss, it is important to realize that even though apparently obvious problems can be corrected, a miscarriage can still occur. This is not to say that attempts should not be taken to correct identified abnormalities that have been historically associated with miscarriage. However, no treatment can be guaranteed. Even with repeated miscarriages, there is still a very good chance of achieving a successful pregnancy. Early pregnancy and pre-pregnancy counseling can help identify risk factors and allow the practitioner to provide any special care that may be needed.


This information is provided by MedicineNet.