As many as 40 percent of all pregnancies may end in miscarriage, often before a woman even realizes that she’s pregnant. A miscarriage is a pregnancy loss that occurs before the 20th week of pregnancy.
Most miscarriages occur in the first trimester (prior to 13 weeks). Of the 1 percent to 5 percent of pregnancies that result in miscarriage in the second trimester (between 13-19 weeks), these losses are oftentimes related to problems with the uterus or a weakened cervix that prematurely dilates.
The causes of first-trimester miscarriage are not thoroughly understood. Most often, miscarriages occur when a pregnancy is not developing normally. Some of the most common reasons for miscarriage include a chromosomal abnormality in the fetus, a blighted ovum, certain maternal health conditions and certain lifestyle or environmental factors.
More than 50 percent of first-trimester miscarriages are caused by a chromosomal problem in the fetus. Each person has 23 pairs of chromosomes, which are thread-like structures, in each cell.
If an embryo has the wrong number of chromosomes, the pregnancy usually results in an early miscarriage. A blighted ovum, when a pregnancy sac contains no fetus, is also often attributed to chromosomal abnormalities that either prevented the embryo from forming or halted embryo development.
Miscarriages are less likely for women who receive early, comprehensive prenatal care and avoid environmental hazards (such as X-rays, drugs and alcohol, high levels of caffeine, and infectious diseases).
Although certain maternal health conditions such as thyroid disease, infections, exposure to environmental toxins, certain hormonal problems, bloodclotting disorders and autoimmune disorders can increase the risk of miscarriage, treating these conditions before and during pregnancy can reduce the risk and/or prevent miscarriage. Women who smoke and/or use illicit drugs may increase their risk of miscarriage.
Vaginal bleeding, menstrual-like cramps or abdominal pain are signs that a miscarriage may occur. While many women experience spotting (light vaginal bleeding) in early pregnancy, most do not miscarry.