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What causes premature birth?

Risks of premature birth

Approximately 40,000 babies are admitted to Neonatal Intensive Care Units (NICUs) or Special Care Nurseries (SCNs) each year for a variety of reasons. Approximately 21,000 of these babies are born prematurely. About 6000 babies require critical and intensive life support each year.

So what causes premature birth and what are the major risk factors for experiencing a premature birth?

What is premature birth?

A normal pregnancy lasts for approximately 40 weeks. Premature or preterm birth is defined medically as a baby being born earlier than 37 weeks of pregnancy.

The major risk factors for premature birth

Many causes of pre-term birth are unexplained and unknown, however there are many risk factors that increase the chances of babies being born early.

There are many different factors, medical and otherwise that may make some women more high risk.  Here are some of the major risk factors for premature birth:


Approximately half of all twin pregnancies end in pre-term delivery and almost all higher order multiples are born prematurely. Multiple pregnancy can also result in TTTS (Twin to Twin Transfusion Syndrome).

Previous premature birth

Women who have had a premature delivery previously have an increased risk of having another pre-term delivery.

Short period between pregnancies

A period of less than six to nine months between the birth of one baby and the start of the next pregnancy increases the risk of premature delivery.


Preeclampsia is the development of elevated blood pressure and protein in the urine after the 20th week of pregnancy. It may be associated with swelling of the face and hands. Symptoms can include oedema (swelling of face and hands), high blood pressure, vision disturbances, headaches, nausea and vomiting. This condition can lead to serious complications for the mother and baby including premature birth. The only known cure is delivery of the baby.

HELLP Syndrome

Hemolysis, Elevated Liver enzymes, Low Platelet count.  HELLP is a serious complication related to high blood-pressure during pregnancy. Red blood cells start to break down causing anemia, liver cells are damaged and this affects liver function, low platelet count which affects the ability of blood to clot. Symptoms may include a general unwell feeling and/or feeling weak, increased blood pressure, protein in urine, abdominal pain, swelling, headaches, nausea and vomiting.


There are many well-known risks associated with smoking during pregnancy, including low birth weight babies and premature deliveries.

Uterine or cervical abnormalities

This includes stretching or abnormally shaped uterus or cervix, as well as fibroids or even having too much or too little amniotic fluid. Women with an incompetent cervix may require a procedure known as a cerclage. Performed at about 12 weeks gestation, this procedure involves placing a stitch in the cervix to prevent it from opening up too soon. The stitch is then removed when the baby is closer to full term.

Recurring infections

Infections affecting the vagina, kidneys, bladder and urinary tract can all lead to pre-term birth. Also, any infection with a high fever of more than 38 degrees during pregnancy can lead to an early birth.

Chronic illnesses

High blood pressure, diabetes, inflammatory bowel disease (IBS), kidney disease or lupus.


History of second-trimester abortion or more than two first-trimester abortions. The cervix may be damaged during these procedures.

Mother’s age

Younger than 18 or older than 40 years.

Clotting disorder


Alcohol and drug abuse


Risk may increase if your mother or her mother experienced premature births.


Preterm Premature Rupture of Membranes.  This is where the amniotic sac ruptures/breaks before the onset of labour. Many causes of PPROM are infections but some of the causes remain unknown or unclear.

Placental risks & causes

Placenta Previa and Placental Abruption.  Placenta Previa  is where the placenta is obstructing the cervix. The placenta may be completely covering the cervix, partially covering the cervix or sitting close to the edge of the cervix. Placenta Previa can cause bleeding, and an increased risk of slow fetal growth, preterm birth and birth defects. There is no treatment for Placenta Previa, however steps can be taken to reduce the health risks to mother a baby. This can include bed rest, steroid injections to strengthen baby’s lungs in readiness for pre-term birth, and caesarian delivery. Placental Abruption is defined as the premature separation of the placenta from the uterus. This can cause minor bleeding or serious haemorrhage, depending on the degree and location of the separation. Direct trauma to the uterus is one known cause and other risk factors may be smoking, high blood pressure and multiple pregnancy. Symptoms can include vaginal bleeding, feeling faint, nausea and abdominal pain.

The material provided here is for informational purposes only and should not replace, or be used as a substitute for, professional medical advice.


Visit our Pregnancy and Birth Information Hub and  Premmie Baby Information Hub for more articles, support and resources.


– this article has been written with information provided by representatives of the National Premmie Foundation

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