It is fairly standard practice in maternity care to weigh pregnant women, and measure (or ask about) their height, and to calculate their body mass index. The body mass index, or BMI, is used to divide people up into categories of “underweight” (less than 18.5), “healthy weight” (18.5 to 24.9), “overweight” (25 to 29.9), and various levels of “obesity” (above 30)*. BMI is not a great tool and doesn’t correlate with health outcomes in a clear way. (Sara Wickham has written about some of the issues complicating the BMI – health relationship here and here if you are interested to read more.) BMI assessment is commonly used to divide women up into lower and higher risk categories in maternity care and to offer different care options on the basis of this distinction.

What do the guidelines say about BMI and CTG use?

Here in Australia, women with a BMI of 40 or more will be advised to have continuous intrapartum CTG monitoring, and women with a BMI in the 30 to 40 range will be given the same recommendation if they have one other minor risk factor (like being aged 40 to 42, or being between 41 weeks and 41 weeks and 6 days pregnant) (RANZCOG, 2019). The latest version of the NICE (UK) guidelines doesn’t specifically mention BMI or weight as an indication for CTG monitoring, but it perhaps is advised in practice on the basis of the recommendation to “consider continuous CTG monitoring if, based on clinical assessment and multidisciplinary review, there are concerns about other antenatal factors not listed above that may lead to fetal compromise” (p. 12). Similarly, the “Physiological CTG Interpretation” guideline (Chandraharan, et al., 2018) doesn’t specifically mention BMI or weight but says that “any condition which is thought to increase the risk of fetal hypoxia mandates continuous electronic fetal monitoring”. [Please note that despite the language used here, CTG monitoring is a choice, and it cannot ever be mandated.]

The Irish National guideline advises CTG monitoring for women with a BMI of more than 35 (p. 11). Canadian guidelines (Liston et al, 2018) advise CTG monitoring for women with “morbid obesity”) (typically defined as BMI in the range 40 to 45) (p. e309). The FIGO guidelines don’t have a list of risk factors for which CTG monitoring is advised, instead they list “conditions required for considering and maintaining intermittent auscultation in settings where cardiotocography is available” (Lewis, et al., 2015, p. 11) with no mention of weight or BMI. Some may argue the advice that women should have “no serious previous maternal health conditions” should exclude an offer of intermittent auscultation for a woman with a higher BMI.

So – there is a range of different approaches with some clearly including women with higher BMI in advice for intrapartum CTG monitoring, while others remain unclear. When CTG monitoring is advised, the BMI cut offs for offering this range from 30 to 40.

There must be evidence of higher BMI being associated with worse outcomes if it is being recommended, right?

Before we tackle this one, let’s make sure we are all starting from the same beginning. The goal of intrapartum fetal heart rate monitoring (of any type) is to prevent death or injury during labour from low oxygen levels (hypoxia). So, for the recommendation to offer intrapartum CTG monitoring to women with higher BMIs to make sense, you would want to see evidence that this group of women are more likely to experience one or more of the following:

• Stillbirth during labour due to hypoxia
• Neonatal death due to hypoxia in labour
• Neonatal seizures or other signs of brain injury due to hypoxia in labour
• Long term neurological problems due to hypoxia in labour

The only guideline offering evidence in support of their recommendation for intrapartum CTG monitoring for women with a higher BMI is the RANZCOG guideline. They cite two sources. The first was a narrative literature review calling for action on stillbirth prevention (Flenady, et al., 2011). The literature review mentioned a BMI of more than 25 as a “potentially modifiable risk factor” (p. 1704) with an emphasis on weight management during pregnancy, rather than using CTG monitoring to modify this risk.

The other source of evidence was a research paper (using BMI more than 30) (Locatelli et al., 2010). The aim of this research was to identify a broad range of antenatal and intrapartum risk factors for hypoxic-ischaemic encephalopathy affecting babies born at term. The study was small and collected data about births from 1993 to 2003 in one hospital in Italy. Locatelli et al. found women who gave birth to an infant who developed hypoxic-ischaemic encephalopathy were more likely to have a BMI of over 30 (2 of the 100 of the women with healthy babies and 4 of the 27 of the women with babies with hypoxic-ischaemic encephalopathy had a BMI over 30). What they don’t tell us is the proportion of women with a BMI over 30 who give birth to an infant with hypoxic-ischaemic encephalopathy, and the proportion of women with a BMI of 30 or less who give birth to an infant with hypoxic-ischaemic encephalopathy. These are the numbers needed to counsel women about the size of any additional risk.

