Is transabdominal ultrasound scanning of cervical measurement in mid-trimester pregnancy a useful alternative to transvaginal ultrasound scan?

OBJECTIVE

The aim of this study is to assess the correlation between transabdominal and transvaginal ultrasound measurements of the cervix in pregnancy. If transabdominal ultrasound measurement of cervical length is found to provide effective information, it could be used in patient counselling and when making clinical decisions.

MATERIAL AND METHODS

One hundred and twenty seven pregnant patients between 18-26 weeks of pregnancy were enrolled in this prospective study for measuring cervical length, both by transabdominal and transvaginal ultrasound scan after bladder emptying. Transabdominal and transvaginal measurements were compared and correlated.

RESULTS

In patients with transvaginal ultrasound scan (TVS) cervical length ≤32 mm, TVS cervical length was found to be shorter than by transabdominal ultrasound scan (TAS). Most of these patients needed >3 cm of vertical pocket of urine in the bladder for adequate visualisation of the cervix. In patients with TVS cervical length >32 mm, the TVS measurement of the cervix was longer than the TAS measurement of the cervix. In these patients, the cervix could be seen by TAS when there was either ≤3 cm vertical pocket of urine in the bladder or an empty bladder. Statistical tests showed that there is a significant difference between TAS and TVS cervical measurements and that there is a significant association between these two measurements.

CONCLUSION

Most of the patients needed variable degrees of bladder filling for adequate visualisation of the cervix. Although minimal bladder filling does not influence TAS measurements of cervical length, moderate fullness of the bladder does cause an apparent increase in TAS measurements of cervical length. If the cervical length is ≥30 mm by TAS, regardless of urine content in the bladder, the patient can be assured vis a vis their risk of preterm labour as far as cervical length is concerned. However, in patients with TAS cervical measurement <30 mm and where the bladder needed a moderate amount of urine for adequate visualisation of the cervix, TVS cervical measurement may be close to the critical value of 25 mm. These patients need to be counselled and offered TVS for better assessment of cervical length.