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The causes of miscarriage and recurrent miscarriage (the loss of two or more consecutive pregnancies in the first or second trimester OR the loss of three or more pregnancies before 20 weeks gestation) are not thoroughly understood. However, in most cases, miscarriages occur because of chromosomal abnormalities in the fetus.

Chromosomes are tiny structures in each cell that carry genes and determine an individual’s physical traits and how the internal organs work. Each person has 23 pairs of chromosomes (46 total), with one chromosome per pair supplied by the mother and the other supplied by the father.

Most chromosomal abnormalities result from a faulty egg or sperm cell. Before pregnancy, immature egg and sperm cells divide to form mature cells with 23 chromosomes. Sometimes, the cell splits unevenly, resulting in egg or sperm cells with too many or too few chromosomes (nondisjunction). If a cell has the wrong number of chromosomes, the embryo has a chromosomal abnormality and is usually miscarried.

These abnormalities are randomly occurring events that surface during cell division and are not inherited from the genes of either parent. They may also result in a blighted ovum, either because the embryo did not form or because it stopped developing very early. According to the American College of Obstetricians and Gynecologists (ACOG), recent studies show that chromosomal problems may cause nearly 50 percent of recurrent miscarriages.

Other conditions that may cause one or more miscarriages include:

• Uterine and/or cervical abnormalities. Some women are born with a uterus that is abnormally shaped, or partly or completely divided. Others develop noncancerous tumors (fibroids) or scars in the uterus from past surgery. These abnormalities can limit space for the fetus to grow or interfere with the blood supply to the uterus. They may also affect the ability for the embryo to implant properly. In addition, a weakened or incompetent cervix (opening of the uterus) can lead to a miscarriage. According to the ACOG, uterine and cervical abnormalities may account for 10 to 15 percent of recurrent miscarriages. However, some of these structural abnormalities can be surgically corrected to improve the chances of a future pregnancy.
  
  
• Chronic illness. Women with conditions such as systemic lupus erythematosus (lupus) and other autoimmune disorders, congenital heart disease, severe kidney disease, uncontrolled diabetes mellitus, thyroid disease and intrauterine infections have a higher risk of miscarriage. If a woman is diagnosed with these conditions, she should get proper treatment to control them before becoming pregnant.
  
  
• Hormonal problems. When the body produces too much or too little of certain hormones, a miscarriage may occur. Researchers believe that insufficient secretion of the hormone progesterone by the ovaries may be associated with spontaneous abortion because progesterone is believed to be important in maintaining gestation. This hormonal imbalance is called luteal phase deficiency and it may make the inner lining of the womb (endometrium) unable to support a pregnancy. In addition, conditions associated with hormonal abnormalities (e.g., thyroid disease, polycystic ovarian syndrome) may result in a miscarriage of the fetus.
  
  
• Infections. Maternal health conditions that have been associated with miscarriage include infectious diseases, such as listeriosis (caused by the listeria bacterium found in certain raw meats and dairy products), toxoplasmosis, mumps, rubella, measles, HIV, herpes and syphilis, among others.
  
  
• Fever. Pregnant women who develop fevers of 100 degrees Fahrenheit or more (37.8 degrees Celsius) appear to have an increased risk of miscarriage.
  
  
• Immune system problems. Some people produce certain antibodies (autoantibodies) that can attack their own tissues, causing a variety of health problems. For example, particular types of autoantibodies (e.g., anticardiolipin) cause blood clots that can clog blood vessels in the placenta, causing the fetus to die, such as when lupus is present. Another condition that is associated with blood clots in the veins or arteries and with miscarriage is antiphospholipid antibody syndrome (APS). According to the American Society for Reproductive Medicine, between 3 to 15 percent of recurrent miscarriage is due to APS.
  
  
• Blood incompatibility. In some cases, the fetus’ and mother’s blood type do not match, causing the mother to develop antibodies to the fetus. This type of incompatibility between mother and fetus (Rh incompatibility) may result in a spontaneous abortion.
  
  
• Previous miscarriages. Risk of miscarriage increases if a woman has a history of recurrent miscarriage.

