What’s the chance of having another PPH, if you’ve had one before?
For some women, that’s an important question. Women who have had a previous postpartum haemorrhage (PPH, or excessive bleeding after birth) are often advised to have their future babies in a hospital in case they have another PPH.
But many want to give birth in midwifery-led settings instead, and there are significant advantages to doing so:
“For women with uncomplicated pregnancies, planning birth in a midwifery unit is considered “particularly suitable”, because it is associated with benefits for the woman in terms of a lower chance of intervention during labour and birth, with no difference in neonatal outcomes, compared with planning birth in a hospital ‘consultant-led’ obstetric unit or labour ward.” Morelli et al (2022).
So it would be useful to know the actual chance of PPH for a woman who has had one before. These sorts of numbers help those who want to make an informed decision about where they will plan to give birth.
A study of midwifery units
Morelli et al (2022) have helped by asking this very question.
They looked at the outcomes for women who had a PPH in a previous pregnancy and who went to an alongside midwifery unit for care in labour. These are midwifery units that are in the same building as an obstetric unit, and they are sometimes called birth centres.
The UK-based researchers used data from the UK Midwifery Study System (UKMidSS), which is a research infrastructure linking all 123 alongside midwifery units in the UK that allow for data on specific outcomes to be collected.
What to measure?
The primary outcome measure for the study’s main analysis was whether a woman had a PPH requiring transfer to obstetric care.
This is a much more useful measure than a set volume of blood loss, like 500ml or 1000ml, although the researchers did look at the estimated volume of blood loss and other related measures as well. But we know that estimation of blood loss is hard, and therefore often inaccurate.
It’s also more useful to look at whether someone requires transfer because some women cope better with higher blood loss than others, because of individual factors such as height, weight, and health status (Edwards & Wickham 2018).
The outcomes of women who had a previous PPH were compared to ‘comparison women,’ who had given birth before but who hadn’t had a PPH.
What did they find?
The results confirm that, relatively speaking, women who had a previous PPH are significantly more likely to have a PPH requiring transfer to obstetric care than the comparison women:
“Women who experienced a previous PPH were significantly more likely than comparison women to: have a PPH requiring transfer to obstetric care (4·2% vs. 2·4%, aRR=1·65, 95% CI 1·14–2·38), be transferred to obstetric care for any reason (17·8% vs 11·9%; aRR=1·41; 95% CI 1·09–1·83), and have any PPH≥ 500 ml (22·7% vs 11·1%, aRR=1·86, 95% CI 1·49–2·32).” Morelli et al (2022).
But, as the researchers themselves point out, that’s not the whole story, for two very important reasons that require us to look in a bit more depth at the numbers.
Digging into the numbers
First, the absolute risk (chance of PPH) is small and more than 95% of the women did not have another PPH for which they needed transfer.
Or, to put it another way, for every twenty women who had a PPH in a previous pregnancy and gave birth in a midwifery-led unit, nineteen did not have a PPH requiring obstetric transfer.
These figures show why it’s really important to look at the absolute chance of something as well as the relative risk. Yes, the relative risk of having a PPH requiring transfer is higher in women who have had a PPH before, compared to women who have had a baby without having a PPH. Its 65% higher, and that can sound serious. But there’s still a 95% chance that you won’t have one.
The second issue is that, when the researchers compared the data from this study to the rate of PPH in general, they showed that, actually, the PPH rate for women who had a PPH before is similar to the rate in women who haven’t had a baby before.
In other words, even if you’ve had a PPH before, the chance of having a PPH again is the same as it was the first time. Your risk doesn’t go up from the first time. But the risk goes down in a woman who has had one baby and who didn’t have a PPH, so it seems higher when we compare the two.
Why numbers and context matter
This is why it’s so important to understand the numbers, or to make sure you’re learning from and following people who do.
Risk figures can sound scary. But they don’t always mean what we think they might, and they can be deceptive if we don’t use them well. It’s important to know that not everyone who has a medical or midwifery degree, or who shares numerical data and information on their website, social media, or in their classes actually understands the statistics they might be sharing.
There’s also always a bigger picture, and it’s important that women and families can weigh up all the relevant information before deciding what’s right for them. In this case, the context of the numbers is an important part of that.
Other important information includes that, as above, there are significant benefits to giving birth in an alongside unit. It’s also important to know that, even if someone does have a PPH, midwives are trained to deal with this while transfer is taking place.
The open access (freely available) paper by Morelli et al (2022) contains some other useful discussion as well, and I would recommend that midwives and birth workers read it in full. You can find it here.
Morelli A, Rogers J, Sanders K et al (2022). Outcomes for women admitted for labour care to alongside midwifery units in the UK following a postpartum haemorrhage in a previous pregnancy: A national population-based cohort and nested case-control study using the UK Midwifery Study System (UKMidSS). Women and Birth https://doi.org/10.1016/j.wombi.2022.11.002
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