What other evidence is there?

I don’t think either of these papers presents a compelling argument for increased risk for this group of women. So I went to PubMed and searched to see what I could find for myself. It was tricky to find current (past ten years) studies, conducted in high income countries (because that is where CTGs are in use) where the cause of death and absolute risk numbers were given, and therefore provide enough detail to answer the question of whether women with higher BMI are at increased risk for one or more events that intrapartum fetal heart rate monitoring has been proposed to prevent. Some studies used pre- or early pregnancy BMI, others looked at weight gain, and some used populations of women whose BMI changed (or didn’t) from one pregnancy to the next. Some corrected for other known risk factors, like diabetes and hypertension, that can complicate the pregnancies of women with higher BMI and are in themselves associated with higher rates of poor outcomes, most did not.

Suffice to say – getting good information to provide an accurate risk assessment for a woman who has a higher BMI, but no other complications in her pregnancy, was quite the challenge. This is what I ended up with…

Intrapartum Stillbirth

Using a population of women from the USA, based on pre-pregnancy BMI, Bodnar, et al (2015), established a rate of intrapartum stillbirth of 2 per 1000 births for women with a BMI of under 25. While the rate rose slightly with higher BMI, it did not become statistically significant. The rate of intrapartum stillbirth was 2.2 per 1000 for women with a BMI between 25 and 29.9 (risk ratio 1.2), 2.5 per 1000 for women with a BMI between 30 and 34.9 (risk ratio 1.2), and for women with a BMI of 35 or over the rate was 3.3 per 1000 (risk ratio 1.4).

The figures from this study still overestimate the risk for the causes of stillbirths that can be modified (theoretically at least) by CTG monitoring, as they included non-hypoxic causes like infection and congenital abnormalities, and some like abruption that evolve rapidly, are clinically obvious without CTG use, and where there may be no time for clinical management to make a difference to the outcome.

As Mohammed et al. (2020) point out, the findings from research linking stillbirth with BMI are inconsistent. In their study, they did not break their findings down by the timing of the stillbirth (therefore including fetal deaths prior to labour onset), but they did look at outcomes for women with higher BMIs but no comorbidities. The stillbirth rate increased from 2 per 1000 in women with BMIs in the 18 to 24.9 rate to 3 per 1000 for women with a BMI of 35 and over, an increase that was not significantly different.

Neonatal death

I was unable to locate any current research where deaths were stratified by cause to focus only on those secondary to hypoxia. From a population of women from low and middle income countries, Wu et al. (2020) reported a neonatal death rate of 13 per 1000 for women with BMIs in the 18 to 24.9 range, rising to 14 per 1000 for women with a BMI of 30 to 34.9 (an odds ratio of 1.32 and a statistically significant increase), and 15 per 1000 for women with a BMI of 35 or over (an odds ratio of 1.56, also statistically significant).

In the USA, Yu et al (2020) looked at the relationship between change in BMI from one pregnancy to the next and neonatal deaths. They noted an additional 2.4 deaths per 1000 births for women whose BMI rose to over 30, compared to those whose BMI did not change from one pregnancy to the next. Interestingly, the neonatal death rate also increased for women whose BMI fell into the underweight range, and the rise in mortality happened for a smaller change in BMI than was the case when BMI increased. We don’t see guidelines recommending CTG monitoring for women who lose weight from one pregnancy to the next.

