Controversy over cervical-length screening

4 minute read

Should there be universal ultrasound screening for a short cervix, or is it a poor predictor of preterm birth?

Debate over the value of cervical-length screening for all pregnant women is heating up, after two recently published articles suggesting conflicting points of view.

On the pro side, Australian researchers are calling for universal screening, saying there is now “compelling evidence” to support it as a means to avoid the significant complication rate from preterm birth.

According to an opinion piece in the Australian and NZ Journal of Obstetrics and Gynaecology, identifying, via ultrasound, women with a short cervix at 18 to 20 weeks and treating them with vaginal progesterone compared favourably with other perinatal interventions.  Eleven women would need to be treated with vaginal progesterone to prevent one preterm birth before 33 weeks’ gestation, the opinion authors said.

Routine reporting of the cervical length should be undertaken in all Australian women mid-pregnancy,  Mrs Michelle Pedretti, chief sonographer at WA’s Department of Health, said.

However, a US study of 9410 nulliparous women published in JAMA found that transvaginal cervical measurement didn’t adequately predict preterm birth and therefore could not be recommended as a routine screening test in low-risk women.

The cervical measurement only identified 8% of women at 16-22 weeks who later went on to have a preterm birth, the researchers found.

So at what point does the prevention rate justify the expense of routine screening?

Professor Stephen Robson, president of the Royal Australian and NZ College of Obstetricians and Gynaecologists, said the college’s new position statement on the subject was expected in the next few weeks.

“It has been estimated that about 125 women have to undergo formal cervical length measurement to prevent one birth before 34 weeks, and about 225 have to be screened to prevent one case of major harm to a baby,” he said.

“At the moment, it is probably cost-effective to have the additional specific measurement of cervical length for women who don’t have any risk factors at all, because a very early preterm birth is a prolonged and expensive thing for the health system to manage, and a trauma for women.”

The downside of universal screening was that it added some complexity to a routine ultrasound assessment, Professor Robson said.

And the question of the specific type of imaging approach also needs to be considered, according to  Mrs Pedretti. “Transabdominal cervical-length screening has been recommended as the approach for low-risk women, though emerging data is now suggesting that this may not be as accurate as transvaginal cervical length screening,” she said. “Transvaginal cervical-length screening is recommended for women at increased risk of preterm birth.”

While transvaginal ultrasound is more accurate than transabdominal ultrasound, it requires different equipment to that used routinely.

“Transvaginal screening in addition to routine pregnancy ultrasound is more expensive, time-consuming, and adds strain to busy ultrasound services,” Professor Robson said.

“It remains unclear as to whether the transvaginal ultrasound cervical length measurement should be universal or selective, following transabdominal assessment,” the ANZJOG authors said.

In Australia, transabdominal cervical-length screening at the same time as routine mid-pregnancy scan had no additional costs for the woman, Medicare or the imaging provider, Mrs Pedretti said.

“Should, however, the progression to a transvaginal ultrasound be required, due to inadequate visualisation of the cervix or a short cervix on transabdominal views, then this will create a time and cost impost,” she said.

There was still a need for standardised protocols for measurement and treatment regimes, and the availability of imaging and treatment options.

“This is particularly important in rural and remote locations where regular imaging services may be limited or there may be compliance or storage issues with treatment options for women with a short cervix,” she added.

Experts are looking closely at WA, which has been trialling a preterm birth prevention program which involves every pregnant woman in the state having transabdominal cervical-length screening, and those determined to have a short cervix being given vaginal progesterone.

Initial results suggest the program may have reduced the number of preterm births by 8%, although other factors may have contributed to the reduction, including better resources to care for women at high risk of preterm birth.

ANZJOG 2017; online 10 March

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