The risks of estimating an unborn baby’s weight

29056964601_04052ef07bWhat are risks of estimating an unborn baby’s weight? Any why on earth would this even carry risks?

An important paper has analysed the outcomes of over 64,000 women and their babies. And the analysis shows that the practice of estimating the baby’s weight before it is born is associated with a higher chance of the woman having a caesarean section.

Yes, just the act of estimation.


Here are the actual figures:

18.5% of women who had their baby’s weight estimated by ultrasound and documented in their notes had a caesarean section.

13.4% of women who had their baby’s weight estimated clinically and documented in their notes had a caesarean section. (In other words, a midwife or doctor estimated the baby’s weight by feeling the baby with their hands).

11.7% of women whose baby’s weight was not estimated/documented had a caesarean section.

All of the women were had reached 37 or more weeks of gestation, were planning a vaginal birth with a single, live, healthy, vertex (head-down) baby and had not had previous caesarean sections. Even after the researchers adjusted to take other variables into account, the differences between the groups remained significant.

As with all research, there are things that might have affected the results and we always have to interpret the findings with caution. For instance, we might ask ourselves why clinicians are more likely to estimate the weight of some babies, and whether that is because of another factor which might also increase the likelihood of an operative birth. But researchers do try to control for such variables, and we must not underestimate the effect that our human perceptions have on decisions. If clinicians think that a baby might be larger than average, for instance because a sonographer has recorded an estimated weight that is above the norm, then they might be quicker to suggest a caesarean section than if the fetal weight hasn’t been discussed, measured or documented.

This isn’t simple or clear-cut, but if fetal weight estimation carries a risk, then the one thing that concerns me most is this: women usually aren’t asked whether they want to have someone estimate their baby’s weight, and they certainly aren’t told about the possible risks and benefits of this screening test before it is performed on them and then document in their medical records. And, given the ramifications of this screening test, not to mention the fact that our estimation of fetal weight is often not that accurate (for who doesn’t have a friend whose baby was thought to be tiny when it was inside but turned out to look like a mini All Black upon arrival), I think we need to give some serious thought to whether it is justifiable to continue such a practice or whether it might be something that women and their families should be informed and asked about before it is performed.


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Froehlich, RRJ, Sandoval, G, Bailit, JL et al, for the Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD) Maternal-Fetal Medicine Units (MFMU) Network (2016). Association of Recorded Estimated Fetal Weight and Cesarean Delivery in Attempted Vaginal Delivery at Term. Obstetrics & Gynecology 128(3): 487-94. doi: 10.1097/AOG.0000000000001571


The Research

OBJECTIVE: To evaluate the association between documentation of estimated fetal weight, and its value, with cesarean delivery.

METHODS: This was a secondary analysis of a multicenter observational cohort of 115,502 deliveries from 2008 to 2011. Data were abstracted by trained and certified study personnel. We included women at 37 weeks of gestation or greater attempting vaginal delivery with live, nonanomalous, singleton, vertex fetuses and no history of cesarean delivery. Rates and odds ratios (ORs) were calculated for women with ultrasonography or clinical estimated fetal weight compared with women without documentation of estimated fetal weight. Further subgroup analyses were performed for estimated fetal weight categories (less than 3,500, 3,500–3,999, and 4,000 g or greater) stratified by diabetic status. Multivariable analyses were performed to adjust for important potential confounding variables.

RESULTS: We included 64,030 women. Cesarean delivery rates were 18.5% in the ultrasound estimated fetal weight group, 13.4% in the clinical estimated fetal weight group, and 11.7% in the no documented estimated fetal weight group (P<.001). After adjustment (including for birth weight), the adjusted OR of cesarean delivery was 1.44 (95% confidence interval [CI] 1.31–1.58, P<.001) for women with ultrasound estimated fetal weight and 1.08 for clinical estimated fetal weight (95% CI 1.01–1.15, P=.017) compared with women with no documented estimated fetal weight (referent). The highest estimates of fetal weight conveyed the greatest odds of cesarean delivery. When ultrasound estimated fetal weight was 4,000 g or greater, the adjusted OR was 2.15 (95% CI 1.55–2.98, P<.001) in women without diabetes and 9.00 (95% CI 3.65–22.17, P<.001) in women with diabetes compared to those with estimated fetal weight less than 3,500 g.

CONCLUSION: In this contemporary cohort of women attempting vaginal delivery at term, documentation of estimated fetal weight (obtained clinically or, particularly, by ultrasonography) was associated with increased odds of cesarean delivery. This relationship was strongest at higher fetal weight estimates, even after controlling for the effects of birth weight and other factors associated with increased cesarean delivery risk.

photo credit: Thiméo.G Shooting via photopin (license)

4 comments for “The risks of estimating an unborn baby’s weight

  1. Sarah
    September 8, 2016 at 3:22 pm

    I went through a week of tests, because I refused a sweep at 38wks when registar said I was having a big baby, I pointed out that fundal height charts were not especially accurate, ultrasounds in previous week’s hadn’t suggested a bigger baby than last time and I hah had two short unassisted second stages of labour. Midwife seemed bemused when I turned up for fetal heart monitoring and son 2 turned up at 41 wks at a whopping 8lb 2oz, 2oz more than son 1. But, I came out of initial consultation wondering if I was risking my babies health and crying on phone to my other half

  2. September 9, 2016 at 12:55 am

    Some one has finally noticed. The speech given for Caesarean birth is even more sinister and terrifies parents. ‘Do you want your (small,) baby to die. Get stuck( larger). The criminality of the Caesarian rate is obscene. I hope your research is read and digested. The $ sign is the powerful reason alone 37 weeks is so. OBGYN does not have to get up for a labouring woman. 37 weeks is normal.

  3. Dominique Mylod
    September 9, 2016 at 9:59 am

    Dear Sarah

    Thank you for publishing this study because it reflects my concerns with the NPEU GROW project. I am seriously worried at the predicted optimal weights of babies and NPEU has not answered my question about the cohort of women they used. The estimation, from my view, is flawed because it is so crude. They anticipate that bigger women will have bigger babies and since the average body size has increased over the last few decades, they’re predicting optimum weights of 4000g for some primips (!) and we’ve even had a 3200g baby put on the hypoglycaemia protocol for being SGA. In addition, many of the women have their ethnic background incorrectly classified. So on the one hand women are being serially scanned and induced for ‘SGA’ babies; on the other hand they’re being projected to have bigger babies with increased CS, 3rd / 4th degree tears etc. This strikes me as a) junk science b) iatrogenesis. I would welcome your thoughts.

    • Lisa
      September 9, 2016 at 9:46 pm

      WOW induction for SGA. You’d think they’d allow the babe more growing time inutero.
      Check out “The Whole Nine Months” a Western Australian research project stating pregnancy should be maintained until a minimum of 39 weeks gestation for neurodevelopment & prevention of behavioural concerns in the child. This is of course in a normal, healthy pregnancy with no risk factors.

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