The risks of estimating an unborn baby’s weight

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

An important paper 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.

How can this be?

Let me explain a bit more. In this study:

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.

And even after the researchers adjusted to take other variables into account, the differences between the groups remained significant.

Are other factors at play?

As with all research studies, 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 documented in their medical records.

It’s VERY clear to me from my research on Plus Size Pregnancy that being weighed or having your weight estimated has a FAR more significant effect on care, outcomes and experiences than most people realise. 

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.

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

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

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