Fetal macrosomia: what to do if the fetus is large

Source: Shutterstock

If the fetus is large

I am close to term and my son is estimated at 4 kg. Better to do a caesarean?

It is a question that many women with this condition ask themselves. After all, when the fetus is large - technically we speak of macrosomia - in view of delivery fears and worries increase, because in fact there may be some extra risk for both the baby and the mother.

Let's see what risks it is and under what conditions it is decided to intervene, with the induction of childbirth or the planned caesarean section. We do it with the help of the gynecologist, an expert in the pathology of pregnancy at the Sacco Hospital in our city.


Pictures with the development of the fetus from 10 to 40 weeks

go to the gallery

The images with the development of the fetus from the tenth week of gestation to the fortieth week. The development in 12 images recreated on the computer.

In this article

  • The fetus is large if ...
  • Because a fetus can be large 
  • How a fetus is determined to be large   
  • The risks of a fetus that is too large
  • What to do when the child is estimated to be large
Read also: The development of the fetus in the 40 weeks

The fetus is large if ...

To begin with, you have to understand when we can really talk about macrosomia. Until recently, an estimate of 4 kg at term was often indicated as the threshold beyond which a fetus was considered large. "Different guidelines still give different definitions, but they tend to be we speak of macrosomia for an estimated weight over 4,5 kg"he says. This is, for example, the position of ACOG, the American College of Obstetricians and Gynecologists.

According to what was reported by the English gynecologist Stuart Campbell, one of the leading experts in obstetric ultrasound, macrosomia affects 1,5% of births in developed countries.


Childbirth: how to deal with it with serenity

go to the gallery

There are subjective and objective variables to take into consideration when the time comes for a baby to be born. How to deal with childbirth with serenity? We have...

Because a fetus can be large

"The very first cause of macrosomia is the mother's diabetes, whether gestational or prior to pregnancy "she explains." In this case, which is precisely the most frequent, macrosomia has to do with alterations in maternal glucose metabolism. These are alterations that can also occur in other conditions of the mother, such as obesity or overweight, which therefore in turn can lead to children who are much larger than the norm ".

Then there are also situations that do not have to do with maternal illnesses, but with genetic and ethnic aspects: if the parents have "by nature" a very robust constitution - and therefore we are not speaking only of fat, but also of bone structure - also the child could have an estimated weight much higher than the average.

The baby's weight

There are several factors that can affect the weight of a fetus and a newborn. Between these:

  • gestational age at birth: a baby born at 41 weeks will tend to weigh more than a baby with similar characteristics born at 38 weeks;
  • parity, that is number of previous parts: generally subsequent children tend to be a little older than the previous ones;
  • ethnicity of the parents;
  • height of mom and height of dad;
  • mom's weight before pregnancy e weight gained during pregnancy itself
  • any maternal diseases, such as diabetes;
  • quality of maternal nutrition;
  • any malfunctions of the placenta, which can slow the growth of the fetus.
Read also: The development of the fetus between the 25th and 32nd week

How a fetus is determined to be large

The classic method is based on the evaluation of fetal biometry - that is the set of parameters that define the size of the child, such as the abdominal circumference or the length of the femur - performed with aultrasound around 34 weeks of pregnancy and at the latest within 36 weeks. In fact, later on, the measurements become less reliable: we therefore rely on the previous ones to estimate what the weight of the baby will be at term.

The problem is that these estimates offer an important indication of what the baby's weight might be, but by definition involve a certain error. In other words: if from the biometric data it is estimated that the fetus will be 4,3 kg at full term, it does not mean that that will really be its weight.

Another method used is the so-called measurement of the symphysis-fundus distance, in practice, the distance between the highest point of the uterus (bottom) and the starting point of the pubis (pubic symphysis), which is taken manually. However, this too offers an estimate and not an accurate figure.

The risks of a fetus that is too large

Knowing how much a baby could weigh at term of pregnancy is not a simple curiosity: it is useful information, because a condition of true macrosomia - therefore a weight over 4,5 or even 5 kg - can involve risks.


Delivery room: the 50 most exciting photos seen on Instagram

go to the gallery

The first images after a birth are the most exciting: they express the strength of the act of birth together with the tenderness for the new life. But above all they mean the beginning of ...

