Miscarriage (Spontaneous Abortion) Causes and Dangers

A miscarriage can be devastating for a woman and it is important to look for any preventable cause so that future pregnancies are not jeopardized, although in a large number of cases no definite cause will be found. A miscarriage in early pregnancy is thought to be nature’s way of dealing with some abnormality within the fetus (genetic defects) by not allowing the pregnancy to proceed any further.

What is a miscariage?

Spontaneous abortion or miscarriage is the spontaneous termination of pregnancy before 20 weeks of gestation or delivery of a fetus weighing 500 grams or less (a non-viable fetus). Spontaneous abortion encompasses threatened, incomplete, inevitable, complete, missed and septic abortions and is the most common cause of vaginal bleeding in the first trimester (first 12 weeks) of pregnancy.

It is estimated that more than 20% of pregnancies terminate in spontaneous abortion but the figures may actually be much higher since many early miscarriages are undiagnosed and are taken as unusually heavy periods. A miscarriage occurring before 12 weeks of pregnancy is known as early abortion, while one occurring between 12 and 20 weeks of pregnancy is called late abortion.

The terms threatened, inevitable, incomplete and complete abortion may be taken as four stages of spontaneous abortion or miscarriage, where one stage may progress into the other.


Types of Abortion

Threatened Abortion

In threatened abortion, the pregnancy may be at risk of being terminated but in more than half the cases, the pregnancy can be saved. It is most common in the first trimester of pregnancy and usually presents as vaginal bleeding in early pregnancy with little or no pain, although slight abdominal cramping may be present.

There is no cervical dilatation and the most significant finding on examination is a closed cervical os (opening of the cervix). Ultrasonography may show a continuing intrauterine pregnancy. A threatened abortion needs to be differentiated from an ectopic pregnancy (a more dangerous situation) by assessing the beta-human chorionic gonadotropin level (Quantitative HCG Blood Test) in conjunction with the ultrasonography report.

Inevitable Abortion

In this case, conditions are such that the pregnancy cannot be saved. In addition to vaginal bleeding in early pregnancy (which may be more severe than that seen in threatened abortion), low back pain and severe abdominal cramps are more likely to be present. There is cervical dilatation and on examination the os is found to be open, with or without protrusion of the products of conception (fetal tissue, placenta and membranes). Ultrasonography may show the products of conception in the lower uterine segment or the cervical canal.

Incomplete Abortion

In an incomplete abortion, there is usually persistent and severe vaginal bleeding with abdominal cramps. Some, though not all, of the products of conception are expelled from the uterus through the dilated cervix. Some of the tissues may be seen in the vagina during examination or the patient may give a history of passing some products. Ultrasonography will confirm the presence of some products of conception still retained within the uterus.

Complete Abortion

In a complete abortion, all the products of conception have been expelled from the uterus. In the initial stage of the abortion or miscarriage, there is severe abdominal pain and vaginal bleeding, but after the passage of all the products, the pain and bleeding diminishes. On examination, the os will be found to be closed. Ultrasonography will show an empty uterus.

Missed Abortion

This is a type of abortion where the fetus is dead but the products of conception are retained within the uterus. There may be no symptoms at all or a brownish vaginal discharge or brown vaginal bleeding may occasionally be seen. Pain is unlikely and os will be closed.

Since no symptoms may be present, the patient is usually unaware that a miscarriage has occurred. Loss of pregnancy symptoms following a period of amenorrhea (cessation of periods), a decrease in size of the uterus, or inability to detect a fetal heart beat at the expected stage of pregnancy may raise a suspicion of missed abortion, which can be confirmed by ultrasonography.

Septic or Infected Abortion

Infection within the uterus or the products of conception (usually a bacterial infection) may occur if any tissue from the placenta or fetus remains in the uterus after an abortion. Fever, chills, foul-smelling vaginal discharge, persistent vaginal bleeding, abdominal cramping, and pelvic pain or discomfort are common findings.

Recurrent or Habitual Abortion

Sometimes, a woman may have miscarriages in successive pregnancies. When three consecutive pregnancies end in spontaneous abortion, it is known as habitual or recurrent abortion.


Causes of Spontaneous Abortion

There are certain causes, as well as risk factors, which may lead to a spontaneous abortion.

Chromosomal Abnormalities

Chromosomal abnormalities account for a large proportion of abortions in the first trimester of pregnancy. Most spontaneous abortions are associated with abnormal products of conception and may occur even before the diagnosis of pregnancy is made. Some are lost before implantation of the fertilized ovum into the uterine wall (first week of gestation), some during implantation (second week of gestation), while others after the first missed period.

  • Chromosomal abnormalities are less common in second trimester abortions.
  • Chromosomal abnormalities may not be associated with defective maternal or paternal genes, but in recurrent spontaneous abortions genetic abnormalities should be excluded.
  • A blighted ovum is caused by chromosomal abnormality where there is a gestational (pregnancy) sac but no fetus within it.
  • In molar pregnancy, the uterus is filled with abnormal tissue instead of a fetus.

Advancing Maternal Age

Maternal age over 35 is associated with more risk of chromosomal abnormalities and first trimester abortion. It has been seen in miscarriages occurring after conceiving with donor eggs that the age of the donor is the deciding factor in causing abortion and not the age of the recipient. An aging oocyte or egg is more likely to be the cause of miscarriage.

