By Zaira Salvador BSc, MSc (embryologist).
Last Update: 06/01/2018

A miscarriage or spontaneous abortion is the involuntary loss of a developing fetus before the 20th week of pregnancy.

The incidence rate of miscarriages is 1 out of 5 pregnancies, that is, 20 percent of pregnancies end up in miscarriage. Most of them occur during the first trimester, particularly between weeks 6 to 12 of pregnancy.

There exist many types of spontaneous abortion based on several factors, as one shall see in the sections below.

What is a miscarriage?

The different kinds of miscarriage can be classified into various groups. First and foremost, one should learn the differences between:

Not induced by the woman voluntarily, they occur naturally.
Induced by the pregnant woman, typically in cases of unplanned pregnancy or pregnancy as a result of rape, for instance.
Therapeutic abortion
Abortion is induced, but due to medical reasons.

Throughout the following sections, we will focus on the different types of miscarriage, which is to say, those pregnancy losses that occur against the will of the woman.

Miscarriage classification

The World Health Organization (WHO) defines miscarriage as the spontaneous or deliberated termination of a gestation before the 20th week of pregnancy, or when fetal weight is below 500 g.

A miscarriage may be due to both problems related to the fetus or the mother. To learn more, read: What Causes a Miscarriage?

Broadly speaking, a miscarriage can take place at different stages of pregnancy. The first sign is known as threatened miscarriage, which typically appears in the form of a mild vaginal bleeding and pain in the abdomen.

In such cases, the woman should visit a doctor as soon as possible. He or she will perform an ultrasound scan to see if the cervix has started dilating. If that’s the case, your doctor will recommend bed rest.

The signs and symptoms of a threatened miscarriage may become worse and turn into heavy bleeding if it turns into an actual miscarriage (known as inevitable miscarriage in these cases). Pain will be more intense and the cervix will open to expel the fetus.

Miscarriages are classified in different types according to the following factors:

By week of pregnancy

There exist two types of miscarriage based on whether it occurs as an isolated event or if is recurrent:

Sporadic miscarriage (SM)
It occurs as a single, sporadic event. It’s common amongst first-time moms and doesn’t translate into infertility.
Habitual or recurrent miscarriage (RM)
When three or more miscarriages occur, regardless of whether they are consecutive (primary RM) or non-consecutive (secondary RM).

Recurrent miscarriage, also known as recurrent pregnancy loss (RPL), requires further examination to find the cause and recommend the most adequate treatment. Get more info by clicking the following link: What Is Recurrent Miscarriage? – Causes, Symptoms & Treatment.

By gestational age

As explained earlier, by miscarriage we refer to a pregnancy loss that occurs before the woman is 20 weeks pregnant. During this time period, we can distinguish two types of miscarriage based on the precie moment when they take place:

Early miscarriage
When it occurs before the 12th week of pregnancy. It’s the most common type of miscarriage.
Late miscarriage
If it occurs after the 12th week of pregnancy.

Early miscarriage can take place before the woman is even aware that she’s pregnant, that is, before getting a positive pregnancy test. For this reason, early or very early miscarriages often go unnoticed and are confused by just a late menstrual period.

A chemical pregnancy, also known as biochemical pregnancy, is a type of early miscarriage that translates into a very early embryo loss, right after having gotten a positive test.

Chemical pregnancies are often due to defects that take place during fertilization, abnormal cell divisions in the embryo after fertilization, or abnormalities during the implantation process.

You may also enjoy some further information reading this: What Is a Chemical Pregnancy? – Symptoms, Causes & Treatment.

By gestational sac

There exist two types of miscarriages based on whether an embryo can be seen within the gestational sac or not:

Anembryonic sac or blighted ovum
The gestational sac appears empty, without an embryo inside, due to early embryonic arrest.
Embryonic sac
The gestational sac contains an embryo, but no fetal heartbeat can be detected due to late embryonic arrest.

About 50% of miscarriages that occur in the first trimester are anembryonic pregnancies (blighted ovum), caused in most cases by genetic abnormalities in the embryo. Related: What Causes a Blighted Ovum? – Symptoms, Diagnosis & Treatment.

Ectopic or extrauterine pregnancy can be included in this section as well. It is a type of pregnancy where the embryo attaches to a place other than the uterine lining, usually to the Fallopian tube. Obviously, this pregnancy is unviable and ends up in miscarriage.

Read this next: What Causes an Ectopic Pregnancy? – Symptoms, Diagnosis & Treatment.

By clinical type

After a pregnancy loss, it is necessary for the body to remove the remaining embryonic tissue naturally or, in case it’s not possible, it should be surgically removed, typically using a technique known as D&C (Dilation & Curettage).

Based on whether embryonic tissue is removed naturally or surgically, we can differentiate between the following types of miscarriage:

Complete miscarriage
All pregnancy tissue has left the uterus after fetal death. No further surgery or treatment is required, and the bleeding typically disappears eventually, along with pain. The cervix contracts to return to normality as well.
Incomplete miscarriage
Pregnancy tissue leaves the uterus partially after fetal death. Thus, the remaining tissue should be removed surgically to prevent bleeding and infections that may affect the woman’s health.
Missed miscarriage
The embryo dies but the gestational sac remains in the body for weeks or even months. It usually happens between weeks 8 and 12 of pregnancy. Pregnancy symptoms disappear eventually, the uterus stops growing, and pregnancy tests are negative 10 weeks after fetal death approximately.
Miscarriage due to embryo arrest
Miscarriage occurs so early that fetal tissue doesn’t form. The embryo stops growing after implantation and is completely removed with menstrual flow.

Both incomplete and missed miscarriage require a surgical procedure to perform a D&C and remove all pregnancy tissue from the uterus. In case tissue is removed partially, it may end up in a septic abortion due to an infection, which would be highly dangerous for the woman.

FAQs from users

How long does miscarriage bleeding last?

By Zaira Salvador BSc, MSc (embryologist).

It varies from woman to woman and the characteristics of each type of miscarriage. Broadly speaking, when it’s a complete miscarriage, vaginal bleeding can last about a week or two approximately.

Can you have a miscarriage before missed period?

By Zaira Salvador BSc, MSc (embryologist).

Yes. In that case, it is called subclinical very early miscarriage, that is, the woman didn’t even know that she was pregnant. Bleeding is confused with menstrual flow.

Is D&C necessary in all cases after a miscarriage?

By Zaira Salvador BSc, MSc (embryologist).

No, in cases of complete miscarriage, or when embryo development stops at the earliest stages, the remaining pregnancy tissue is removed with menstrual flow. In other words, D&C is not necessary in these cases.

Suggested for you

In most cases, before the woman has a miscarriage, she has a threatened miscarriage, which can lead to an actual miscarriage or stop and lead to a good outcome. Check out this for information: Threatened Miscarriage – What Are the Odds of Miscarriage?

Bleeding during pregnancy can be caused by a number of factors and are associated with complications in all cases. To learn more about the potential causes of each type of bleeding, read: Causes of Bleeding During Pregnancy – What’s Normal & What’s Not?

Authors and contributors

 Zaira Salvador
BSc, MSc
Bachelor's Degree in Biotechnology from the Polytechnic University of Valencia (UPV). Embryologist specializing in Assisted Procreation, with a Master's Degree in Biotechnology of Human Assisted Reproduction from the University of Valencia (UV) and the Valencian Infertility Institute (IVI). More information
License: 3185-CV