In particular, during a vaginal birth, the mother may be at risk of perineum injury and damage to the pelvic floor, with fecal and urinary incontinence and uterine and vaginal prolapse. Not surprisingly, some believe that fetal macrosomia may be a condition that justifies the episiotomy, the famous "cut" of the perineum.

For the child, the most significant risk is that of shoulder dystocia, a condition that can lead to further complications such as asphyxiation and brain damage. Of course, we're always talking about risks, not events that are sure to happen if the baby is big. It should be clearly reiterated that the vast majority of heavy children are born without dystocia, and that about half of all births with shoulder dystocia involve children weighing less than 4 kg.

These possible complications mean that in the case of fetal macrosomia the risk of undergoing an emergency caesarean section or operative delivery with suction cup increases. For this reason, a scheduled caesarean section or induction of delivery at 39 weeks may be recommended in some cases to avoid the baby having time to grow further.

Read also: The development of the fetus between the 33th and 40nd week

What to do when the child is estimated to be large

"If a real diagnosis of fetal macrosomia is made, pregnancy is considered to be at risk, and then the expectant mother will have to do some more control. And of course, an evaluation of his nutritional status must be made immediately ", he says.

Having said that, there is still some debate in the scientific field about what to do operationally in view of childbirth.

To answer the opening question, in general 4 kg of the baby's estimated weight is certainly not a sufficient condition to establish the need for a caesarean section.

"After all, let's never forget the caesarean is a surgery for which the benefits and risks must always be carefully evaluated, from time to time," says the gynecologist.


The guidelines on caesarean section of the Higher Institute of Health recommend surgery if the fetus has an estimated weight of over 4,5kg, but only in the case of a diabetic mother. Also for mothers with diabetes, some scientific societies argue that it is possible to think about the intervention even for slightly lower weights, over 4,25 kg.


If there is no diabetes, the recommendation falls: in this case, for example, even in the case of an estimated weight of 5 kg, the American College of Obstetricians and Gynecologists limits itself to considering the caesarean as an option that "can be considered".


In short, contrary to what happened not so long ago, it is no longer certain that in the case of a large fetus it is necessary to have a planned caesarean section. "In addition to diabetes, which makes the situation more critical, the parameters to be taken into consideration for this choice are different," she says. "For example, if there have been previous births and how they went: if a woman has already given birth without difficulty two children weighing 4,5 kg, there is no reason to assume a priori that with a third grown child one should think about the whole thing. 'intervention".


"Or if there have been previous cases of shoulder dystocia, perhaps even with younger children. In this case it may make sense to evaluate the intervention. Or, again, the obstetric situation as the term approaches, that is the set of parameters such as the length and consistency of the cervix or the position of the fetal head ".


The other option for an estimated 4,5 kg or more baby without maternal diabetes is to induce delivery at 39 weeks., to avoid that the little one has time to grow again. Again, however, there are no rigid and rigorous indications. According to a recent article published by the Cochrane Collaboration, an international research group dedicated to the critical review of information on health interventions, induction can certainly bring benefits, but it also has some disadvantages. For this reason, it is an option that must be carefully evaluated, case by case, and discussed in depth with the child's parents, so that they are aware of the risks and benefits.


Other sources for this article: Linee guida ACOG sulla macrosomia fetale; Estimation of Fetal Weight, articolo su Medscape; Fetal macrosomia: a problem in need of a policy, articolo pubblicato su Ultrasound in Obstetrics & Gynecology; Shoulder dystocia: risk factors and planning delivery of at risk pregnancies, articolo su Up to Date.



Losing weight after childbirth: 20 tips on how to lose weight

go to the gallery

After giving birth, it is normal to feel weighted and out of shape. As Dr. Roberta Spiti, Dietitian at the GEPO Center (www.gepoconsultorio.com) explains, a few kilos in ...

Questions and answers

Where does fetal macrosomia come from? 

The most common cause is maternal diabetes, both gestational and pre-pregnancy mellitus.

When can we really talk about fetal macrosomia? 

Tendentially when the estimated weight at the end of pregnancy is greater than 4,5 kg.

In the case of fetal macrosomia, what are the risks involved in childbirth?

As for the time of delivery, during a vaginal birth the mother can run the risk of injury to the perineum and damage to the pelvic floor.

  • macrosomia
  • large fetus
  • operative delivery
  • diabetes
  • fetal size
add a comment of Fetal macrosomia: what to do if the fetus is large
Comment sent successfully! We will review it in the next few hours.