Tuesday, December 19, 2006

What is the neonatal death rate in the correct comparison group?

Johnson and Daviss described their study as a prospective cohort study. They are comparing the cohort of all women who gave birth at home with a direct entry midwife in the year 2000 to the cohort of women who gave birth in the hospital. In order for the results to be valid, the cohorts must be matched very carefully in all possible variables. The correct cohort with which the homebirth group SHOULD have been matched is low risk white women with vertex babies at term, and from which congenital anomalies have been excluded (since Johnson and Daviss removed congenital anomalies from the homebirth group).

As we have seen, Johnson and Daviss did not use the correct group for comparison. They created their own group by amalgamating data from a series of out of date homebirth studies. This is an unacceptable comparison and renders the conclusion of their study invalid. What would the correct comparison have looked like?

In order to make the correct comparison, we would need to find the data from low risk white women with vertex babies at term who gave birth in the hospital in the eyar 2000. Then we would need to subtract out the deaths caused by congenital anomalies. All the relevant data is available from the National Center for Health Statistics in Maryland. It would take some work to calculate the exact figures, the answer can be calculated. We know that Johnson and Daviss had access to this data, because they used it to create the comparison group for their conclusions about interventions at homebirths.

We are spared from doing the tedious calculations because someone has already done it for us. MacDorman et al. did these calculations as part of their study of neonatal mortality and C-sections . I have argued that their numbers are artificially high because the group is higher risk than they claim. I'll ignore that problem for the moment, and use their numbers. The real numbers are undoubtedly lower.

MacDorman et al. performed their calculations on all birth from the years 1998-2001. The neonatal mortality rates were similar in each of those years, so it can serve as an acceptable proxy for neonatal mortality in 2000. Keep in mind that we are not using someone else's control group, as Johnson and Daviss did. We are simply using the same birth certificate data that anyone must use.

MacDorman et al. reported 3,571,332 births to low risk white women with vertex babies at term. Within that group, there were 2344 neonatal deaths from a total neonatal death rate of 0.65/1000. MacDorman et al. also found that 55% of the neonatal deaths were caused by congenital anomalies. We need to subtract the congenital anomalies from our group because Johnson and Daviss subtracted the congenital anomalies from their group. The final neonatal death rate is 0.3/1000. As I have remarked elsewhere, this number is almost certainly higher than the true number since the MacDorman "low risk" group undoubtedly included some high risk women.

The neonatal death rate for low risk white women giving birth to vertex babies at term (and excluding congenital anomalies) is 0.3/1000 or less. According to Johnson and Daviss:
After we excluded four stillborns who died before labour but whose mothers still chose home birth, and three babies with fatal birth defects, five deaths were intrapartum and six occurred during the neonatal period (see box). This was a rate of 2.0 deaths per 1000 intended home births. The intrapartum and neonatal mortality was 1.7 deaths per 1000 low risk intended home births after planned breeches and twins (not considered low risk) were excluded.
I am not convinced that removing the breeches and twins from the group is warranted, but for the sake of argument, I will accept their lower rate of 1.7/1000.

The bottom line is this: Johnson and Daviss quote a neonatal death rate at homebirth of 1.7/1000. The neonatal death rate in the comparable hospital group is 0.3/1000 or less. The appropriate conclusion drawn from the Johnson and Daviss data is that the neonatal death rate at homebirth is more 5 times higher than the neonatal death rate at hospital birth! Not only have Johnson and Daviss NOT shown that homebirth is as safe as hospital birth, they have actually shown that homebirth is considerably more dangerous for babies than hospital birth.

I have been participating in a similar discussion of this issue over on the Lamaze blog. Neither Henci Goer, nor the 6 other homebirth advocates have responded to a direct, yes or no question about the control group. I take that as an acknowledgement that Johnson and Daviss used an inappropriate comparison (cohort) group. I have now posted the above information on the Lamaze blog to demonstrate that the neonatal death rate at home in the study is more than 5 times higher than the neonatal death rate in the hospital. It will be interesting to see if there is an acknowledgement of the true numbers.

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