Big Babies – The Curse of (mis) Diagnosing a Macrosomic Infant Part.1

Thousands of women are diagnosed with suspected big babies (or ‘macrosomia’ to use the medical terminology) every year, but this one simple label can have a profound effect on their birth.
Obstetric consultants often take the results of growth scans as gospel, scheduling elective C-Sections or inductions as a result of them, but how accurate are these estimates? and does early induction or elective C-Section really prevent problems if a baby is extra large? – you might be surprised at the answer!
…and even IF the baby is big, what’s to say that the mother will have problems giving birth? Some women have a great deal of trouble giving birth to a small six pounds baby, whilst others, like myself, can birth an eleven pound baby with ease. Then there is the issue of the nocebo effect – the creation of a medical problem when there was none initially, in short if you go looking for problems with ‘a big baby’, you dent the mother’s confidence, you induce her before her body and baby are ready and keep her immobile by monitoring constantly throughout her labour – the chances are her birth will be problematic, not because of the size of her baby – but because of the management of her labour. Yes the nocebo effect is particularly strong in modern day obstetrics.
So, even if the growth scan estimate is accurate how can anyone possibly know how YOU will birth? The only sure thing is that nobody knows how much your baby will weigh until it is placed in the scales after birth, or how your birth will go until after the event. 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!
So How Accurate are Ultrasound Estimates of Foetal Weight?

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 patientsestimates 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, arbitrary measure, research suggests that “mother’s intuition” is more accurate!
So What to Do if You are Offered a Growth Scan?
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? For instance, will the results be used to benefit you and your baby? You could ask how accurate they are at predicting birth weight? Or 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.
Why Are Some Babies Much Bigger Than Others?

If you believe the media you will probably think it is because we all eat too many chips. In reality there’s an awful lot more to it than that. It is true that women who develop Gestational Diabetes are more likely (a twofold risk in fact) to give birth to a larger infant, however this site is interested in those women who have not been labelled with any medical disorder and who seem to “just make big babies”. There are several known risk factors, aside from Gestational Diabetes, which can highlight certain women who are more predisposed to birthing a macrosomic baby, these are:
  • Multiparity (not your first baby)
  • Birthing a previous macrosomic baby
  • A prolonged period of gestation
  • Carrying a male baby
  • Parental Stature
  • Excessive weight gain during pregnancy
Genetics does seem to play a role though and often women will give birth to a very large baby with none of the above risk factors – in fact according to Boyd, Usher & McLean and their report “Fetal macrosomia: prediction, risks, proposed management” – 34% of macrosomic babies are indicated to be born to mothers with NO risk factors! similarly very obese women are just as likely to deliver smaller infants. Finally even with TWO of the above risk factors present, women are only 32% likely to have a macrosomic infant.
Why are We So Terrified of Birthing a Big Baby?
The female pelvis is PERFECTLY designed for birth… matter the size of the baby… is designed to open up to make more space for the baby…….and a baby’s head is PERFECTLY designed to gently mould, to make it smaller, so that it passes through the female pelvis with ease. Babies know how to get into a good position for birth, tucking their head tightly so that the smallest part presents first.
But when a women reduces her pelvic capacity, by being immobile on a bed, perhaps due to an induction – for a “big baby” or an epidural, because of the pain caused by the induction, or the fear caused by the constant “big baby” conversations everything gets a lot harder, words like”cephalopelvic disproportion” (CPD) – where the baby’s head is too big to pass through the pelvis and “shoulder dystocia” – where the baby’s shoulders get stuck – get used – needlessly.

If a woman enters labour free from fear and anxiety oxytocin (the hormone of labour) will be free to flow, her uterus will contract efficiently, endorphins (natural morphone like pain relievers released during labour) will flood her body, adrenaline will be kept to a minimum ensuring that her uterus is well oxygenated and making her as comfortable as possible. She will move instinctively into positions which freely open her pelvis, such as a squatting position (where the pelvis is said to have up to 30% more capacity) or perhaps on all fours – both superb positions for birthing a big baby.

The size of a baby then in a normal, physiological birth – where anxiety and “big baby” talk is not present – is largely irrelevant, it doesn’t make it more painful and it doesn’t make it harder! The following article, by midwife Gloria Lemay, is a MUST read for anybody worrying about birthing a large baby and wondering whether their pelvis is “big enough” Pelvises I have known and loved.




So, Is an Induction of Elective C-Section Not Necessary for a Suspected Big Baby Then? 

You may have been advised that it would be best for you and your baby if you are induced early? or perhaps an elective C-Section has been advised. You might have been told that you baby is much more likely to get stuck (also known as “shoulder dystocia”), but does the research agree?

