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Preterm Birth: Part 1 – Who is at risk?

Pregnancy Dr Minke Burke Director
Preterm Birth is by its nature, unexpected. You haven’t got the babies name sorted, your bag isn’t packed yet and you may not have been to your antenatal classes. Most of all, you worry about your baby, will he or she be all right, it’s too early! To answer the question “why is this happening to me?”…

What is preterm birth?

Preterm birth (PTB) is defined as delivery of your baby prior to 37 weeks gestation. Unfortunately, the majority of the time we cannot explain the mechanism for why PTB occurs.  In about 70% of cases, women break their waters prematurely or start to have contractions. The other 30% have a (somewhat) planned delivery which is indicated because of either concerns to the baby (like growth restriction) or mum (like severe preeclampsia, a pregnancy condition of high blood pressure and other abnormalities).

One of the frustrating things about PTB, is that the percentage of women who have their baby early has remained stable for many years despite attempts to predict and reduce this number.  Outcomes for babies have significantly improved but that is mainly because of advances in neonatal intensive care and not because of any change in antenatal care.  The PTB paradigm is keeping scientists, researchers and clinicians searching for ways to predict, manage and above all prevent PTB.

The role of the cervix

In our recent blog “The Cervix” we learned what features have put the cervix on the map for gynaecologists.  In addition to this, it’s the ‘Extracellular Matrix’ (ECM) that places the cervix on our obstetric map.  The cervix mainly consists of collagen (80%), water, and other “filler” proteins (like proteoglycans, hyaluronans and elastin).  It’s the ECM that gives the cervix its mechanical properties and the complex architecture of the collagen strands determines its strength.  A change in organisation and architecture (‘remodelling’) of the collagen strands is what leads to cervical softening and ripening in the labour process.

What initiates this remodelling of the ECM, and are these factors also responsible for PTB?  There are 4 discrete pathways that individually lead to cervical shortening, softening and eventually ripening.  These are very complex pathways that involve factors and hormones from mum, placenta and your baby. The pathways include:

  1. Placental Corticotropin Releasing Hormone production (CRH) activating a hormonal response in the mother;
  2. The inflammatory and infection pathway (interestingly, inflammation can occur without infection, but infection is a major component in premature labour);
  3. Bleeding;
  4. Uterine distension (for example in women with twins or multiples or an excess of amniotic fluid called polyhydramnios).

These pathways have their own unique epidemiological, genetic, and clinical characteristics but are not mutually exclusive.

The inflammatory pathway has received a lot of attention. Bacteria can dissolve the fetal membranes by producing proteins. Some bacteria are more harmful than others (like bacterial vaginosis and chlamydia). The detection of bacteria can be difficult as current culture media are not always sufficient. Another contributing factor could be that the mother’s immune response may not be sufficient to manage an altered bacterial environment. The effects on the cervix from inflammation are usually slow, occurring over many weeks and not involving uterine contractions.

In addition to the above factors, the cervix may consist of a weak ECM or connective tissue which is the case in conditions like Ehlers-Danlos syndrome.  Cervical weakness due to intrinsic factors is rare.

Consequences of PTB

PTB (that is prior to 37 weeks gestation) affects 7% of women in Australia. Of this group, the babies that are born under 34 weeks pose the biggest concerns in relation to their health and survival. The risks to the baby include death, infection, and breathing and feeding problems, often requiring intensive care admission for several weeks. To you this means being separated from your little one, increasing your risk of post partum depression and putting your family through stress which can cause relationship problems. To our society PTB comes at an enormous cost due to inpatient stays in the intensive care as well as ongoing health issues for these babies as children.

Are you at risk?

If you have had a preterm delivery previously, there is an increased risk of it happening again in your subsequent pregnancies. Risk factors for PTB are divided into modifiable and non-modifiable risk factors. Non-modifiable risk factors are things we unfortunately cannot change or influence. Examples include previous PTB, maternal age under 18 or over 40, low BMI (Body Mass Index), low social support or no antenatal care, uterine abnormalities, previous cervical surgery, certain connective tissue disorders (like Ehlers-Danlos) and uterine over distension (like carrying twins).

Potential modifiable factors that are associated with PTB are genital tract infections like Bacterial Vaginosis (BV), Group B streptococcus (GBS) and chlamydia, smoking, high maternal stress, chronic medical conditions (like kidney disease, auto immune conditions) and anaemia.

Of all maternal history risk factors, previous PTB is the most important.  With one previous preterm birth your risk of subsequent PTB increases by 15-30%.  Having had 2 or more increases this risk to 60%.

A large study in 1996 revealed that if you have a short cervix, which is defined as 25 mm or less, you have a 6-14 times increased risk of PTB.  Since then, research of the short cervix and associated management options became a ‘hot topic’ in the academic world.  In 2012 several studies concluded that intervention with progesterone reduces the risk of PTB in women with a short cervix. Based on these findings it was recommended to consider universal screening of cervical length in pregnancy.

Traditionally during your morphology ultrasound the cervix was assessed trans-abdominally, with the ultrasound probe on your belly. It is very difficult to observe the cervix properly in this way as your full bladder will stretch the cervix which will give an over measurement. If your bladder is empty, the cervix can barely be seen. Women with a short cervix may be missed when you scan trans-abdominally only.

Universal cervical length screening was implemented in Australia in 2013.  As part of your morphology ultrasound the sonographer will perform an internal scan (which will take a few minutes) to observe your cervix over time and measure its length. The cervix can be seen close up and this is a very reliable measurement. A cervical length measurement over 25 mm is associated with a low risk of PTB, and in general does not require any follow up assessments.

If during your antenatal booking visit risk factors are identified for PTB, like a previous birth prior to 37 weeks, it is worthwhile to commence cervical length screening earlier. A measurement at 16 weeks followed by a measurement at your morphology scan will help determine if the cervical length is stable or if there is a trend downwards. If the cervix is short, treatment can be considered.

Treatment options include vaginal progesterone or insertion of a cervical suture (cerclage).  We will discuss these options in more detail in our next blog “Preterm Birth: Part 2 – What can we do?”

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