Hypoxic ischaemic encephalopathy

I was able to locate two studies to provide information about the risk of hypoxic ischaemic encephalopathy. Studying a USA based population, Monaco-Brown et al (2022) reported a rate of hypoxic ischaemic encephalopathy of 0.9 per 1000 births for women with a BMI between 18 and 24.9, and 1.6 per 1000 for women with a BMI of 30 or over, a statistically significant increase with an adjusted odds ratio of 1.73. Also from the USA, Kureshi et al. (2022) reported a rate of hypoxic ischaemic encephalopathy of 2.0 per 1000 for women with a BMI of under 30, and of 1.9 per 1000 for women with a BMI or 30 or over, not significantly different.

So where does that leave us?

There might be an increase in the risk of intrapartum stillbirth, neonatal death, and hypoxic ischaemic encephalopathy for women with higher BMI, but the findings are not consistent across studies. The absolute difference in risks, even when you add all three outcomes together, is under one percent. It is likely that women with health conditions complicating their pregnancy contribute more to this increase than healthy larger women, but I simply can’t find sources of evidence to sort that out reliably.

Having established there is a marginal increase in risk (maybe), to make a difference at a population level to the uncommon issue of poor perinatal outcome for women with higher BMI, you would need a potently effective intervention. So, this brings us to the point where someone has now recommended intrapartum CTG monitoring. Is there evidence CTG monitoring is the potent tool that can reverse any increased risk?

What do the RCTs say?

The Alfirevic et al 2017 Cochrane review summarises all the randomised controlled trials that have compared CTG use in labour with intermittent auscultation. There were eleven in total. I pulled each one out of the depths of my Endnote library and re-read them to see whether they made any mention at all about BMI or weight. With the exception of one trial, none used the term BMI anywhere in the paper. It was never used to distinguish women into low or high risk categories, even though these studies span low and high risk populations. Nor did they describe body weight anywhere.

The only trial mentioned that weight was the 1986 Copenhagen based trial published by Neldam et al. In a table titled “clinical condition of mothers” weight and height were listed along with age and the number of previous pregnancies the women had. The mean weight for women allocated to CTG monitoring was 58.6 kg, and for those allocated to intermittent auscultation it was 59.4 kg. Based on the height also given in the table, I calculated the average BMI of the two groups to be 21.

Further down the same table, listed as a reason for categorising some women as high risk was the term “adipositas”, which I assume refers to adiposity or “fatness”. The term was not further defined so I don’t know whether this was based on weight or BMI, nor at what number they distinguished between people who made it on this list and those who didn’t. Women in this category made up 16% of those allocated to CTG monitoring and 14% of those allocated to intermittent auscultation. Outcomes from women in this category were not reported separately from the total population. This trial showed no differences in perinatal mortality, seizures, Apgar scores at 5 minutes of age, nursery admissions, or the use of fetal blood sampling between those allocated to CTG monitoring and those allocated to intermittent auscultation.

But CTG monitoring is harmless, why not do it just in case it works?

As you have seen me write over and over – overall the evidence regarding CTG use in women considered as higher risk shows no convincing evidence of benefit for the baby and possibly long term harm in those born preterm. It is possible that we might make outcomes worse for babies born to larger women by using CTG monitoring, but we don’t know, because no one has ever bothered to research that question. Not knowing there is harm is not proof there is no harm.

What we do have clear evidence for is the association between CTG use and caesarean section. The Cochrane review reports a statistically significant risk ratio of 1.63 (in other words a 63% increase in the use of caesarean section for women in the CTG arms of trials), and a risk ratio of 1.91 for women considered to be at higher risk. Recent literature reviews place the increased risk of caesarean section for larger women at somewhere between two to three times higher than women in the 18 – 24.9 BMI range (Marchi, et al., 2015). It isn’t possible to unpick the degree to which CTG monitoring contributed this increase, as it is likely that most women with higher BMI were exposed to CTG monitoring. Caesarean section poses additional risks for women and their babies, both in the short term and the long term. Larger women are more likely to develop short term perioperative complications, such as wound infection (Marchi, et al., 2015).

CTG monitoring is challenging in women with higher BMI

The aim of continuous intrapartum CTG monitoring is to be continuous. That is, to record every single heartbeat of the fetus from the time the woman presents for care, to when she gives birth. In theory at least, gaps in recording the heart rate might increase the possibility that abnormal heart rate patterns remain undetected and therefore go unmanaged. There is evidence that externally worn Doppler (heart rate) and tocodynamometer (contractions) sensors are less reliable in women of higher BMI (Cohen, 2017; Euliano, et al., 2017).

