Causes and risk factors of Prematurity

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Approximately 1 in every 10 pregnant women will have a premature labour. In Spain, the prevalence is around 6-7% but in countries like the USA more than 12% of childbirths are preterm. Despite prenatal care, these percentages have not varied over recent decades and have even increased slightly. This is caused, among other reasons, by the increase in assisted reproduction techniques, as well as the age of the pregnant women, which have consequences including a greater percentage of multiple gestations and more labours induced before week 37 due to maternal or foetal complications.

One of the main challenges in managing any pregnancy is identifying of those women with a higher risk of premature labour since, for this population, there are specific therapeutic strategies shown to be useful in reducing preterm births.

The main risk factors for premature labour are summarised below:

Pregnant woman

Women with a history of premature labour prior to week 34. This is the most important risk factor. The number of premature labours and the gestational age at which the preterm babies were born points to risk in the new pregnancy.

Pregnant woman undergoing gynaecological uterine surgery

Uterine factors. Uterine surgery, such as conisation or trachelectomy; uterine malformations, such as a septal or bicornuate uterus; myomas that deform the endometrial cavity.

Expulsion of the mucus plug in pregnancy

Complications in the current pregnancy. Multiple gestation; bleeding in the 2nd or 3rd trimester; alterations in the volume of amniotic liquid (too much or too little); risk of premature labour; premature rupture of membranes; systemic infection such as pyelonephritis, pneumonia, or appendicitis.

People of different skin tones

Women of African-American or Afro-Caribbean origin have twice the risk of a premature labour than white women.

Line graph

Nutrition defined by body mass index (BMI). Very thin women with a BMI less than 19 have a higher risk of prematurity, possibly related to the decreased uterine flow due to malnutrition. Similarly, obesity is also considered a risk factor since adipose tissue produces inflammatory proteins that trigger spontaneous delivery. In addition, these women have a greater risk of maternal complications including high blood pressure and diabetes, which can cause labour to be induced prematurely.


If the period between the birth of one child and a new pregnancy is less than 6 months, the risk of premature labour doubles.

Pregnant woman smoking

Consumption of tobacco and other toxins. Smokers have a greater risk of premature labour and delayed intrauterine foetal growth.

Person with symbols on the head representing stress, anxiety, confusion, disorientation, headache or migraine

Social factors. Lower socioeconomic and educational levels, and psychosocial factors such as stress or depression, slightly increase the risk of prematurity.

When to go to the doctor?

The clinical symptoms that warn us of the risk of premature labour are:

In addition to a physical exam, the medical consultation will involve various diagnostic procedures, including a vaginal ultrasound, cardiotocographic, or blood analysis, to rule out any complications related to prematurity.

How to avoid prematurity?

Although most of the time we do not know the ultimate cause of premature labour, there are a series of recommendations that can help reduce the risk of suffering this in a new pregnancy.

Basic infant screening

Initial evaluation in a specialist Prematurity Unit to improve the obstetric result of the pregnancy. In this way, any pregnant woman with a history of gestational loss beyond week 17 of pregnancy or a history of premature labour or premature rupture of membranes before week 34, should be referred to a specific Prematurity Unit. These women benefit from monitoring in specialised Prematurity Units with personalised management involving a series of follow-up ultrasounds, assessment of the influence of infection on prematurity, and the possibility of specific treatment according to the clinical situation (e.g., progesterone, cervical cerclage, pessary, or antibiotics).

Pregnant woman

If there is a history of prematurity, it is recommended that there is a period between childbirth and a new pregnancy of at least 12 months.

Cigarette crossed out on a "no smoking" poster

Quitting smoking and avoiding consumption of toxins before, or at any point in the pregnancy. Stopping smoking during the pregnancy reduces the risk of premature labour by up to 20%.

Blood test tube with a low red blood cell count or anemia

Correction of nutritional shortfalls and anaemia. It is very important to maintain adequate iron levels as anaemia in the 1st or 2nd trimester can increase the risk of premature labour. We suggest an iron supplement only in cases of anaemia.

Oil in an oilcan

Omega-3 fatty acid supplements. We advise a low-fat diet rich in Omega-3 fatty acids (present in olive oil and oily fish), as well as the consumption of fruits and vegetables. If there is a history of premature labour, commercial preparations containing omega-3 fatty acid supplements may be recommended.


Probiotics. The consumption of dairy products rich in probiotics could reduce the risk of premature labour due to their action on the vaginal flora and the prevention of bacterial vaginosis.

Substantiated information by:

Ana Herranz Barbero
Erika Sánchez Ortíz
Maria Teresa Cobo Cobo
Marta Arnal Ahulló
Mª Dolors Salvia Roiges

Published: 20 February 2018
Updated: 20 February 2018

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