Ultrasound to estimate size – what of it?

Late pregnancy ultrasound to estimate your baby’s size is sometimes presented as a routine part of antenatal care. So what of it?

Evidence tells us that for low risk women, routine late pregnancy ultrasound has no benefit for mother and baby (Cochrane). Estimating baby’s size in utero is notoriously inaccurate and can be out by as much as 15%.

What we do know, however, is that if you measure large for gestational dates, the ‘big baby’ label alone – whether it’s accurate or not -  “can increase your chance of interventions during birth, and of experiencing complications caused by those interventions, even if the baby is not actually big” (Dr Rachel Reed). 

Before consenting to a late pregnancy ultrasound, it’s important to consider what may be the potential flow-on effects and how this may impact your birth experience.

For many women planning a vaginal birth, being told your baby is big or has a large head may cause unnecessary anxiety. How does this serve your preparation for birth? 

In terms of the impact on birth outcomes, unfortunately we know that induction is routinely offered for suspected big babies despite the fact it is not a medical reason to induce. One study showed women who were suspected of having a big baby were three times more likely to have an induction or caesarean, and were four times more likely to have complications such as severe perineal tearing and postpartum haemorrhage (Sedah-Mestechkin et al 2008).

Another study in the American Journal of Obstetrics and Gynacology found care providers were more likely to diagnose slow progress during labour and recommend caesarean among women with suspected big babies.

Many women enjoy ultrasounds as an opportunity to ‘see’ your baby, but it’s important to be aware of the potential cascade of intervention particularly of late pregnancy ultrasound.

Know your options, know the evidence. The choice is yours to make.

For further reading on suspected big babies and the impact of care provider fear, visit midwifethinking.com