Guess how many sweets are in the jar, or better yet, guess how much the sweets inside the jar weigh. Not including the jar itself but including all the sweets you can’t actually see in the photo….

I’ll give you a little help, you see that green one near the top left? Well the distance from the highest tip of that sweet to the lowest tip of the red stripy one down the bottom is… let’s say 25cm. NOW tell me how much the contents of the jar weigh.

What? You mean you can’t work it out?

No, I couldn’t either, neither can my mathematically gifted husband. Although he does argue that he is familiar with these kinds of sweet jars and would therefore be able to estimate that the sweets weigh 1kg. But he is missing my point a bit.

From looking at this picture alone, with one arbitrary measurement and without being able to see maybe 80% of the sweets in the jar, there is absolutely no way that with this information alone anyone would be able to estimate with any degree of accuracy how much the sweets weigh. My husband is right though, using his life experience, he can guess the weight fairly accurately. But we do know with some degree of certainty that these exact sweets were weighed before putting them in the jar prior to sale. The same cannot be said of a baby in the womb.

So why do so many people believe a sonographer when they say that their unborn baby weighs xlbs in utero? It’s the same sort of equation. An ultrasound gives the sonographer a view of a small section of the uterus at any one time and gradually builds up a picture as the transducer is moved across the abdomen. Three measurements are taken during a growth scan, the biparietal diameter (distance between the 2 sides of the head), femur length and the abdominal circumference. These measurements are entered into a computer program which then calculates the estimated weight based on charts created by researchers. You would think it would be fairly accurate. All that science.

The problem is that for one, the scan is only as good as the sonographer. All humans are capable of error. What one sonographer measures one day may be totally different to what another sonographer measures the next (on the same woman and baby). I have even heard of one sonographer giving a woman and her obstetrician quite a fright by writing down measurements that implied that her baby had SHRUNK! That was human error, babies don’t shrink, as the obstetrician she subsequently saw quite rightly pointed out, and later apologised to her and admitted that a mistake had been made.

But also, who made these charts and from what research? My best guess is that researchers have measured aborted and miscarried fetuses and premature babies in order to establish average lengths, diameters and circumferences at different gestational ages. I wonder how many samples they looked at before coming up with these charts? From how many different ethnicities? Did they account for growth abnormalities in unhealthy fetuses?

There is a wonderful bit of research that demonstrates the complete unreliability of growth scans in late pregnancy:

Pregnancy outcome following ultrasound diagnosis of macrosomia. 
AUTHORS: Delpapa EH; Mueller-Heubach E
AUTHOR AFFILIATION: Department of Obstetrics and Gynecology, University of Pittsburgh, Magee-Women’s Hospital, Pennsylvania.
SOURCE: Obstet Gynecol 1991 Sep;78(3 Pt 1):340-3
Extract: “In 66 of 86 women (77%) delivering within 3 days of ultrasound examination, estimated fetal weight (EFW) exceeded birth weight. In only 41 of these 86 women (48%) were the EFWs within the corresponding 500-g category of birth weight.”

So basically, this study (admittedly only a small one), found that 77% of growth scans predicted a higher birth weight three days BEFORE birth, so if these scans really are as accurate as we are led to believe then 77% of babies shrink in the last days of pregnancy. See above. More alarming is that 48% of scans, i.e. less than half, were accurate to within 500g, which is more than 1lb. What a large margin for error! The difference between a 9lb baby and a 10lb baby can be a HUGE difference to the mother and her care providers both psychologically and in terms of her care during late pregnancy and birth. To look at it the other way, more than half of scans were LESS accurate than that. 52% of women scanned were expecting either much bigger or much smaller babies than they got.

Why does it matter?

Because if a midwife sends a woman for a growth scan because of concerns late in pregnancy, and the obstetrician that the woman sees after her scan decides that, based on this one scan, her baby is too big or too small, then interventions may be advised that are simply not necessary. For example, if a baby is suspected to be too big, early induction or c-section will be advised. Except in the current NHS system the woman will come away thinking that it is not advice that she has been given, but an order to comply with.

Why does it matter?

Because interventions without clinical need are a waste of resources and may have long-lasting negative effects on the mother and baby. Caesarian sections are known to leave babies with more breathing problems, greater risk of diabetes and cause risk factors for the woman in future pregnancies, never mind the risk of post operative infection and post traumatic stress disorder.

Why put women through that? Why subject a woman at the end of her pregnancy to fears about the wellbeing of her baby? Even when the prediction of a too big or too small baby is accurate, how often does it actually mean ill health for the newborn? According to one study, “Our ability to predict macrosomia is poor. Our management policy of suspected macrosomic pregnancies raises induction of labor and cesarean delivery rates without improving maternal or fetal outcome.” (My emphasis).

The Delpapa and Mueller-Heubach study above also concludes that “A trial of labor resulted in vaginal delivery in 76 of 106 women (72%). There were five cases of shoulder dystocia but no birth trauma. Estimated fetal weights and birth weights were not significantly different between the women who had a trial of labor and those who did not. Our results do not support cesarean delivery or early induction as a means of preventing infant morbidity when fetal macrosomia (weight of 4000 g or more or the 90th percentile for gestational age) is diagnosed by ultrasound.”

So ultrasound is NOT an accurate estimate of fetal weight and even when a baby is genuinely large (4000g+), spontaneous labour and vaginal birth are still the safest option.