Mori et al.1 in their study of intrapartum-related perinatal mortality (IPPM) attempted to use the ‘best available data’ to ascertain the safety of planned home birth. The validity of this study hinges on being able to determine accurately the numbers of planned home births. The authors took the number of actual home births and adjusted these using estimates for the numbers of both unintended home births and transfers. However, they have used inappropriate assumptions and have compounded these mistakes by making errors in their calculations.
The authors use two ways to determine the numbers of unintended/unplanned home births, Calculation A and Calculation B, producing widely differing answers of 66 265 and 20 206. ‘Calculation A’ estimates unintended home births as a percentage of all home births (50.7%) and ‘Calculation B’ as a percentage of overall births (0.32%). There is no reason to suppose that the number of unplanned home births are affected by a rise or fall in planned home births. However, it is likely that a small consistent proportion of pregnant women concealed their pregnancy or had a precipitate birth at home. Indeed, Murphy et al.2 reported from 1970 to 1979 that unintended home births formed a relatively constant percentage of all births, around 0.35% (range 0.27–0.46%). In contrast, unintended home births increased from 17 to 57% when expressed as a percentage of all home births. This demonstrates that Calculation B is more reliable, yet Mori’s conclusions are based on Calculation A.
In addition, calculations of the numbers of births and IPPM rates using Calculation A are subjected to a number of errors and are therefore invalid. The Murphy study2 data applicable to Calculation A are included in table 1 (34.1%) but omitted from the calculation of both the weighted mean and the sensitivity ranges used to create table 2. Using a revised weighted mean and lower range reduces the IPPM in table 2 for booked home birth, whether completed or not, and increases the range in which the true rates could lie. Furthermore, the study by Redshaw et al.3 is included in Calculation B, but not Calculation A, adding to the inaccuracy of table 2. There is also an error in table 2 in the completed home birth group, where 31 intrapartum-related deaths for 83 343–111 126 gives a range of 0.28–0.37, not 0.28–1.15 as reported.
The authors inappropriately make a direct comparison between women who ‘planned home birth but transferred to hospital’, with ‘all women giving birth’. However, if such a comparison is made, it should be with a matched group of women who booked hospital birth and developed complications. The key finding that ‘there was no evidence of difference in the IPPM rate for the booked home birth group compared to the overall rate’ was not reported in the abstract as it should have been.
This study1 illustrates that the ‘best available data’ are, in this case, not ‘good enough’ and are unacceptable for providing evidence-based information for professionals, service users, the press, or national guidance.4 We shall be submitting a detailed critique for publication after peer review.