We read with interest the study by Pennell et al.1 on induction of labour in nulliparous women with an unfavourable cervix, in which single-balloon catheters, double-balloon catheters and prostaglandin E2 (PGE2) gel were compared.
As the authors emphasise, the sample size is relatively small. The sample size calculation was based on the results of previously reported studies by Atad et al.2,3 In the observational study by Atad et al.,2 a caesarean section rate of 16% is reported. This very low rate of caesarean section is however not found in the randomised trial,3 where a caesarean section rate of 23% is reported in the double-balloon group, compared with 30% in the PGE2 group. Furthermore, the sample size of this study was calculated using data from studies conducted in Israel where the overall caesarean section rate is 18% compared with over 30% in Australia.
Also, in contrast to the current study, the study groups in these studies consisted of both nulliparous and multiparous women. As the authors described, nulliparous women are known to have a less favourable outcome than multiparous women when labour is induced, and the groups are as a result not comparable with the studied group in this article. It is therefore surprising that the authors used a caesarean section rate of 15% in their sample size calculation. In our opinion, much larger groups are needed to assess the impact of the mode of induction on the caesarean section rate.
A second point of concern arises when the authors conclude that cord blood gases were worse in the PGE2 group. Although the P-value was 0.05, the difference in cord blood pH was only 0.01, a difference not relevant from the clinical point of view. It would be of more interest to compare the number of children born with clinically relevant asphyxia, for example the number of children with a pH below 7.05 and a base deficit below −12.