How the consent process introduces bias into RCTs

Part 1: Why I remain sceptical of RCTs concerning obstetric interventions in normal labour and birth

Another blog post in response to a Twitter debate .. this time concerning various RCTs currently evaluating IOL vs expectant management. We’ve been discussing three trials:

The questions are valid. We know stillbirth is increased, especially in certain populations, the longer a pregnancy continues. In order to make an informed decision, many women will want to know the most likely outcomes and effects of opting in or out of proposed interventions. RCTs are considered the most unbiased way of settling these issues, unsullied by the biases of women or health professionals.

The problem is, these interventions are eventually applied to a population that is, due to being human, inherently biased. Some women feel a strong preference in one direction, some in another; and some want their doctor to decide for them. Women need to consent to be randomised into RCTs, and women who are most averse to the proposed intervention simply decline consent. Therefore the population recruited becomes slightly biased towards a preference for the intervention being investigated.

Is this important? Does it matter? I don’t know. Recent research by Wu et al suggests that women with a strong preference for vaginal birth were more likely to have a vaginal birth. RCTs cannot tell us the effect of women’s preferences on the outcomes they measure. Yet in theory their results are used to offer women an option they will almost certainly have an opinion about.

If I were contributing to the design of these trials, I would want to collect observational data alongside the main trial data. Things like:

  • Why do women consent or decline to participate in the RCT? Are the women who declined to participate due to a strong preference against induction more or less likely to have a normal birth? Are the outcomes for their babies significantly different than those in either arm of the trial?
  • What are the long-term outcomes? Especially in the over-35 population, an increase stillbirth rate may be due to inherent weaknesses in the baby. Significant long-term differences are often not detectable until 2 years of age.
  • And finally .. would they do the again? Would they recommend it to a friend?

These are questions best answered using quantitative techniques, but women may have different questions or priorities, which we will only discover using qualitative investigations.

See Part 2: Why midwives are sceptical.


11 thoughts on “How the consent process introduces bias into RCTs

  1. Pingback: Why midwives are sceptical | The midwife, the mother and the breech

  2. Denise Hynd

    Correction; All ‘studies’ of labour and birth are non-physiological birth as labour is driven by the shy hormone oxytocin and therefore requires that the woman feel private and unobserved. As Michel Odent in ‘ The Functions of the Orgasms; the Highway to Transcendence.’ published in 2000 says on page 13 “those who understand the fetus ejection reflex understand how useless it is to exchange views with others on subjects such as breech birth,posterior position of the baby’s head or perineal lacerations. All scientific studies in the medical literature about the best way to ‘manage’ particualr obstetric situationsor particualr phases of labour are conducted in environments where the fetus ejection reflex is ignored or inhibited. No fetus ejection reflex occurs when the birth process is ‘managed’.
    Reply ↓

    1. midwifeshawn

      Hi Denise .. thanks for this. I struggle with this dilemma when it comes to breech. I think the same is true as with all other births .. when you take people into hospital, birth becomes more complicated and more interfered with. But I also think that the occasional risks with breech are too high for the answer to be staying at home in order to avoid interference. Not sure I have the answer yet, still trying to create as warm and welcoming a space as possible in various hospital environments.

      1. Denise Hynd

        I did not mention home birth! However when you have experienced undisturbed birth (which mostly happens at home) you learn that there is a multitude of ways to disturb the intrinsic hormonal flows of labour and birth, which results in a range of problems that the majority do not recognise as a reflection of our highly intervetionist maternity culture. Problems like “Failure to Progress which actually need less not more intervention! The need to optimise nature’s means (hormonal flows and instinctual movements by the mother) to assist the baby’s efforts to birth are even more relevant for the baby coming breech or posterior. However suspecting let alone finding a breech almost automatically triggers a cascade of psychological and physical events (now in mothers and attendants) to change or save this baby deemed ‘abnormal’ or at risk by virtue of its possibly finding the most accomodating positon in the womb?

