Various factors increase the risk of spontaneous abortion in pregnancy including:

Age. Women older than age 35 have a higher risk than do younger women. At age 35, you have about a 20 percent risk. At age 40, the risk is about 40 percent. And at age 45, it's about 80 percent.

. Previous abortion-Women who have had two or more consecutive miscarriages are at higher risk of miscarriage.

Chronic conditions. Women who have a chronic condition, such as uncontrolled diabetes, have a higher risk of miscarriage.

Uterine or cervical problems.  uterine conditions or weak cervical tissues (incompetent cervix) might increase the risk of abortion

Smoking, alcohol and illicit drugs. Women who smoke during pregnancy have a greater risk of miscarriage than do nonsmokers. Heavy alcohol use and illicit drug use also increase the risk of miscarriage.

Weight. Being underweight or being overweight has been linked with an increased risk.

Invasive prenatal tests. Some invasive prenatal genetic tests, such as chorionic villus sampling and amniocentesis, carry a slight risk of spontaneous abortion