The use of ultrasound to estimate fetal weight

Despite the evidence suggesting that Ultrasounds are quite inaccurate at estimating baby’s weight while in utero, clinicians are still turning to ultrasound weight estimate as a form of assessment of the unborn babies wellbeing:

I am currently caring for two clients who have been directly affected by inaccuracies of ultrasound estimation of fetal weight.  My first client had her first baby 2 years ago with an obstetrician who suspected my clients baby may be small by fundal height measurements in cm.  The obstetrician ordered three ultrasounds from 34 weeks which showed average fetal growth.  At 41 weeks, when in early spontaneous labour, my client’s unborn baby showed signs of fetal distress and her baby was born by caesarean section, and he weighed approximately 2100g, which was 1-1.2kg difference from what the ultrasounds had estimated.  Her baby was born requiring significant resuscitation, and gladly he is well and developing normally post birth.

Recently I cared for a client who at 38+5 weeks gestation measured around 34 weeks gestation by clinical fundal height measurement.  An ultrasound was ordered and her baby was estimated to weigh 2100g, which is below the 5th centile for his gestation.  So my client was induced and two days after the ultrasound, she gave girth to a 2990g baby, a completely normal size for his gestation.  Although the induction of labour went well, it was an intervention which clearly could have been avoided if there was a more accurate way of assessing fetal growth and weight in the womb.

It is well known and has been well researched that estimating fetal weight by ultrasound is inaccurate in its estimation but it is widely used and relied upon for the estimation of fetal weight.  This inaccuracy can be either way ie estimating that the baby weighs more or less than it actually does.

The literature supports that it is not uncommon for ultrasounds to be between 8-35% inaccurate in assessing fetal weight  This inaccuracy seems to be greater in women with a high BMI, and for babies that are higher in birth weight >4000g

The literature has also studied the various ways fetal weight can be estimated, ie AC, FL (see article to see whether there is one method more reliable than the other.

Low and high birth weight is associated with adverse outcomes for Mothers and babies and it is important for the clinician to be able to determine accurately whether or not a fetus is growing well or potentially compromised within the womb to prevent serious adverse outcomes from occuring.

From the available evidence clinicians are not able to rely on ultrasound to be a very accurate method of determining fetal weight; .  At this stage as there is no reliable method for assessing fetal weight in utero, clinicians need to continue with some kind of clinical assessment of fetal weight and wellbeing (fundal height measurements and fetal movements), and if there are concerns about fetal wellbeing it is important to assess the overall clinical picture and to assess each woman individually based on her own individual circumstances and not to rely on the findings of ultrasound alone;

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