What Causes Miscarriage?

By Sally Robertson, B.Sc.Reviewed by Yolanda Smith, B.Pharm.

A miscarriage is the loss of a pregnancy before 24 weeks, when most developing babies would be unable to survive outside of the uterus.

Miscarriage is common, occurring in about 25% of pregnancies, usually in the first 12 weeks of pregnancy. Some women experience recurrent miscarriage, which is the loss of three or more pregnancies one after the other. However, this is uncommon and only happens to around 1% of couples.

The main sign that a woman is experiencing a miscarriage is vaginal bleeding, possibly followed by pain and cramping in the lower abdominal area. However, it is relatively common for vaginal bleeding to occur during the first trimester of pregnancy and does not always mean a miscarriage is occurring.

In many cases, no cause of the miscarriage can be established and a great deal remains unknown about why this happens. Therefore, many women who miscarry never find out why they lost their baby, even after investigations have been carried out.

Miscarriage in the first trimester

A first trimester miscarriage is one that occurs during the first 12 weeks of pregnancy. If a miscarriage occurs during this period, it is usually caused by an a developmental abnormality due to a problem with the chromosomes of the fetus. Around three-quarters of miscarriages occur during the first trimester.

Chromosomes are the blocks of deoxyribonucleic acid (DNA) that carry the instructions that control the baby’s development and determine the baby’s individual features. Sometimes, a problem can occur during conception and the fetus then receives either too many or too few chromosomes. The fetus is then unable to develop normally and the pregnancy ends in a miscarriage. This problem is unlikely to recur and it does not mean that the mother or father has faulty chromosomes.

Another factor that may lead to miscarriage during the first trimester is a problem with the placenta. The placenta is the structure that connects the mother’s blood supply to that of her baby’s and problems with the development of this organ can lead to a miscarriage.

Risk factors

There are several factors that increase the risk of miscarriage during the first trimester and these include the following

• The age of the mother – among women aged under 30 years, miscarriage occurs in around 10% of cases, while among women aged between 35 and 39 years, the proportion is 20% and for women older than 45 years, it is more than 50%.
• Caffeine – Drinking more than 200 mg caffeine per day can increase the risk of miscarriage
• Alcohol – Drinking more than two units of alcohol per week also increases the risk
• Obesity
• Smoking during pregnancy
• Drug misuse during pregnancy

Miscarriage during the second trimester

A second trimester miscarriage is one that occurs during week 14 and week 26 of a pregnancy and is sometimes caused by a health problem with the mother or by infection that has affected the baby. Rarely, it is caused by the cervix dilating too early on in the pregnancy.

Long-term underlying health conditions that can increase the risk of miscarriage during the second trimester include the following:

• Diabetes
• Kidney disease
• Severe hypertension
• Overactive or underactive thyroid
• Lupus
• Celiac disease

Examples of infections that may increase the risk include the following:

• Cytomegalovirus
• Bacterial vaginosis
• Rubella
• Chlamydia
• Gonorrhoea
• Malaria
• Syphilis
• HIV

Other factors that may lead to a miscarriage during the second trimester include food poisoning, the use of certain medications and abnormal womb structure.

Food poisoning

Food poisoning as a result of eating contaminated food can increase the risk of miscarriage. Examples include salmonella, caused by eating raw or partially cooked eggs; toxoplasmosis, caused by eating undercooked infected meat, especially pork, venison or lamb and listeriosis, caused by consuming unpasteurised dairy items such as stilton. Expectant mothers should avoid eating these foods.

Medications

Medications that increase miscarriage risk during this period include retinoids, misoprostol, methotrexate and non-steroidal anti-inflammatory drugs. Mums-to-be should always check with their doctor, pharmacist or midwife before taking any medication.

Problems with the womb

Abnormalities of the womb can increase the risk of second trimester miscarriage and examples include an irregularly shaped womb that has restricted space and growths in the womb called fibroids.

Recurrent miscarriage

Often, when a woman experiences a miscarriage, she worries that she will go on to miscarry if she becomes pregnant again. However, most miscarriages are a one-off event. In around 1% of cases, women experience recurrent miscarriage and over 60% of those women do eventually go on to carry a pregnancy to full term.

Further Reading

• All Pregnancy Content
• Early Signs of Pregnancy
• Is it Safe to Exercise During Pregnancy?
• Pregnancy: 0-8 weeks
• Pregnancy: 9 - 12 weeks