Preterm birth is a daunting thought for expectant parents. Dr Martin Sowter explains why it happens, what to expect if you’re at risk, and reassures that help is at hand.
Going into labour before your due date can be a real inconvenience, with work tasks unfinished or anticipated time off not enjoyed, but women who go into preterm birth can have much more serious concerns. They are often overwhelmed with worry about the long-term health of their baby or, if very premature, uncertainties that their baby will even survive.
Preterm birth is defined as giving birth before 37 completed weeks of pregnancy. In 2012, eight per cent of New Zealand babies (just over 4,700 babies) were born before 37 weeks, and 800 babies were born very prematurely at less than 32 weeks.
Is there a cause?
There are a multitude of possible causes for premature delivery. Most risk factors only slightly increase the chances of delivering early and, for most women, no definite reason can be found for why their labour began before it should have. Being pregnant in your teens or in your forties, conceiving after a long period of infertility, smoking and even challenging social circumstances can all increase your risk, yet most women in these situations will still deliver at a normal time. The most important risk factor to consider is a history of previous preterm birth, with about 30% of women with a history of one preterm birth having another preterm baby. This statistic rises to over 50% if she has had two preterm births.
Treatments for pre-cancerous cells of the cervix can also cause concern for triggering early labour. Older treatments, where a knife was used to cut out pre-cancerous areas, have long been known to increase the risk of preterm birth, but even modern treatments have some risk of triggering delivery. While it is uncertain how significant this risk really is, recent studies suggest that when larger areas of tissue are removed, the risk of preterm delivery may be doubled. However, if a woman has only had small amounts of tissue removed, the risk of preterm delivery is much the same as for women who have had no treatment. This means that, for most women, their doctor can reassure them that their risk of preterm delivery is still low, although an assessment can only be made if it is possible to find out details of how much tissue was removed as part of that woman’s treatment.
Friends and family may advise women to avoid stress, thinking that it may increase the risk of them delivering early or that it is just somehow going to be ‘bad for baby’.
While this is good advice, it is also important not to ‘worry about being worried'. There is a modest link between stress and pre-mature labour, with high levels of stress possibly increasing the risk of preterm delivery by about a third (so still maybe only a 10 to 15% risk). However, to quote the conclusions of one very large American study, “the effects of maternal stress on risk of preterm birth may, for the most part, vary as a function of context”. Which is a medical way of saying that stress is associated with a range of other social, economic and environmental risk factors for preterm birth. Trying to reduce some of those stressors in your pregnancy will be more effective in reducing your risk of preterm delivery than trying not to ‘stress about your stress’.
Exercise in pregnancy is often mentioned as a risk factor for preterm birth, but there is very little evidence to show that exercising regularly in pregnancy will negatively impact women with no other risk factors – in fact, many studies suggest that regular exercise in pregnancy reduces your risk of preterm delivery, although this may be because women who exercise regularly are more likely to be looking after all aspects of their health, diet and lifestyle.
Another piece of advice you may be given is that enjoying sex during pregnancy increases the risk of preterm birth, but there is really no evidence at all to say that this is true. Even for women with proven risk factors, it is difficult to unravel all the evidence to say whether or not abstaining from sex definitely reduces the risk of an early delivery. To borrow from the rather dry words of the Canadian guidelines for managing women with an increased risk of preterm birth: “there is no evidence to suggest a clear benefit from restricted sexual activity; however, this is a simple intervention that causes no harm and may be a reasonable recommendation until better evidence emerges”. This means that, if you are at risk, your LMC will probably advise you to not have intercourse until after the baby is born.
