This week I’m returning to an exploration of the literature about water birth and the perineum. The recently published paper by Sanders and colleagues (2024, in press) has already generated two letters to the editor. I want to start by looking at their criticisms and considering their validity, before making some conclusions about what I think the POOL study can teach us.
Letter number 1
The first of the letters to the editor came from a team based at a department of obstetrics and gynaecology in China (Tang, Liu, & Lan, 2024). They argued that the authors didn’t provide information about whether any of the women had a multiple pregnancy, or about labour onset, use of pharmacological analgesia, labour duration, or the duration of water immersion. It seems they didn’t read the fine print.
There are supplementary materials attached to the paper, available for download. These list the medical and obstetric risk factors (there are a LOT of them) that led to exclusion from the trial data set. And yes – multiple pregnancy and induction of labour are listed as exclusion criteria. The body of the paper also includes the use of epidural analgesia as an exclusion criteria. Labour duration isn’t described in the paper – but women who used oxytocin for labour augmentation, and who had CTG monitoring were excluded . Prolonged labour is an indication for CTG use (according to the NICE guideline) and for labour augmentation. This presumably would remove most women who had a longer labour from the analysis.
The concerns raised by these letter writers can’t be sustained after actually reading the POOL study in detail.
Letter number 2
The next letter was authored by a team of people from a variety of European countries (de Leeuw et al., 2024). They made the statement that rates of severe perineal trauma in the study are “remarkably if not unacceptably high”. Let’s take a look to see if that holds up to scrutiny. To remind ourselves of what we are comparing with – the POOL study data for severe perineal trauma rates are below.
| Water birth | Birth out of water | |
| Women who gave birth for the first time | 4.8% | 5.3% |
| Women who had given birth before | 1.1% | 1.7% |
Here are some recent studies to compare with:
- Lee and colleagues 2024 paper on the impact of the introduction of the Australian perineal care bundle reported severe perineal trauma rates of 4.7% in women giving birth for the first time, and 1.1% for women who had given birth one or more time before, after the bundle was introduced.
- Thorne and colleagues 2022 study examining the impact of previous birth mode on the rate of severe perineal trauma at this birth reported a rate of 6% at first births, 0.6% in women who had a previous vaginal birth, 12.9% in women giving birth vaginally after a previous elective caesarean section, and 17.3% in women giving birth vaginally after a previous “emergency” caesarean section.
- Pierce-Williams and colleagues 2021 literature review about hands-on or hands-off approaches to perineal trauma prevention reported a 2.6% rate of third degree perineal trauma with a hands on approach, and 0.7% with a hands off approach. (They found no randomised controlled trials providing data about fourth degree trauma.)
- The findings of the “OASI” bundle in the UK, reported on by Gruel-Urganci and colleagues in 2020 reported a reduction in severe perineal trauma from 5.2% to 4.9% at first births, and 1.7% to 1.5% for women who had given birth before.
Therefore – the rates reported in the POOL trial are neither “remarkably” nor “unacceptably” high.
The letter writers went on to criticise the POOL trial for not reporting on what approaches were used to prevent trauma, like whether a hands-off or -on approach was used. They are correct that this was not reported on in the paper. I presume the participating hospitals were operating under the “OASI” bundle, where the interventions include a hands on approach (more about that here). If anyone can verify or refute this, let me know.
The letter writers concluded by making reference to the Montgomery trial recommendations that clinicians should inform women of the risks and benefits of all birth options, then said “… one should question, why the risk of childbirth in the POOL study was so unacceptably high and still remained undiscussed?”
Sigh.
I’m a HUGE fan of giving women accurate information and supporting their decision making. It’s kind of my whole vibe. But creating an argument like this makes me want to slap them with a wet fish. How do they know the risks of perineal trauma were not discussed with women, using appropriate risk estimates? This letter reads of fear-mongering to me.
What I think the POOL study teaches us
If you want to understand the precise mechanisms that cause one woman’s perineum to have extensive trauma and not another woman’s – then you don’t do a cohort study like this. This trial can’t tell us whether it was the duration of immersion, the temperature of the perineal tissues, the woman’s position, the position or size of the fetal head, or so on, that had an impact. If you want to know for sure that it is being in a birth pool, ONLY being in a birth pool and no other aspect of care that is more common when a woman is in the birth pool, that makes (or doesn’t make) a difference to the rate of severe perineal trauma, then you don’t do a cohort study like this.
But if you are giving birth in one of the hospitals in this study, or one quite like it, and you want to have a rough idea about whether the kind of care you get while having a water birth compared to the kind of care you get when you give birth out of the bath will make a difference to your chance of severe perineal trauma – then this research is just the ticket. Cohort studies are nothing but pragmatic. This looks at actual births happening in the messy real world and what actually happened.
And that’s why I think this one is a useful addition to the body of literature about water birth.
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References
de Leeuw, J., Laine, K., Manresa, M., Raisen, S., Kalis, V., Rušavy, Z. (2024). Delivering in or out of water, the OASI rates in the POOL cohort study are disturbingly high. BJOG, in press. https://doi.org/10.1111/1471-0528.17933
Gurol-Uganci, I., Tidwell, P., Sevdalis, N., SIlverton, L., Novis, V., Freeman, R., Hellyer, A., van der Meulen, J., & Thakar, R. (2020). Impact of a quality improvement project to reduce the rate of obstetric anal sphincter injury: A multi centre study with a stepped-wedge design. BJOG, 128(3), 584-592. https://doi.org/10.1111/1471-0528.16396
Lee, N., Allen, J., Jenkinson, B., Hurst, C., Gao, Y., & Kildea, S. (2024, Feb). A pre-post implementation study of a care bundle to reduce perineal trauma in unassisted births conducted by midwives. Women & Birth, 37(1), 159-165. https://doi.org/10.1016/j.wombi.2023.08.003
National Institute for Health and Care Excellence. (2022). Fetal monitoring in labour. www.nice.org.uk/guidance/ng229
Pierce-Williams, R. A. M., Saccone, G., & Berghella, V. (2021, Mar). Hands-on versus hands-off techniques for the prevention of perineal trauma during vaginal delivery: a systematic review and meta-analysis of randomized controlled trials. Journal of Maternal-Fetal & Neonatal Medicine, 34(6), 993-1001. https://doi.org/10.1080/14767058.2019.1619686
Tang, W-Z., Liu, T-H., & Lan, X. (2024). Evaluating water birth risks: A deep dive into the POOL cohort study’s research gaps. BJOG, in press. DOI: 10.1111/1471-0528.17934
Thorne, E. P. C., Durnea, C. M., Sedgwick, P. M., & Doumouchtsis, S. K. (2022, Dec). Influence of previous delivery mode on perineal trauma risk. International Journal of Gynaecology & Obstetrics, 159(3), 757-763. https://doi.org/10.1002/ijgo.14218
Categories: New research, Obstetrics, Reflections
Tags: Perineal trauma, water birth
Birth Small Talk 
Waterbirth is usually hands off.
Theoretically to avoid stimulating the newborn to take a breath while still submerged. The trigeminal nerve is thought to be stimulated through touch or cool air.
In Ireland the guidelines are for hands off.
Love your posts Kirsten
Ruth Banks SECM
+353 87 133 3476
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