The term "missed abortion" refers to an inevitable miscarriage in a woman who has not yet experienced any vaginal bleeding or uterine cramping. In a missed abortion, the pregnancy will not progress because, although the woman retains embryonic and/or placental tissue in her uterus, the embryo either never formed or died of natural causes. Rather than waiting for a miscarriage, the doctor can recommend medication or surgery to remove the tissue from the uterus.
Medically, "abortion" means loss of the products of conception from any cause, natural or induced, before 20 weeks of pregnancy. In this light, the term "missed abortion" makes more sense. The woman’s body will eventually expel the contents of her uterus, but for now, the body has missed the signs that the embryo died or never existed and hangs on to the products of conception. The Mayo Clinic uses the slightly less confusing term “missed miscarriage” instead.
Most cases of missed abortion result from severe genetic abnormalities that prevent the formation of the embryo or stop its development early in pregnancy. In most cases, says the Mayo Clinic, the genetic abnormality arises in the first few cell divisions and often the exact cause is never identified. There is nothing a woman can do to prevent a missed abortion.
When a fertilized egg develops a placenta and membranes with no embryo inside it, doctors call it a "blighted ovum." Although there is no embryo, the woman has symptoms of pregnancy because the placenta produces the pregnancy hormone hCG. Blighted ovum usually results from a genetic abnormality and causes about half of all miscarriages in the first 12 weeks of pregnancy, according to the Mayo Clinic. It can cause a spontaneous miscarriage or a missed abortion.
Molar pregnancies rarely occur; the Mayo Clinic cites a frequency of one in 1,000 pregnancies. In a molar pregnancy, or gestational trophoblastic disease, the placental cells grow out of control to form a cystic mass. An embryo, if present, will not survive. Like a blighted ovum, a molar pregnancy can cause a spontaneous miscarriage or a missed abortion.
In cases of missed abortion, allowing the tissue to remain in the uterus until is it naturally expelled can take three or four weeks after diagnosis, risking heavy bleeding or infection. Instead, a doctor might prescribe medications to bring on a miscarriage at home, or if bleeding is a concern, the doctor might recommend dilation and curettage (D&C) to remove the placental tissue in a more controlled way. In this minor surgery, the doctor dilates the opening of the uterus and suctions or gently scrapes out the tissue.
The routine use of ultrasound has increased the diagnosis of missed abortion, allowing women to protect their future fertility with medical treatment, rather than suffering more dangerous late miscarriages. Because most genetic abnormalities that cause missed abortion develop in the fetus rather than being inherited from the parents, most women can go on to have successful pregnancies after a missed abortion.