A spontaneous abortion or 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.
Causes of Spontaneous Abortion
There are certain causes, as well as risk factors, which may lead to a spontaneous abortion.
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.
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.
Prenatal genetic test procedures such as amniocentesis or chorionic villus sampling may increase risk of miscarriage.
Implantation of the fertilized ovum in some site other than the uterus (usually the fallopian tube) will ultimately lead to early pregnancy loss.
- Cigarette smoking, drug and alcohol abuse. Caffeine in high doses has been implicated too as a risk factor for causing spontaneous abortion.
- History of more than one spontaneous abortion.
- Multiple pregnancies such as twins or triplets or more.