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The colloquial term ‘miscarriage’ is commonly applied to any loss of an embryo or fetus that occurs as part of a natural process. Medically, this usually corresponds to a spontaneous abortion which we must distinguish from an induced abortion, a termination of pregnancy that is initiated on purpose through either a minor surgical procedure or medical treatment. Terminology can get a little confusing for a couple of reasons. One reason is that the term spontaneous abortion applies only to pregnancy loss occurring prior to a gestational age of 20 weeks, the halfway point of pregnancy. Beyond 20 weeks, pregnancy loss is called a stillbirth, yet often the colloquial miscarriage is applied to pregnancy loss beyond 20 weeks, at least through the second trimester, which ends at the close of the 26th gestational week. The closer the loss occurs to expected time of delivery, the more you will hear people refer to a pregnancy loss as a stillbirth rather than a miscarriage.
The overwhelming majority of pregnancy losses, however, occur during the first half of pregnancy, and usually during the first trimester, when they are called early pregnancy loss. In such cases, the medical term spontaneous abortion always applies. However, it actually can occur in several different ways, requiring different amounts of help from medical professionals. So in a certain sense, spontaneous abortion is a kind of spectrum.
At one end of the spectrum is a situation called a threatened abortion. This part of the spectrum technically is not a kind of pregnancy loss, because it only means that a spontaneous abortion is gearing up to occur, but may not. There is a threat that it may occur, because there are symptoms (pain and/or bleeding) and on ultrasound it looks as though the uterus is getting ready to expel the products of conception. But there is time to intervene with the possibility of preventing the expulsion, because the cervical os, the passageway between the uterus and the vagina, remains closed.
Next on the spectrum is an inevitable abortion, which we can include within the realm of pregnancy loss, since inevitable means that the abortion will happen no matter what, because the cervical os is open. Spontaneous abortion also includes a missed abortion, meaning that the embryo or fetus is no longer viable, but the uterus forgot to expel it. This is dangerous, because the products of conception can cause in infection of the uterus, such as endometritis. If the products of conception do not come out on their own soon, doctors need to intervene, medically or surgically, to remove them. An abortion that causes an infection is called a septic abortion (term that can apply either to a spontaneous abortion or an induced abortion, as either can become infected). There also can be an incomplete abortion. In this situation, the embryo or fetus becomes non-viable and is only partly expelled. Pieces remain unexpelled, requiring intervention to protect the mother from potential infection and bleeding. In contrast, there is a complete abortion, in which all products of conception exit the uterus. Often, this requires no intervention other than supportive therapies.
As for causes of spontaneous abortion, there are various particular known causes, such as cocaine use, smoking, alcohol consumption (not just a few drops, but a lot). Various diseases, such as diabetes, celiac disease, and autoimmune conditions —notably anti-phospholipid antibody syndrome— can trigger spontaneous abortion, plus advanced maternal age (especially being more than 45 years old) and having a previous early pregnancy loss are risk factors. However, about half of all cases of spontaneous abortion are though to result from the fetus having chromosomal abnormalities. Most early embryos resulting from conception contain problems with genes and chromosomes that might be incompatible with life at the time of birth, but usually they do not implant. They are simply expelled in the woman’s period. Sometimes they do implant, but are aborted very early, often before the woman knowns that she is pregnant. Occasionally, however, they persist longer in the uterus, leading to a spontaneous abortion that is noticed, because the pregnancy itself has been noticed, and also because there is bleeding and pain. In such cases, the woman’s body is ridding itself of a non-viable product of conception than should have been expelled much earlier.