Treatments and prevention
Premature birth has a huge social and economic cost. Looking after those 800 New Zealand babies born before 32 weeks every year costs tens of millions of dollars for their immediate healthcare alone, and many millions more in lost income and on-going care costs for each baby’s family. Research into new treatments and drugs that might prevent preterm birth is being undertaken in many countries, but at the moment, only two interventions are commonly used. These are hormone therapy with progesterone and cervical cerclage (putting a stitch into the cervix). Progesterone therapy has not been widely used in New Zealand and it is uncertain how it actually works to reduce the risk of preterm labour. It is thought to inhibit the effects of some of the hormones that initiate labour and a number of studies suggest it may be effective for some women in reducing their risk of premature delivery. Women who have had a previous preterm delivery, or women expecting their first baby but with signs of a shortened cervix on ultrasound scanning, are likely to lower their risk if they start using progesterone from early in the second trimester. There’s little evidence that progesterone reduces the risk of women with twins delivering early, and it is uncertain whether women who have been admitted to hospital with an episode of threatened preterm labour would benefit from the use of progesterone. The dosage and administration of progesterone is also uncertain – some countries use weekly intramuscular injections and others use daily vaginal pessaries. There are a number of on-going studies that are attempting to answer these uncertainties and it is likely that progesterone therapy is going to be more widely used in New Zealand for women at risk of preterm delivery.
Cervical cerclage, or a cervical suture, is widely used in New Zealand. It appears to be an effective treatment for women who have previously had two or more very preterm deliveries or late miscarriages. However, there is uncertainty about its use in women who have had only one previous preterm birth, or who have had surgery for pre-cancerous cells in their cervix, and there is no evidence that it should be used for women expecting twins. There is a small risk of infection, or of a woman’s waters breaking prematurely with a cervical suture, so often one is only inserted after carefully weighing up how great a woman’s risk of premature delivery is. This will be based on her history and ultrasound measurements of the length of her cervix. Like progesterone therapy, exactly how a suture works is not certain – there may be some limited mechanical effect keeping the cervix closed. It may also help to keep the mucous plug in the cervix, reducing the risk of bacteria entering the cervix or inflammation within the cervix starting labour. The procedure is usually done using general anaesthetic or epidural.
What if it’s me?
If you are referred to a hospital with threatened preterm labour, you may be given oral or intravenous medication to try to stop your contractions. This is usually only done in women without any bleeding or signs of infection, in the hope that labour can be prevented for long enough to give a 24-hour course of steroids. Steroid injections are used to help reduce the risk of a premature baby developing breathing problems in the hours and days after birth. You may also be started on antibiotics if your waters have broken prematurely.
If it weren't possible to prevent a woman from going into preterm labour, a decision would need to be made around how the baby should be born. Most women would go on to deliver vaginally, but if there was infection or bleeding, then some babies may need to be delivered quickly by caesarean section. A high proportion of breech babies are also delivered by caesarean section if a woman labours prematurely.
In recent years, many women labouring very prematurely (before 32 weeks) have been given a short infusion of magnesium sulphate to protect the baby’s brain. Studies suggest that there is some benefit in giving magnesium sulphate to women labouring at 32 to 34 weeks, so this may be discussed with you. Medical staff may refer to this treatment as neuroprotection. Magnesium sulphate has been widely used to treat women with pre-eclampsia and it is a safe drug when it is administered in a hospital setting.
There is also a lot of research being undertaken that will help us better understand when to use the treatments that we currently have, and to look at other drugs, particularly anti-oxidants, that might be given to a woman in premature labour to protect baby or to help baby cope. There are also studies underway to assess new treatments that could prevent preterm birth, but we are still years away from having any completely effective ways of preventing preterm birth entirely.
One exciting new development, however, is that hospitals in larger countries are setting up Premature Birth Clinics. These clinics have been established to provide women with an expert assessment and to explain the risks and treatments available.
So far in New Zealand, only National Women’s in Auckland has a well-established clinic, but referrals are accepted from LMCs across the country. These clinics are likely to be a very cost-effective solution and hopefully, with time, all of New Zealand’s larger hospitals will be able to offer a Premature Birth Clinic to parents facing the early arrival of their precious babies.
Martin Sowter (BSc MB ChB MD FRCOG FRANZCOG) is a part-time specialist at Auckland City Hospital in the fertility and menstrual disorders clinics. He also works privately as part of the specialist team at The Auckland Obstetric Centre. Over the last six years he has also been a regular visitor to the Cook Islands under the auspices of NZAID, helping the local specialists with gynaecological surgery. Martin is married to Alison, an eye surgeon, and has two daughters.