Consultants often take the resuls of these scans as gospel, often booking you in for a C-Section or Induction as a result of them, but how accurate are they REALLY? - you might be surprised!
...and even IF your baby is big, what's to say that you will have problems giving birth? some women have a great deal of trouble giving birth to a small six pounds baby, whilst others, like the editor of this site, can birth an eleven pound baby wih ease.
One thing is for sure though and that is that nobody knows how much your baby will weigh until it is placed in the scales after birth!
I heard a great analogy from an obstetrician once, that likened trying to predict the size of a baby before birth, by ultrasound, to trying to guess the weight of a man, sitting in a bath full of water, in the room next door by measuring his waist and thigh bone. When you look at it like that it really does become apparent how ludicrous these gross measures we use are!
Ultrasound is widely believed to be the most accurate method of estimation of foetal weight.
Yet in 1988, Miller, Brown, Khawli, Pastorek & Gabert in "Ultrasonographic identification of the macrosomic fetus" found that the typical mean error ranges from 300 to 550g (11.6 to 19.4 oz). That's around a WHOLE POUND!
In 1992 Chauhan, Lutton, Bailey, Guerrieri & Morrison In "Intrapartum clinical, sonographic, and parous patients' estimates of newborn birth weight" found that ultrasound was the least accurate of the three methods, i.e: it was less accurate than the educated guess of the obstetrician or mother!
In their study "Pregnancy outcome following ultrasound diagnosis of macrosomia" Delpapa & Mueller-Heubach found that "In 66 of 86 women (77%) delivering within 3 days of ultrasound examination, estimated fetal weight exceeded birth weight. In only 41 of these 86 women (48%) were the estimated fetal weights within the corresponding 500-g category of birth weight" - That's quite some difference!
Limitations in the sensitivity and specificity of ultrasound have been observed in many other studies but sadly despite these well documented limitations, health professionals continue to incorrectly believe that ultrasound is an accurate way of predicting macrosomia, this also despite the UK Government's CESDI (Confidental Enquiry Into Stillbirths & Deaths in Infancy) report stating that "the inaccuracy of ultrasound estimates have been well documented. Indeed, it is possible that estimating fetal weight by late ultrasound may do more harm than good by increasing intervention rates".
I'm sure many people can recount a story of a friend or relative who has been told their baby would weigh 10lbs only to birth a perfectly average sized 8lber, or those, as myself with my first baby (who subsequently weighed 10lbs) who were told "it's completely average, a 7lber" only to find they were expecting a whopper.
As a general rule of thumb growth scans are quite good at plotting the growth rate of a single baby over a time period following several scans, but as a one off, arbitary measure, research suggests that "mother's intuition" is more accurate!
If you are offered a growth scan (note you do not "have to have" one) you could ask what the results will be used for? will the results be used to benefit you and your baby? you could ask how accurate they are at predicting birth weight? you could ask if they have any negative effects or risks attached to them? (such as leading to unnecessary inductions or C-Sections).
Some people also believe that Ultrasound scans themselves carry risks. Above all else, how will YOU feel about the result, how will you feel if you are told your baby is big? will it dent your confidence and potentially inhibit your labour? remember - the scans are being offered to you, as such you can politely decline them after carefully assessing the risks and benefits of them.
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