Actually the vast majority of research into Macrosomic babies indicates that elective C-Section and induction for just a suspected large baby is a bad idea, deeming it unneccessary, needlessly expensive and not effective at preventing birth trauma to the mother or baby. When looking at induction the research is in fact even more scathing, with many suggesting that early inductions only serve to raise caesarean section rates without altering outcomes for the mums or babies.

Finally, just suspecting a macrosomic baby can lead to problems, with one study indicating that the risk of cesarean section was substantially higher (52 versus 30 percent) in pregnancies in which macrosomia was suspected, even after controlling for birth weight and other confounding variables. More importantly, the difference in the cesarean section rate was attributable to a greater proportion of failed inductions for the macrosomia in the group in which it was suspected.

Lets take a quick whistle stop tour of clinical research into suspected foetal macrosomia and birth outcome. Lets start with my favourite, a Cochrane Review looking at “Induction of labour for suspected macrosomia”, April 1998 suggested that elective C-Sections and inductions for a suspected big baby was a bad idea. The summary of the review is as follows. Important points have been highlighted:

“Babies who are very large (macrosomic – over 4500 g) can sometimes have difficult and, occasionally, traumatic births. One suggestion to try to reduce this trauma and to reduce operative births has been to induce labour before the baby grows too big. However, the estimation of the baby’s weight in utero is difficult and not very accurate. Clinical estimations are based on feeling the uterus and measuring the height of the fundus of the uterus, and both are subject to considerable variation. Ultrasound scanning is also not accurate. Induction, if undertaken too early, can lead to babies being born prematurely and with immature organs. The review of trials, assessing induction of women when it was suspected that their baby was above 4 kg, found three trials involving 372 women, none of them with diabetes. There was no evidence of any benefit in terms of caesarean section or instrumental births, or in outcomes for the baby. However, these studies were too small to be sure of the outcomes. Further research is in progress.”

The results section of the Cochrane Review also states: “Perinatal morbidity was not statistically different between groups (shoulder dystocia).”

In the Archives of Gynecology & Obstetrics, September 2008. Sadeh-Mestechkin, Walfisch, Shachar, Shoham-Vardi, Vardi & Hallak, in their study entitled “Suspected macrosomia? Better not tell” noted that:  “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.”

In 1994, in The Chinese Medical Journal, Yan, Chang & Yin, in their article “Elective cesarean section for macrosomia?” studying the births of 207 macrosomic babies concluded that: “Elective Cesarean section on macrosomic infants to prevent dystocia is not recommended because most of them can be delivered vaginally.”

In 2000, in the European Journal of Obstetrics, Gynecology & Reproductive Biology, Mocanu, Greene, Byrne & Turner studied the births of 828 macrosomic babies born over a 5yr period in their report entitled “Obstetric and neonatal outcome of babies weighing more than 4.5 kg: an analysis by parity” they concluded that:  “The poor antenatal predictability of macrosomia, the high rate of vaginal delivery and the low incidence of shoulder dystocia would not support the use of elective caesarean section for delivery of the macrosomic infant either in primigravidae * or multigravidae.”

* Editor’s Note: Primigravidae = First Time Mother, Multigravidae = Second or More time mother.

In april 1995 in Obstetric Gynecology in their research “The outcome of macrosomic infants weighing at least 4500 grams: Los Angeles County + University of Southern California experience” looking at 227 births of macrosomic babies, Lipscomb, Gregory & Shaw noted that:  “Vaginal delivery is a reasonable alternative to elective cesarean for infants with estimated birth weights of at least 4500 g, and a trial of labor can be offered.”.

In 2006 in the American Journal of Obstetrics & Gynecology, Chauhan, Grobman, Gherman, Chauhan, Chang, Magann and Hendrix reviewed the evidence for performing an elective ceasarean or an induction for suspected macrosomia, in their article”Suspicion and treatment of the macrosomic fetus: a review.” they noted that:  “Due to the inaccuracies, among uncomplicated pregnancies suspicion of macrosomia is not an indication for induction or for primary cesarean delivery.”

In 1996, Rouse, Owen, Goldenberg, Cliver, in “The effectiveness and costs of elective cesarean delivery for fetal macrosomia diagnosed by ultrasound” published in JAMA 1996.  estimated that “to prevent one case of permanent brachial plexus injury, 3,700 women with an estimated fetal weight of 4,500 g would need to have an elective cesarean section for suspected macrosomia at a cost of $8.7 million per case prevented.

Hmmm – perhaps that early induction isn’t looking like such a good idea after all? What are your birth options then? READ ON – to see part two – Birthing a Big Baby.


About SarahOckwell-Smith

Sarah Ockwell-Smith, Parenting author and mother to four.
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