What are the chances of having a miscarriage?

By Dr. Derrick Thompson (Obstetrician & Gynaecologist)

I find that all women who become pregnant are concerned that they could suffer a miscarriage. You are most unlikely to suffer a miscarriage if you see on the ultrasound image:

  • a live intrauterine pregnancy
  • the size of the fetus corresponds to the number of weeks you are pregnant
  • a regular fetal heart beat of 110-180 beats per minute
  • the gestation (pregnancy) sac corresponds to your dates ans is of regular shape
  • an intact yolk sac

I recommend that at the first visit to your doctor, preferable at about 6-8 weeks gestation, you should have an ultrasound examination.  An ultrasound involves high frequency sound waves being passed into the uterus.  The reflected sound echoes are then analysed to build up a picture of your baby within the uterus.

At this early stage of pregnancy its best to have a vaginal ultrasound as opposed to a abdominal ultrasound.  By having a vaginal ultrasound, you  are able to see the fetus more clearly, and you avoid the need to have a full bladder.  You are less likely to experience discomfort if you take a deep breath as the vaginal probe is being inserted.

In my experience, the risk of having a miscarriage (following a normal result from the ultrasound examination) is only about 2%, and certainly not more than 5%.  Therefore, it should be very reassuring that 95-98% of the time you will not have a miscarriage.

An ultrasound scan can identify 4 conditions that will result in a miscarriage:

  • blighted ovum or silent miscarriage
  • missed abortion or missed miscarriage
  • ectopic pregnancy
  • hydatidiform mole

Blighted ovum or silent miscarriage.

A blighted ovum is diagnosed when the ultrasound scan shows an empty pregnancy sac within the uterus.  The fetus has not developed, and there is no fetal pole.  The pregnancy sac and the placenta have developed, but the fetus has not developed past the embryonic stage.

If the diameter of the sac is equal to or greater than 2cm, a fetal pole with the heart beat present has to be identified for your to have an ongoing pregnancy.

Missed miscarriage (missed abortion).

A missed miscarriage is diagnosed when a gestation sac is present and a fetal pole is seen, but the fetal heartbeat cannot be identified in the area of the body of the fetus.  In a normal pregnancy at 6-8 weeks gestation, the fetal pole should measure about 4-17 mm long respectively.  the fetal heartbeat is normally seen as a flicker in this band of tissue, at a rate of 110-180 beats per minute.

Ectopic pregnancy

An ectopic pregnancy occurs when the pregnancy develops outside the uterine cavity.  The chance of this happening is about one in every 150-250 pregnancies.  Most commonly, the pregnancy will develop in a Fallopian tube, but sometimes this happens on the surface of an ovary, the cornu of the uterus or rarely in the cervix.  An ultrasound scan will show an empty uterine cavity and a swelling in one of the above sites.  The swelling consists of a blood clot and placental tissue, and a live fetus is rarely seen.  Occasionally, there may be a live fetus in a gestation sac.

Management of an ectopic pregnancy depends on its size, hormone production and symptoms; in other words, the amount of pain or bleeding the woman might be experiencing.    Your carer might suspect an ectopic pregnancy if it has been confirmed that you are pregnant, but you then develop lower abdominal crampy pains on one side and you then begin to bleed.  This is in contrast to the usual form of uterine miscarriage when the first symptom is vaginal bleeding, followed by lower abdominal crampy pains.

If an ectopic pregnancy is diagnosed early, a doctor can manage it by giving an injection of a chemotherapeutic substance called methotrexate.  This chemical affects the dividing and multiplying cells of the pregnancy and, in effect, dissolves the pregnancy.

For an ectopic pregnancy that is more advanced, the usual treatment is to remove the Fallopian tube containing the pregnancy by laparoscopic key-hole surgery known as laparoscopic salpingectomy. The gynaecologist or obstetrician will decide how best to treat you.  Occasionally the ectopic pregnancy can burst through the wall of the Fallopian tube and cause severe internal bleeding.  symptoms that this may have occurred are shoulder-tip pain or feeling that you are going to faint.  You must go to your nearest hospital immediately.

In summary

Miscarriage is very unlikely if your 6-8 weeks ultrasound scan confirms a healthy intrauterine pregnancy.  That is the fetus:

  • is correct size for your dates
  • has a normal fetal heart rate (110-180 bpm)
  • has  a regular, normal sized gestation sax with a normal amount of amniotic fluid around the yolk sac or fetal pole
  • is unaffected by a maternal uterine abnormality, for example, uterine septum or distortion of the uterine cavity by a fibroid.
  • If an ultrasound examination is normal, then the risk of miscarriage is only about 1-2% and certainly less than 5%

Note: Lower abdominal pains or pelvic cramps alone are not a sign of threatened or imminent miscarriage.  If you notice any dark brown vaginal discharge or bright red vaginal bleeding, you should have an ultrasound scan.  Intercourse is vest avoided for one week after the bleeding has stopped.