The POOL study (more information is here if you haven’t been following along with this series) is simply the latest in a long series of studies examining aspects of safety relating to the use, or not, of water immersion at birth. Where does the POOL study evidence fit with what was already known from earlier research? Does it provide something new and controversial, or does it reinforce where the evidence was already headed?
To answer these questions, I looked for recent systematic reviews of the literature that included information about the relationship between water birth and perineal trauma rates. The ones I found set out to examine multiple outcomes, with perineal trauma rates included in the list. I found three that I will make use of. They are:
- Burns et al., 2022 – this review included studies published in 2000 or later. They provided an analysis for the rate of having an intact perineum and for severe perineal trauma. They did not analyse outcomes for women giving birth for the first time separately from women who had given birth before, but did consider first births as a confounding variable when completing the analysis.
- Cristina et al., 2022 – this team searched for all studies from the time each of the databases they used had been established. They provided analyses for first or second degree perineal trauma, and for severe perineal trauma. They didn’t distinguish between whether women had given birth before or not.
- McKinney et al., 2024 – this team searched for all studies from the time each of the databases they used had been established. They reported only on severe perineal trauma, and also did not consider whether women had given birth before or not.
Randomised controlled trials, prospective studies, and retrospective studies from non-randomised groups of women were included in each review. As would be expected – there was significant overlap in the studies included in each analysis and the data analysis approaches used. It is therefore not surprising that they report similar findings.
Having an intact perineum
There have been 17 studies with 59,070 women who reported on whether water birth had an impact on the chance of having an intact perineum (Burns et al.., 2022). Sadly, these authors don’t tell us what proportion of women actually had this outcome in each group (known in statistical language as the absolute risk). Water birth was protective – women were three times more likely to have an intact perineum than women who did not give birth in water (odds ratio 3.03, 95% confidence intervals 1.52 – 6.04).
Having a first or second degree tear
There have been 13 studies with 54,247 women who reported on whether water birth had an impact on the chance of having a first or second degree tear (Cristina et al., 2022). They found that 48.6% of women giving birth in water and 45.3% of women who were not giving birth in water had a first or second degree tear. While the size of the difference in chance was small, it was statistically significant (odds ratio 1.45, 95% confidence intervals 1.16-1.81). The authors of the review did suggest caution in interpreting these findings due to a high degree of variability in the data.
Severe perineal trauma
The Burns et al. 2022 review found 15 studies with 93,690 women, where rates of severe perineal trauma were reported; Cristina et al. 2022 found 20 studies with 168,0001 women; and McKinney et al. 2024 found 23 studies with 287,800 women. Each review team found no significant difference in the rate of severe perineal trauma between birth in or out of water. The two reviews where rates were reported had similar findings:
- 1.8% in water vs 2.3% not in water in the Cristina et al. 2022 review and
- 1.23% in water vs 1.72% not in water in the McKinney et al. 2024 review (from only the non-randomised studies in their review)
The odds ratios across the studies were also similar
- 0.84 Burns et al. 2022
- 0.87 Cristina et al. 2022, and
- 0.82 McKinney et al. 2024 (again from only the non-randomised studies in their review)
So how did the POOL study compare?
For all the women included in the POOL study, the rate of severe perineal trauma for women who gave birth in water was 2.5% and for those giving birth not in water it was 3.8%. These rates are therefore marginally higher than what has been reported in similar research (but as I explained last week, they are consistent with what is seen in practice). There was a significantly lower rate of severe perineal trauma for women giving birth in water in the POOL study. The odds ratio in the POOL study was 0.89 – so the size of the difference is consistent with what has been previously seen, making this a plausible finding.
The POOL study isn’t therefore a radical departure from previous research – it simply adds to the size of the pool of data about water birth and perineal outcomes (it also reported on other outcomes but that’s not been the focus of this series).
Things it would be nice to see more of in research…
What I appreciated from the POOL study was that they did consider women having their first birth separately from women who had given birth before. The rates of severe perineal trauma are higher at first births – so this makes sense. It also helps clinicians to provide slightly more personalised advice when counselling women about the pros and cons of water birth. I wish all research reporting on perineal outcomes would do the same.
The other useful thing the POOL study team did was to provide absolute risk numbers. This was not done in the Burns et al. 2022 review. They didn’t even tuck the event rate away in the small print (or at least, if they did it was so well hidden I couldn’t find it!) so I could calculate it for myself. Absolute risk is a very handy bit of information when counselling women. If there is a 50% relative risk reduction (the thing happens half as often) – it matters whether the difference is between two chances in ten million vs one chance in ten million or two chances in ten vs one chance in ten. I wish more researchers would think about how clinicians might make use of their findings when counselling women – and provide the sorts of information that are most useful for this.
Thank you for following along with me for this brief diversion away from fetal monitoring – I’ll be back onto fetal monitoring topics next week!
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References
Burns, E., Feeley, C., Hall, P.J., & Vanderlaan, J. (2022). Systematic review and meta-analysis to examine intrapartum interventions, and maternal and neonatal outcomes following immersion in water during labour and waterbirth. BMJ Open, 12, e056517. doi: 10.1136/bmjopen-2021-056517
Cristina, T., Mara, T., Arianna, S., Gennaro, S., Rosaria, C., & Pantaleo, G. (2022). Impact of waterbirth on postpartum haemorrhage, genital trauma, retained placenta, and shoulder dystocia: A systematic review and meta-analysis. European Journal of Obstetrics & Gynecology and Reproductive Biology, 276, 26-37. https://doi.org/10.1016/j.ejogrb.2022.06.016
McKinney, J.A., Vilchez, G., Jowers, A., Achoo, A., Lin, L., Kaunitz, A., Lewis, K., Sanchez-Ramos, L. (2024). Waterbirth: A systematic review and meta-analysis of maternal and neonatal outcomes. American Journal of Obstetrics & Gynecology, 230(3-Suppl), S961-979. https://www.ajog.org/article/S0002-9378%2823%2900604-X/fulltext
Categories: New research
Tags: OASI, Perineal trauma, water birth
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