  • Advanced paternal age and extremes of age.
  • Unknown cause. In a large number of cases, no cause can be found.

Endocrine or hormonal factors.

Hormonal factors may play a part in increasing risk of miscarriage, in conditions such as :

  • Cushing’s syndrome
  • Polycystic ovarian syndrome (PCOS)
  • Thyroid disorders

Luteal phase defect – this may be related to failure of corpus luteum to produce adequate amount of progesterone to maintain the early stage of pregnancy. However, luteal phase defect as a cause of miscarriage has not been conclusively proven.

Maternal Systemic Disease

Uncontrolled diabetes or severe hypertension may increase the risk of a spontaneous abortion.


Maternal, fetal or placental infection with certain organisms often leads to a miscarriage :

  • Toxoplasma gondii
  • Listeria monocytogenes
  • Parovirus B19
  • Cytomegalovirus (CMV)
  • Herpes simplex

Rubella (German measles) and malaria in pregnancy are associated with higher risk of miscarriage.

Immunological factors

Collagen vascular diseases such as systemic lupus erythematosus (SLE) and antiphospholipid antibody syndrome may cause miscarriage by making antibodies against a person’s own tissues.

Anatomic or structural defects of the reproductive system

Anatomic defects or abnormalities within the uterus, such as a septate uterus (a septum or wall of tissue dividing the uterus), bicornuate uterus, or a fibroid (a non-cancerous tumor of the uterus) may interfere with implantation of the fertilized ovum within the uterus and lead to a miscarriage.

Cervical incompetence or cervical insufficiency is painless cervical dilation, as a result of weakness of the cervical tissue, which can become the cause of early second trimester miscarriage.

Asherman’s syndrome – vigorous curetting or several dilatation and curettage (D&C) procedures sometimes leads to adhesions within the uterine cavity. It may cause repeated miscarriages.

Trauma or injury

  • Direct injury to the pregnant uterus by penetrating wounds or by the steering wheel of a car in a road traffic accident.
  • A blow or a fall possibly as a result of physical abuse.
  • Indirect injury, such as surgical trauma may occur by removal of an ovary containing corpus luteum of pregnancy or during an appendectomy.
  • An electric shock of high voltage.

Invasive Procedures

Prenatal genetic test procedures such as amniocentesis or chorionic villus sampling may increase risk of miscarriage.

Ectopic pregnancy

Implantation of the fertilized ovum in some site other than the uterus (usually the fallopian tube) will ultimately lead to early pregnancy loss.

Other Causes

  • Cigarette smoking, drug and alcohol abuse. Caffeine in high doses has been implicated too as a risk factor for causing spontaneous abortion.
  • Stress.
  • History of more than one spontaneous abortion.
  • Multiple pregnancies such as twins or triplets or more.

Dangers of a Miscarriage

Emotional Aspect

In any type of miscarriage, grief can lead to depression. Feelings of guilt are common where a woman may feel that the abortion may somehow be her fault. This can lead to anxiety and stress disorders, especially in case of recurrent abortions. Psychotherapy is important especially in women who are severely depressed. Identifying the cause of the miscarriage and reassuring the patient that a future pregnancy is possible is an important consideration for the partner, counselor and doctor.

Complete Abortion

A complete abortion does not usually cause any further problems. Bleeding is generally not severe and gradually diminishes once all the products of conception have been expelled. Infection is a rare possibility.

Threatened Abortion

A threatened abortion in some women may lead to a complete abortion, even after adequate rest and other management. There is also an increased risk of preterm delivery.

Incomplete and Inevitable Abortion

  • Some products of conception may be retained within the uterus following spontaneous abortion or after a suction D&C. This may lead to an infected or septic abortion.
  • Asherman’s syndrome is a complication which may occur following a vigorous curetting or repeat curetting. Adhesions are formed within the uterus and in extreme cases, scar tissue fills up the uterus almost completely. Asherman’s syndrome could be the cause for infertility or miscarriage in the future and usually presents as amenorrhea or decreased flow during periods.
  • Perforation of the uterus may occur following D&C since a pregnant uterus is softer than normal uterus (a surgeon may often feel it as a sudden loss of resistance while performing a D&C procedure). If the perforation is very small it may heal on its own.
  • Excessive and uncontrolled bleeding may lead to hypovolemic shock or disseminated intravascular coagulopathy (DIC). These are serious conditions which need immediate medical attention.
  • Wrong diagnosis could be dangerous. There may be an ectopic pregnancy where passage of a decidual cast may be mistaken for products of conception.
  • Complications of anesthesia while performing a D&C procedure.
  • Injury to cervix and vagina during the procedure.
  • Bowel and bladder injury.
  • Death may occur in extreme cases.

Septic Abortion

  • Bleeding.
  • Infection.
  • Peritonitis.
  • Pelvic inflammatory disease (PID).
  • Septic shock.
  • Inferior vena cava thrombosis.
  • Death.


  1. Early Pregnancy Loss. Medscape

Please note that any information or feedback on this website is not intended to replace a consultation with a health care professional and will not constitute a medical diagnosis. By using this website and the comment service you agree to abide by the comment terms and conditions as outlined on this page