Premature birth risk for those with endometriosis

In February, 2009, I shared an article stating that just having endometriosis doubles one’s risk for delivering a premature baby.

Another study has come out in July 2009 showing similar results after having studied a much larger sample of women.

Endometriosis and birth problems
Friday July 3 2009
NHS Choices UK

Women with a common womb condition are at risk of giving birth prematurely, The Daily Telegraph has reported. It says that research on more than 13,000 women with endometriosis shows that having the condition raised their risk of premature birth by around one-third. Endometriosis is a condition where the tissue lining the womb is found in other areas of the abdomen, often causing pain and infertility.

This was a large and well-conducted study that looked at almost 1.5 million births, comparing pregnancy and delivery outcomes in women diagnosed with endometriosis to those in women without the condition. Women with endometriosis were found to have an increased risk of premature birth as well as several other pregnancy-related complications, even after the influence of other factors believed to contribute to premature birth were taken into account.

As the authors acknowledge, a possible source of error in their study is inaccuracy in the number of endometriosis cases, which were taken from hospital records only. Overall, this study’s findings highlight the importance of providing all pregnant women with appropriate support, care and monitoring. It is also another reminder that pregnant women or those trying to conceive should stay as healthy as possible, for example by eating well, being active, not smoking and avoiding alcohol.

Where did the story come from?

Olof Stephansson and colleagues from Karolinska University Hospital and Institute in Stockholm, Sweden, carried out this research. The study was funded by the Swedish Society of Medicine and was published in the peer-reviewed medical journal Human Reproduction.

What kind of scientific study was this?

This was a retrospective cohort study that compared the rates of premature birth and complications of pregnancy among women previously diagnosed with endometriosis and women who did not have the condition.

Endometriosis is a relatively common gynaecological condition where endometrial tissue (the tissue that lines the uterus) is found outside the uterus and builds up in other areas around the pelvis and abdomen. It often causes painful periods, pain during intercourse and fertility problems.

The Swedish Medical Birth Register was used to identify single babies born between January 1992 and December 2006 (1,442,675 in total). Researchers linked the mothers of these babies to the Patient Register to identify those women who had been diagnosed with endometriosis at any time since 1964. This gave them a total of 13,090 single babies born during the 14-year period to 8,922 women diagnosed with endometriosis.

The authors used the Swedish Birth Register to obtain demographic data on the women and information on their age, BMI, smoking status and reproductive history (for example, previous children and use of fertility treatment). They also looked at the incidence of several complications during pregnancy, delivery and the period following birth, including premature birth, small-for-gestational-age (SGA), stillbirth, haemorrhage prior to birth, pre-eclampsia and the need for a caesarean section.

What were the results of the study?

Of the total 1,442,675 single births, 4,778 were stillborn (a rate of 3.3 per 1,000 births). Of the remaining live births, 71,689 were premature (born at less than 37 weeks), 883 of which were born to women with endometriosis (a rate of 6.78 per 100 births) and 70,806 to women without the condition (4.98 per 100 births).

Compared to women without the condition, those with endometriosis were of higher maternal age and were more likely to be having their first baby. After adjustment for age and other possible confounders (for example, BMI and smoking status), there was a higher risk of premature birth among women with endometriosis (odds ratio 1.33, 95% confidence interval 1.23 to 1.44). There was also a higher rate of premature birth among:

• women of low age (19 or under),
• women of high maternal age (35 or older),
• women with minimal education,
• women with high or low BMI,
• women who smoked, and
• women having their first baby.

Women with endometriosis also had a higher risk of pre-eclampsia, haemorrhage prior to birth and caesarean section. Fertility treatment had been used more often in women with endometriosis (11.9% against 1.4% in women without the condition). Risk of premature birth was increased among women with endometriosis regardless of whether they had used fertility treatment.

What interpretations did the researchers draw from these results?

The researchers concluded that endometriosis seems to be a risk factor for premature birth. Women with endometriosis may also be more likely to suffer from haemorrhage prior to birth or placental complications, to develop pre-eclampsia or to require a caesarean section.

What does the NHS Knowledge Service make of this study?

This was a large and well-conducted study that looked at almost 1.5 million Swedish births over 14 years. A diagnosis of endometriosis appeared to increase the risk of premature birth as well as several other pregnancy-related complications, even after adjustment for possible confounding factors, such as maternal age, BMI, smoking status and previous births.

Women living with endometriosis can experience great physical and emotional distress, particularly considering the difficulties that many experience when trying to conceive. The findings that other pregnancy-related complications are possibly associated with endometriosis may not come as a surprise to women with this challenging condition or to the medical professionals who treat it. As part of their review, the authors discussed the possible inflammatory and biochemical changes associated with birth and endometriosis and proposed factors that may be involved in the association between the two.

However, the authors acknowledge that there is a potential source of error in their results as diagnoses of endometriosis were only taken from hospital or outpatient records. It is not known whether these were confirmed using biopsies and lab testing. There is also the possibility that a number of women with endometriosis may not be referred to hospital with their condition or may put up with symptoms and avoid consulting a doctor. Therefore, the number of cases of endometriosis in this population sample may not be entirely accurate.

It should be noted that premature birth was also independently associated with other risk factors, such as being a smoker, having a low (underweight) or high (obese) BMI, or being of high or low maternal age. The findings highlight the important need for pregnant women to receive dedicated support, care and monitoring and to maintain optimal health, for example through taking appropriate vitamin supplements, eating well, remaining active, not smoking and avoiding alcohol.