Monday, February 04, 2008

How homebirth misinformation propagates across the web

Tina Cassidy, author of The Birth Book, recently thanked Carol Sakala of the Childbirth Connection for alerting her to 2 studies that, when compared, showed Repeated VBACs are safe; repeated cesareans are not. There's just one problem; that's NOT what the studies show. In fact, they look at two very different groups of women, and it was disingenous (at best) and deliberately misleading (at worst) for Sakala to have suggested the comparison.

The two studies are Labor Outcomes With Increasing Number of Prior Vaginal Births After Cesarean Delivery published in the February 2008 issue of Obstetrics and Gynecology and Maternal Morbidity Associated With Multiple Repeat Cesarean Deliveries published in the same journal in 2006. Of note, both studies were done by the same group of investigators.

The 2008 study looks at whether a previous successful VBAC affects the outcomes of subsequent VBACs. In the 2008 study, 33% of the women had already had at least one successful VBAC, and 100% of the women were candidates for attempted VBAC. This study looked at neonatal and maternal outcomes.

The 2006 study looks at the maternal effects of previous C-sections on subsquent C-sections. In this study, it appears that 0% of the women had had a previous successful VBAC. More than 21% of the women in the study were not even candidates for vaginal delivery; they were having primary C-sections for medical indications like placenta previa. This study did not even look at neonatal outcomes.

Let's look at the results of the study in more detail. Here is how the authors describe the 2008 study:

"The relationships between the number of prior VBACs and the probability of successful VBAC attempt or uterine rupture in the current pregnancy remain to be clearly elucidated. It is also unknown if successive labors will place an additive strain on the uterine scar, increasing the risk of uterine rupture when VBAC is attempted. The purpose of this analysis is to evaluate the impact of increasing number of prior VBACs on the likelihood of VBAC success and uterine rupture in subsequent pregnancies."

Here are the main findings of the study:

"The frequency of VBAC success rose with increasing number of prior VBACs, increasing from 63.3% with no prior VBAC to 87.6% and 90.9% for those with one or two or more prior VBACs, respectively. The frequency of uterine rupture declined from 0.87% with no prior VBACs to 0.45% and 0.43% for those with one or two or more prior VBACs, respectively."

When uterine rupture did occur, the consequences were dire. There were 78 uterine ruptures in the group that had never had a successful VBAC, resulting in the deaths of 58 babies and 2 maternal deaths. In addition, there were 15 babies who lived but suffered permanent brain damage, and 21 women required hysterectomy.

There were additional ruptures and deaths in the groups of women who had had previous successful VBACs. There were 24 additional babies who died and no maternal deaths.

The 2006 study did not look at neonatal outcomes. The maternal outcomes for women having their second or higher C-section included maternal death 0.07%, and a hysterectomy rate of 0.42% for second C-sections, 0.9% for third C-sections, and 2.41% for third C-sections.

If you look at women in the 2008 study who attempted VBAC and had never had a previous successful VBAC, the rate of hysterectomy was 0.64% and the rate of maternal death was 0.02%. In contrast, in the 2006 study, when women with one prior C-section chose elective repeat C-section, the rate of hysterectomy was 0.42% and the rate of maternal death was 0.07%. Both the hysterectomy rate and the maternal death rate rose with the number of previous C-sections. Presumably, on that basis, Sakala is claiming that the studies show that VBAC is safer than elective repeat C-section.

The comparison is invalid for two main reasons:

1. The two groups of women are not comparable. In the 2008 study, all the women are candidates for VBAC and 33% already had a successful VBAC. In the 2006 study, we have no idea what proportion of women are candidates for VBAC. We know that at least 21% are not candidates at all. In the 2008 study, less that 5% of the women had had two or more C-sections, in the 2006 study 27% of the women had two or more C-sections.

2. The single biggest risk in attempted VBAC is death of the baby. The 2006 study did not even look at neonatal outcomes.

Therefore, the two studies cannot be compared directly, and it is false to claim that they show VBAC to be safer than repeat C-section. The primary risk of attempted VBAC is death of the baby, and that result was not even presented in the second study. I'll be interested to see if Ms. Cassidy changes her post to reflect the truth about these studies

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