Miscarriage is a term that is applied to pregnancy loss that occurs during the first 20 weeks of gestation; after 20 weeks, pregnancy loss is considered a stillbirth. The terms “spontaneous abortion” and “miscarriage” are used interchangeably by medical professionals.
Approximately 30 to 40% of all conceptions and 10 to 20% of all recognized pregnancies end in pregnancy loss. Many early miscarriages go unrecognized because they occur before parents realize conception has occurred, and they are mistaken for a late menstrual period.
90 to 95% of all pregnancy losses occur during the first trimester (12 weeks) of gestation. At least one-half of first trimester pregnancy losses—and 85% of all miscarriages—are caused by chromosomal abnormalities.
One to five percent of pregnancies are lost between 13 and 19 weeks of gestation, while less than one percent of pregnancies end in stillbirth.
Causes of Miscarriage - First Trimester
Fetal factors
- Chromosomal abnormalities: trisomies 13, 18, and 21 (Down syndrome); monosomy X (Turner syndrome); sex chromosome polysomies (e.g., Klinefelter’s syndrome)
- Congenital abnormalities: neural tube and other fusion defects; amniotic band syndrome; lethal malformations
Maternal factors
- Anatomic anomalies: intrauterine adhesions, fibroids, uterine septum
- Immunologic factors: lupus, antiphospholipid antibodies
- Poorly controlled chronic diseases: diabetes, thyroid, hypertension
- Endocrine abnormalities: polycystic ovary syndrome, Cushing’s syndrome, luteal phase defect (failure of the corpus luteum to secrete hormones that maintain early pregnancy)
- Infection: cytomegalovirus, herpes simplex, rubella, parvovirus, Toxoplasma, etc.
- Thrombophilia (abnormal tendency to form clots): hereditary mutations in clotting proteins; activated protein C; factor V Leiden, etc.
Other Factors
- Ectopic pregnancy; smoking or drug use; exposure to teratogens (e.g., Accutane, ACE inhibitors, Coumadin, tetracycline, anabolic steroids, etc.); trauma
Second TrimesterFetal Factors
- Chromosomal and congenital abnormalities: similar to first trimester
Maternal factors
- Anatomic anomalies: same as for first trimester; incompetent cervix (possibly secondary to previous cone biopsy); retained IUD
- Immunologic factors: similar to first trimester
- Poorly-controlled chronic illness or severe acute illness
- Infection: bacterial vaginosis or intra-amniotic infection
- Placental anomalies: placenta previa; placental abruption or hematoma
- Thrombophilia
Other factors
- Drug use; smoking; preterm premature rupture of membranes (often associated with incompetent cervix or intrauterine infection); teratogen exposure; trauma
Third trimester
Fetal factors
- Chromosomal and congenital abnormalities: similar to first and second trimesters
Maternal factors
- Immunologic factors: same as for first and second trimesters
- Poorly-controlled chronic illness or severe acute illness
- Infection: bacterial vaginosis
- Placental anomalies: similar to second trimester
- Thrombophilia
- Umbilical cord complication: compression, thrombosis, etc.
Feto-maternal factors
- Twin-twin or feto-maternal transfusion
- Fetal growth retardation
- Isoimmunization (transfer of incompatible antibodies from mother to fetus)
Other factors
- Drug use; smoking; trauma; teratogen exposure
(Adapted from Michels T and Tiu A. Second trimester pregnancy loss. Am Fam Phys 2007;76(9):1341-46)
Prevention of Miscarriage
The majority of miscarriages are secondary to conditions that are not preventable (e.g., chromosomal anomalies). Additionally, a specific cause for pregnancy loss cannot be identified in at least 50% of cases.
Pregnancies that end in the second or third trimesters should prompt a thorough evaluation for underlying preventable causes (i.e., infection, cervical incompetence, poorly-controlled chronic illness, etc.).
Women who experience recurrent miscarriage in their first trimester should also undergo complete evaluation to rule out immunologic, chromosomal, hematologic, infectious, or anatomic anomalies.
All pregnant women (and those considering pregnancy) should abstain from smoking, alcohol, and illicit drug use; all medications should be reviewed and discontinued or changed if they are potentially teratogenic.
Folate should be initiated prior to conception and continued throughout pregnancy: 0.4 mg daily for most women; 4 mg daily for women who have had a fetus with a neural tube defect. (The Merck Manual, 18th Edition 2006:2168,2199-2201)
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