In addition, studies have shown several factors associated with a higher rate of miscarriage:

• Age. Advancing age of the mother is the most important risk factor for miscarriage in healthy women. Women over age 40 are at higher risk of miscarriage and recurrent miscarriage than younger women. A woman’s risk of miscarriage increases with age because chromosomal abnormalities become more common with aging.
  
  In addition, advanced age of sperm may also significantly influence the rate of miscarriage. Women with male partners aged 35 years or older have nearly three times as many miscarriages than women who conceive with men younger than age 25, according to a recent large-scale study conducted at Columbia University’s Mailman School of Public Health and the New York Psychiatric Institute. Results from previous research also indicate that the genetic quality of sperm gradually deteriorates as men get older, increasing a man’s risk of infertility, fathering unsuccessful pregnancies (miscarriages) and passing along certain genetic defects to his offspring (e.g., dwarfism).
  
  
• Number of pregnancies. The number of times a woman has been pregnant also affects her risk of miscarriage. Women who have had two or more pregnancies appear to have a greater risk.
  
  
• Folate insufficiency. According to the National Institutes of Health (NIH), pregnant women who have low levels of folic acid (a B-complex vitamin that is essential for cell growth and reproduction) are more likely to experience early miscarriages than pregnant women who have adequate levels of the vitamin.
  
  In addition, women who are underweight prior to becoming pregnant are at greater risk of suffering a miscarriage during the first trimester, according to a recent study published in the BJOG: An International Journal of Obstetrics ande Gynecology. The same study also found that folic acid or iron supplementation and a daily diet incorporating plenty of fresh fruits and vegetables can reduce risk of miscarriage by as much as 50 percent in underweight women.
  
  
• Caffeine. Although evidence is inconclusive, some studies suggest an association between moderate to significant amounts of caffeine (in the range of four to five cups of coffee daily) and an increased risk of miscarriage.
  
  
• Smoking. Excessive smoking (more than 10 cigarettes a day) is associated with an increased risk of miscarriage. In addition, smoking can harm the development of the fetus, even when miscarriage does not occur. Paternal smoking may also be associated with an increased risk of miscarriage.
  
  
• Alcohol. Women who drink are twice as likely to have a miscarriage as women who abstain from alcohol during pregnancy. In addition, alcohol exposure can be harmful to the fetus even in cases where miscarriage does not occur, and often results in low-birth weight babies with learning and/or physical disabilities, among other damaging effects. Alcohol intake can also reduce sperm count in men.
  
  
• Use of certain medications and/or illegal substances. Pregnant women who have taken certain prescription or over-the-counter drugs, including nonsteroidal anti-inflammatory drugs (NSAIDs) like ibuprofen have reported miscarriages. In addition, the use of illegal substances such as heroin, cocaine and ecstasy has been associated with miscarriage.
  
  
• Use of donor eggs. Recent research indicates that women who achieve pregnancy via assisted reproductive technology (e.g., in-vitro fertilization) using donated eggs may be more likely to experience miscarriage. Scientists believe that miscarriage may occur as a result of the mother’s immune system reacting negatively against the foreign egg.
  
  
• Prenatal testing. Certain types of prenatal genetic tests (e.g., amniocentesis, chorionic villus sampling) have been associated with an increased risk of miscarriage due to the invasive nature of the procedures. However, both amniocentesis and chorionic villus sampling appear to carry only a low risk of miscarriage, according to researchers at the University of California, San Francisco who recently conducted a large-scale review of numerous studies over the past 20 years.
  
  
• Trauma. Severe trauma to the uterus (e.g., trauma from a serious accident or fall) can increase the risk of miscarriage. However, activities of daily living, such as exercising, sex, working or lifting heavy objects, do not provoke a miscarriage – even in very active women. Exercise, in general, is healthy for the mother and the developing baby. Pregnant women should consult with their physicians to discuss the extent and types of exercise to ensure the baby’s safety before engaging in such activities.
  
  
• Environmental factors. Exposure to environmental toxins, radiation and immunologic factors has been associated with miscarriage. Lead, arsenic, mercury, some chemicals like formaldehyde, benzene, ethylene oxide, and large doses of radiation or anesthetic gases have been associated with miscarriage.