Consequently, women with high BMI are often advised to use either, or both, a fetal spiral electrode (to record the fetal heart rate) and an intrauterine pressure catheter (to record uterine activity) to achieve a closer to continuous recording of CTG data. Both require the amniotic membranes to be open and require vaginal examination to place them. And both introduce additional risks of complications for the woman and her baby. There are case reports of uterine rupture with intrauterine pressure catheter insertion (Rood, 2012). Maternal infection is more common with fetal spiral electrode use (Kawakita, et al., 2016). Fetal spiral electrodes increase the risk of scalp trauma, cephalohaematoma, necrotizing fasciitis, intracranial abscess, and sepsis for the baby (Davey & Moore, 2006; Fick & Woerdeman, 2021; Kawakita, et al., 2016). While these are all rare, they can be avoided by using intermittent auscultation.

Intermittent auscultation also presents challenges, but as long as the heart rate can be heard over a long enough period and often enough to establish what is going on (typically 60 seconds in every 15 minutes), the Cochrane review findings reassure us that this is as effective as CTG use without the added risks that CTG use presents.

In summary…

Women with higher BMI might have marginally higher risks for poor outcomes relating to low oxygen levels in labour, but there remains some doubt about this, particularly for women with no health problems. There is evidence to suggest CTG monitoring might be harmful for women with a higher BMI, and no evidence to speak of to know whether or not it could possibly improve outcomes for the baby. Guideline advice favouring CTG use for women of high BMI isn’t built on a sound evidence base.

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* I choose not to use these terms, because they are considered problematic by many women with higher BMIs. My issue with the terms mostly relates to the inaccurate assumptions inherent in the term “health weight”. Most women with BMIs above and below this are healthy. And not all women in the “healthy weight” range are healthy. Health and weight are not the same concepts.

References

Alfirevic, Z., Devane, D., Gyte, G. M. L., & Cuthbert, A. (2017, Feb 03). Continuous cardiotocography (CTG) as a form of electronic fetal monitoring (EFM) for fetal assessment during labour. Cochrane Database of Systematic Reviews, 2(CD006066), 1-137. https://doi.org/10.1002/14651858.CD006066.pub3

Bodnar, L. M., Parks, W. T., Perkins, K., Pugh, S. J., Platt, R. W., Feghali, M., Florio, K., Young, O., Bernstein, S., & Simhan, H. N. (2015, Oct). Maternal prepregnancy obesity and cause-specific stillbirth. American Journal of Clinical Nutrition, 102(4), 858-864. https://doi.org/10.3945/ajcn.115.112250

Cohen, W. R. (2017, Nov). Clinical assessment of uterine contractions. International Journal of Gynaecology & Obstetrics, 139(2), 137-142. https://doi.org/10.1002/ijgo.12270

Davey, C., & Moore, A. (2006). Necrotizing fasciitis of the scalp in a newborn. Obstetrics & Gynecology, 107(2), 461-463. https://doi.org/10.1097/01.AOG.0000164094.02571.77

Euliano, T. Y., Darmanjian, S., Nguyen, M. T., Busowski, J. D., Euliano, N., & Gregg, A. R. (2017). Monitoring fetal heart rate during labor: A comparison of three methods. Journal of Pregnancy, 2017(6), 8529816-8529815. https://doi.org/10.1155/2017/8529816

Fick, T., & Woerdeman, P. A. (2021). Neonatal brain abscess development following fetal scalp electrode placement: a rare complication. Child’s Nervous System, 38(1), 199-202. https://doi.org/10.1007/s00381-021-05150-7

Flenady, V., Middleton, P., Smith, G. C., Duke, W., Erwich, J. J., Khong, Y. T., Neilson, J., Ezzati, M., Koopmans, L., Ellwood, D. A., Fretts, R., Frøen, J. F., & Lancet’s Stillbirths Series steering committee. (2011). Stillbirths: the way forward in high- income countries. Lancet, 377(9778), 1703–1717. https://doi.org/10.1016/s0140-6736(11)60064-0

