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  • Nathan Riley, MD

Obgyno Wino Podcast Episode 39 - Macrosomia

"Even though after years of studying I could tell Donna everything about how her kidneys work and what happens to her body when they fail, I hadn't the slightest idea how she should experience dying from kidney failure, or what medications could ease her suffering...I struggled to understand how I could be on the cusp of becoming a physician and lack the words to answer her question, to guide her through the one certain transition every patient of mine, every human being including myself, would experience." — Sunita Puri, MD, physician and author, from her book The Good Night: Life and Medicine in the Eleventh Hour.

2014 Restitution Red Blend from Magistrate Wines


PB#216 - Published January 2020

Five Pearls

  1. Large babies come with a relatively higher risk to mom and baby, but absolute risks remain low.

  2. Shoulder dystocia is generally difficult to. predict, but in diabetic patients with large fetuses, the risk is significantly higher, especially if vacuum or forceps is attempted.

  3. Ultrasound is. notoriously inaccurate at predicting fetal weight (+/- 15%), and it's no better than using your hands or asking your patient to guess based on a previous pregnancy.

  4. Diet, insulin and exercise are helpful in preventing macrosomia.

  5. Inducing labor if fetus found to be macrosomic is not indicated. C-section reasonable if fetus measuring >4500 g in a diabetic patient or >5000 g in a non-diabetic patient

Background and definitions

- macrosomia refers to estimated fetal weight (EFW) of > 4000 g

- large for gestational age (LGA) implies >90%tile for a given gestational age

- big babies are associated w/ relatively increased risk for postpartum hemorrhage, 3rd and 4th degree perineal lacs, and chorioamnionitis


- pregnancies complicated by larger fetuses carry higher risks of some bad neonatal outcomes (eg, low APGARs, need for ventilator support, birth injuries) when fetus weighs >4000 g, with even higher risks (including mortality) when >4500 g and >5000 g

- large newborns also have higher risk for hypoglycemia, polycythemia, meconium aspiration, and NICU admission



Side bar: Remember that these are relative risks. An odds ratio relates the likelihood of two events happening. When looking at studies that want to compare the likelihood of an event happening in one group compared to another group, an odds ratio of 2.0 means "double the risk". This also means that if the risk of your head exploding later today 1/100,000,000 then an odds ratio of 2.0 if you eat green eggs and ham implies that the risk doubles to 2/100,00,000 (or 1 in 50,000,000). This is still a low absolute risk.


- below is a table of the odds ratios for bad outcomes related to fetal macrosomia:


PMID: 12748514

PMID: 18455528



- there's also an increased c-section rate and shoulder dystocia risk as fetuses get bigger

- shoulder dystocia carries risk of clavicle fracture and brachial plexus injury (most common C5-C6 roots, which leads to Erb-Duchenne palsy)

- risk of neonatal clavicle facture is ~0.5% of all births; 10-fold increase in macrosomic infants

- risk of neonatal brachial plexus injury is 1.5/1000 across all births; 20-fold increase if fetus >4500 g

- up to 90% of these injuries resolve by 1 year of life, though more time may be required for infants >4500 g


PMID: 9731856

PMID: 9731856

Risk factors for macrosomia

- preexisting or gestational diabetes mellitus (38% in preexisting DM; 30% in GDM) (esp high link w/ fasting hyperglycemia)

- obesity

- excessive weight gain in pregnancy (especially in women who entered pregnancy with obesity)

- history of prior macrosomic newborn

- postterm pregnancy (risk of >4500 g birthweight is 1.3% at 39-39w6d; 3% at >41wga)

- taller women (compared to shorter women, even after controlling for weight)

- male fetuses


How might one diagnose macrosomia?

- ultrasound or hands

- ultrasound utilizes fetal biometry: biparietal diameter, head circumference, femur length, and abdominal circumference (calculators are available online)

- ultrasound is notoriously inaccurate (15% error on average)

- using your hands to palpate fetal parts through the abdominal wall doesn't fare much better


Side bar: Leopold's grips (aka Leopold's maneuvers) were first described by German gynecologist Christian Gerhard Leopold as a means of determining fetal position!


- interestingly, parous women are just as good at predicting the weight of their soon-to-be newborn as clinicians or ultrasound


How can fetal macrosomia be prevented?

- exercise during pregnancy

- low glycemic diet in women with GDM

- for GDM, addition of twice-daily insulin to low-sugar diet may increase the likelihood of a birth weight > 90th percentile from 45% to 13% compared to those treated with diet only

- pre-pregnancy bariatric surgery in women with class 2 or class 3 obesity (decreased risk for LGA, but increased risk for SGA)

Erb-Duchenne palsy

Should I offer induction of labor for fetuses at risk of macrosomia?

- the data sucks: some studies suggest higher c-section rate, some suggest decreased risk of shoulder dystocia, others show a possibly decreased c-section rate or no change at all, a few suggest lower clavicle fracture risk

- these studies were limited by small sample size

- additional studies have attempted to determine if IOL is helpful, and the jury is not out (even after several meta-analyses)

- ACOG's stance: insufficient evidence to recommend IOL for macrosomia


Is scheduled c-section ever indicated?

- the question is: will arranging for c-section decrease risk to mom and baby when compared to vaginal delivery in macrosomic fetuses?

- important to remember that, although risks are relatively increased, the absolute risks remain low, so many big babies are still born without any issues

- scheduling c-section won't eliminate ANY risks, and this is especially challenging due to the inaccuracy in predicting EFW

- data is literally all over the place. for example: NNT for one brachial plexus injury is between 155 and 1026 c-sections (!!!)

- it's also so challenging to predict shoulder dystocia

- ACOG feels that it's reasonable to offer c-section if EFW >4500 g in diabetics or >5000 g in non-diabetics


How might you manage labor and delivery differently?

- 1st and 2nd stage might be longer than expected, which leads to unnecessary c-sections (relax and grab a smoothie, knifey)

- if you feel the need to intervene operatively, remember that forceps or vacuum assistance increases the risk for shoulder dystocia

- consider the whole picture: if patient is diabetic and fetus thought to be large, maybe vacuum or forceps is not the best idea (one study found an odds ratio of 33 with this triad!!)

- insufficient data to steer TOLAC counseling in the setting of suspected macrosomia re: VBAC success rate or risk of uterine rupture

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