  3. jimgthornton

    Shawn, I agree, randomising women, who already have a strong preference, could potentially alter results. But it might work in either direction. I, an NHS obstetrician, see mostly women requesting induction, while you see mostly the opposite. And who declines to join may also work either way. You assume only women who want induction would agree to be randomised. But not all women who decline randomisation choose to wait. Many decline and choose induction!

    Fortunately fully informed consent means that only women without a strong preference participate. You may argue that they are not representative of the general population of pregnant woman, and you may be right. But the results will still be valid.

    And of course choice bias affects observational studies even more!

    Re. need to study those who decline (can be tricky ethically), and long-term follow-up – could not agree more!

    Are you doing any RCTs at James Paget? If not induction. Cord clamping? Hypnotherapy?

    1. midwifeshawn

      But Jim, I LIKE observational studies. I just like them, and you’re not going to talk me out of it! A really good set of observational data, and it’s like looking at a big painting – rather than a photograph, noticing something different each time, making connections, developing new wild assumptions, intuitions (all those softy softy things midwives like). RCTs sometimes seem so .. prosaic! But the world needs both, right? 🙂

      I stand by my wild assumption that most women who decline prefer not to be induced .. but it would not surprise me at all to find many women return to you requesting induction anyway. (I intended to do this naturally, but I’m now 39 weeks, the heartburn is killing me, my husband has developed insomnia and permanent low-level man-flu, my mother keeps calling with news of yet another study she’s found about the increase in stillbirth after 39 weeks, and I can’t put on my own socks. Help! – Fictional, mostly.) It will be good when you’ve done your trial and can advise this poor woman appropriately on whether it’s a good idea or not.

      I see similar in my breech clinic, where I sometimes find myself explaining why we cannot (without further cause) schedule the ELCS she has chosen before 39 weeks.

      Yes! We are doing BUMPES at JPUH. It’s our first bit of externally funded research in the maternity department. You’ll be pleased to hear I pushed so hard for us to take it on and recruit that the OB requested I be listed as co-PI 🙂 We’ve got a good R&D dept keeping an ear to the ground for future studies appropriate to our set-up, and Martin Cameron at Norwich has been inspirational in our LSG.

      1. jimgthornton

        We’re not that far apart are we? 🙂

        BUMPES, of course. That’s sitting v lying down in 2nd stage for women with epidural isn’t it? Brill! How’s it going?

        Note to self. Why aren’t we joining in that? 🙁

        1. midwifeshawn

          No, we’re not that far apart. If we’re similar enough that you wake up one morning with an irrational urge to help design a sound RCT of upright breech birth, do let me know! 😉

          BUMPES has gone very well for us. We’re winding down to the closing date of 31 Jan, but have already exceeded our recruitment targets significantly so pleased with our first effort. It suits our small unit, easy to keep in touch with delivery suite midwives, who are responsible for consenting. We have no research midwife – just me and another midwife who has picked up a few hours to help process the paperwork. I and the anaesthetist hoped participating would encourage midwives that women with epidurals can be mobile, but that hasn’t quite happened, for a number of reasons. However, taking on the study has helped convince some other key people that signing up to research isn’t just altruistic, as BUMPES income has funded the training of our PROMPT instructors this year.

  4. Margaret Jowitt

    I like observational studies as well, I believe we still have far too little knowledge on the physiology of normal labour, and we need observational studies at home where there is no extraneous pressure.

    Can anyone tell me, is induction done for the fear of ante-partum stillbirth? Is the study making a distinction between ante-partum and intra-partum stillbirth and neonatal mortality? What bothers me is that putting a fetus already at risk for some reason through the extra stress of induction might be dangerous in itself.

    I’m not convinced by this relatively new method of graphing the risk of ongoing pregnancy which seems to be driving induction. If only Hospital Episode Statistics were linked to infant outcome, we would have the largest possible observational/epidemiological study which, to my mind, would outrank even the largest RCT but of course still would be ranked lower than RCT evidence.


What are your thoughts?