Miscarriage—the loss of a baby in the first 20 weeks of pregnancy—is one of the most common pregnancy-related issues. In fact, according to the American College of Obstetricians and Gynecologists (ACOG), about 15 to 20 percent of pregnancies end in miscarriage. It's smart to learn everything you can about why it happens, and how to tell if it's happening to you.

What are the most common miscarriage causes?

Many women blame themselves or their behavior if they have a miscarriage, but in most cases, there's nothing you've done to cause it—and there's nothing you can do to prevent it. Miscarriage is not caused by moderate exercise, sex, or a small daily cup of coffee. The most common cause, according to American Pregnancy Association (APA), is a chance genetic abnormality in the embryo.

Issues that could cause miscarriage include:

• Chromosomal problems. During fertilization, the sperm and egg each bring 23 chromosomes together to create 23 perfectly matched pairs of chromosomes. This is a complex process, and a minor glitch can result in a genetic abnormality that prevents the embryo from growing. Researchers blame genetics for most miscarriages. As you age, these glitches are more likely to occur.
• Hormone imbalance. About 15 percent of all miscarriages are attributed to unbalanced hormones, such as insufficient progesterone levels that prevent your fertilized egg from implanting in your uterus.
• Uterine problems. Uterine fibroids inside the uterus can interfere with implantation or blood supply to the fetus. Some women are born with a septum, an uncommon uterine defect linked to miscarriage, and others develop bands of scar tissue in the uterus from surgery or second-term abortions that can keep an egg from implanting properly or may hamper blood flow to the placenta. A doctor can determine uterine defects through specialized X-rays; most can be treated.
• Chronic illness. Chronic illnesses like autoimmune disorders, lupus, heart disease, kidney and liver disease, and diabetes cause as many as 6 percent of recurring miscarriages. If you have a chronic illness, find an obstetrician experienced in caring for women with your condition.
• High fever. No matter how healthy you are normally, if you develop a high fever—a core body temperature over 102 degrees—during early pregnancy, you may experience a miscarriage. A high core body temperature is most damaging to the embryo before 6 weeks.

What are the most common miscarriage symptoms?

Spotting in early pregnancy is fairly common, but it should never be considered normal, and it always warrants a call to your OB/GYN, Dr. Landy says. Although the presence of spotting does not always indicate a miscarriage, it is a sign that something abnormal may be going on in the pregnancy, she adds. "Even if everything looks normal, the fact that the patient's had spotting will be kept in the back of our minds throughout her pregnancy."

Some women do not experience any symptoms of miscarriage at all; however, possible miscarriage signs besides bleeding include:

• Mild to severe cramps
• Pain in your back or abdomen
• Loss of pregnancy symptoms, such as nausea or vomiting
• White-pink mucus
• Passing tissue or clot-like material