Macrosomia, aka fetal growth acceleration

and large for gestational age (LGA) 

reference: ACOG Practice Bulletin Number 22, Reaffirmed 2013 



Definition of Large for Gestational Age

LGA = birth weight greater than or equal to the 90th percentile for a given gestational age

Definition of fetal macrosomia = fetal weight in excess of 4000 to 4500 g (around 10 pounds ) 

Upon first glance, it might seem like the bigger the better, when it comes to babies. But this is very much not the case. Macrosomia, fetal growth acceleration and large for gestational age status are all associated with increased complications for both mother and baby.

Of course determining large for gestational age requires that we have a very accurate idea about gestational age. This requires early presentation to prenatal care and usually early examination and dating ultrasound. Once this is established we can follow growth rates very easily. Furthermore we have a pretty good idea of the risk factors for macrosomia and large for gestational age. They are as follows:


Risk factors for LGA: 


A prior history of macrosomia

Maternal prepregnancy weight

Weight gain during pregnancy

multiparity ( having many babies ) 

 male fetus

gestational age greater than 40 weeks

Maternal height

Maternal age younger than 17 years

Positive 50 g glucose diabetic screening test with a negative result on the three hour glucose tolerance test

Pre-gestational diabetes

Gestational diabetes

Hispanic race


Most all of these risk factors have one thing in common: they are factors which mean that mom's physiology can pack excessive weight onto the baby and in disproportionate ways. Those who are obese, diabetic and or carbohydrate intolerant will have higher circulating glucose levels which pass through the placenta to the baby and stimulate the baby to lay down fat especially in the abdominal and shoulder areas. This produces not only a big baby but a disproportionate and big baby that is especially difficult to deliver due to the disproportionate increase in abdominal and shoulder girth. Per ACOG (American College of Obstetricians and Gynecologists), "Although both diabetes and maternal obesity increased risk of fetal macrosomia, most agree that maternal obesity place a greater role." What are the risks?


Maternal risks of macrosomia:

Labor difficulties and trauma such as significant vaginal laceration, Third and fourth degree lacerations

Increased risk of cesarean delivery

Postpartum hemorrhage


Fetal risks of macrosomia:

1. Shoulder dystocia– This is an Obstetrics emergency where in the head of the baby is able to be delivered, but the rest of the baby is stuck under the pelvic bone by wide shoulders and or abdomen. In this instance the baby is not able to oxygenate properly because he can neither take a breath because his chest is compressed, nor can he receive flow from the umbilical cord which is also compressed. It is a problem that requires an immediate solution to avoid oxygen deprivation to the baby.

Shoulder dystocia drills are routinely practiced by obstetric teams. There are multiple effective measures which we can use to resolve shoulder dystocias. In many cases this involves repositioning the patient, having the nurse supply suprapubic pressure, and cutting an episiotomy. Sometimes mother nature resolves the matter herself and a clavicle (collar bone) on one side of the baby will fracture, thus reducing the diameter of the shoulders. This is much preferable alternative to producing a major stretch on the nerves of the arm called the brachial plexus. Clavicle fractures heal almost right away, but in rare cases,  brachial plexus injuries can persist leading to nerve problems with the arm. 

2. increased risk of depressed five minute Apgar Score

3. increased rate of admission to neonatal intensive care unit


Strategies to address Macrosomia:

The principal strategy for macrosomia is to prevent it. We can do this through control of pre-pregnancy obesity and weight gain during pregnancy. We can also ensure effective management of pre-gestational diabetes and gestational diabetes.

Should babies, once they reach a certain size, be delivered by C-section? Should mothers whose baby's reach a certain size be induced?

These are very reasonable questions. First of all we can never be too certain about the baby's weight before we deliver the baby. Accuracy of estimated fetal weight at term is an inexact science at best where clinical palpation is just as good as ultrasound. As you know, we practice evidence-based medicine. You may also remember from earlier sections of the website that all evidence is not the same. To answer these questions about C-section delivery and induction, we look to the literature which can provide us with only level B and level C evidence:   "Limited or inconsistent scientific evidence shows with suspected fetal macrosomia is not an indication for induction of labor because induction does not improve maternal fetal outcomes" , according to the existing data. Having said that, if a patient with a macrosomic baby has a very favorable cervix and she's past 39 weeks buy good dating criteria, It seems reasonable to do what amounts to essentially an elective induction, as long as the mother understands the risks of induction itself. Level C evidence, which means consensus and expert opinion, indicate that " prophylactic cesarean delivery maybe considered for suspected fetal macrosomia with estimated weights greater than 5000 g in women without diabetes and greater than 4500 g in women with diabetes."