Health Service Executive National Women and Infants Programme. (2022). National Clinical Guideline for Intrapartum Fetal Heart Rate Monitoring. https://www.hse.ie/eng/about/who/acute-hospitals-division/woman-infants/clinical-guidelines/national-clinical-guideline-for-intrapartum-fetal-heart-rate-monitoring-2021-.pdf

Kawakita, T., Reddy, U., Landy, H., Iqbal, S., Huang, C.-C., & Grantz, K. (2016). Neonatal complications associated with use of fetal scalp electrode: a retrospective study. British Journal of Obstetrics & Gynaecology, 123(11), 1797-1803. https://doi.org/10.1111/1471-0528.13817

Kureshi, A., Khalak, R., Gifford, J., & Munshi, U. (2022). Maternal obesity-associated neonatal morbidities in early newborn period. Frontiers in Pediatrics, 10, 867171. https://doi.org/10.3389/fped.2022.867171

Lewis, D., Downe, S., & Panel for the FIGO Intrapartum Fetal Monitoring Expert Consensus Panel. (2015, Oct 01). FIGO consensus guidelines on intrapartum fetal monitoring: Intermittent auscultation. International Journal of Gynecology and Obstetrics, 131(1), 9-12. https://doi.org/10.1016/j.ijgo.2015.06.019

Locatelli, A., Incerti, M., Paterlini, G., Doria, V., Consonni, S., Provero, C., & Ghidini, A. (2010). Antepartum and intrapartum risk factors for neonatal encephalopathy at term. American Journal of Perinatology, 27(8), 649–654. https://doi.org/10.1055/s-0030-1249761

Liston, R. M., Sawchuck, D., & Young, D. (2018, Apr 01). No. 197b – Fetal health surveillance: Intrapartum consensus guideline. Journal of Obstetrics and Gynaecology Canada, 40(4), e298-e322. https://doi.org/10.1016/j.jogc.2018.02.011

Mahomed, K., Chan, G., & Norton, M. (2020, Dec). Obesity and the risk of stillbirth – A reappraisal – A retrospective cohort study. European Journal of Obstetrics & Gynecology and Reproductive Biology, 255, 25-28. https://doi.org/10.1016/j.ejogrb.2020.09.044

Marchi, J., Berg, M., Dencker, A., Olander, E. K., & Begley, C. (2015). Risks associated with obesity in pregnancy, for the mother and baby: a systematic review of reviews. Obesity Reviews, 16(8), 621-638. https://doi.org/10.1111/obr.12288

Monaco-Brown, M., Munshi, U., Horgan, M. J., Gifford, J. L., & Khalak, R. (2022). Association of maternal obesity and neonatal hypoxic-ischemic encephalopathy. Frontiers in Pediatrics, 10, 850654. https://doi.org/10.3389/fped.2022.850654

National Institute for Health and Care Excellence. (2022). Fetal monitoring in labour. www.nice.org.uk/guidance/ng229

Rood, K. (2012). Complications associated with insertion of intrauterine pressure catheters: an unusual case of uterine hypertonicity and uterine perforation resulting in fetal distress after insertion of an intrauterine pressure catheter. Case Reports in Obstetrics & Gynecology, 2012, 517461. https://doi.org/10.1155/2012/517461

Royal Australian and New Zealand College of Obstetricians and Gynaecologists. (2019). Intrapartum fetal surveillance clinical guideline. 4th Edn. https://ranzcog.edu.au/statements-guidelines

Wu, H., Liu, F., Zhao, M., Liang, Y., & Xi, B. (2020, Nov). Maternal body mass index and risks of neonatal mortality and offspring overweight and obesity: Findings from 0.5 million samples in 61 low- and middle-income countries. Pediatric Obesity, 15(11), e12665. https://doi.org/10.1111/ijpo.12665

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Categories: CTG, EFM, IA, Obstetrics, Perinatal brain injury, Perinatal mortality, Stillbirth

Tags: BMI, caesarean section, Evidence, guidelines, obesity