There are two terms applied to excessive fetal growth: “large for gestational age” (LGA) and “macrosomia.” Large for gestational age generally implies a birth weight equal to or more than the 90th percentile for a given gestational age.
‘Macrosomia’ is the medical term for birth weight of more than 4,000 g-4500g
(8 lb, 13 oz-9 lb, 15 oz). The term macrosomia literally means 'big body' - Greek words “macro,” meaning big, and “somia,” meaning body.
Despite research and evidence suggesting otherwise, it is still common for care providers to request routine third trimester ultrasounds to estimate fetal weight/diagnose fetal macrosomia.
So what does the research say? Let's break it down!
According to researchers who performed a systematic review and meta-analysis of diagnostic test accuracy:
“Ultrasonic [estimated fetal weight] is relatively poor as a predictor of shoulder dystocia, given that the actual birth weight of the baby is also not strongly predictive of the outcome. The majority of cases of shoulder dystocia involve a normal birth weight infant.”
Shoulder dystocia (SD) is an obstetric emergency which is unpredictable and complicates approximately 0.5–1% of vaginal births weighing less than 3.9kg (8lbs 8oz), 5% for fetuses 4–4.5 kg, and about 10% for fetuses weighing >4.5 kg. Despite fetal macrosomia being the most significant factor for shoulder dystocia, more than 50% of SD cases occur in pregnancies with a normal birth weight fetus.
Researchers investigated the diagnostic effectiveness of universal ultrasonic fetal biometry in predicting the delivery of a macrosomic infant and shoulder dystocia. They sourced more than 10,000 titles and identified 41 studies including 112,034 patients that offered third-trimester ultrasounds for the prediction of macrosomia. (PMID: 33048935)
Here are the conclusions/recommendations from this study:
Universal third-trimester will not have a clinically significant effect at predicting shoulder dystocia. There is not enough evidence on the effect of ultrasound screening on neonatal morbidity
Recommend caution prior to introducing universal third-trimester screening for macrosomia, as it would increase the rates of intervention, with potential iatrogenic harm, without clear evidence that it would reduce neonatal morbidity.
The American College of Gynecology (ACOG) is a professional organization that produces practice guidelines for health care professionals. ACOG made the following recommendation based on good and consistent scientific evidence (Level A) regarding ultrasonic measuring:
“The diagnosis of fetal macrosomia is imprecise. For suspected fetal macrosomia, the accuracy of estimated fetal weight using ultrasound biometry is no better than that obtained with clinical palpation (Leopold's maneuvers).”
What does this mean? There is no definitive or accurate way to predict SD or fetal weight, and there is no advantage to ultrasound biometry over clinical palpation in terms of accuracy.
In fact, research states that the overall margin of error of a third trimester ultrasound between estimated fetal weight (EFW) and actual birth weight (ABW) is 15%.
Based on this large margin of error, ACOG made the following statement:
“In order to utilize our EFW measurements for antenatal management and delivery recommendations, improvements in our biometric measurements should strive for improved error rates of <5–10%.”
If we understand that ultrasounds are not an accurate tool to measure fetal weight, what are the recommendations for 'suspected big babies'?
ACOG made the following recommendations based on limited or inconsistent scientific evidence (Level B):
Suspected fetal macrosomia is not an indication for induction of labor, because induction does not improve maternal or fetal outcomes.
Labor and vaginal delivery are not contraindicated for women with estimated fetal weights up to 5,000 g in the absence of maternal diabetes.
With an estimated fetal weight more than 4,500 g, a prolonged second stage of labor or arrest of descent in the second stage is an indication for cesarean delivery.
Are there any accurate tools or metrics to predict SD or macrosomia?
Weighing a baby after birth is the best and most accurate way to determine their weight!
Research has consistently shown that the care provider’s perception that a baby is big can be more harmful than an actual big baby by itself. Care providers are more likely to diagnose slow progress during labour and recommend a caesarean if they suspect the baby is big.
“Fetal macrosomia is difficult to predict, and clinical and ultrasonographic estimates of fetal weight are prone to error. Elective cesarean section for suspected macrosomia results in a high number of unnecessary procedures, and early induction of labor to limit fetal growth may result in a substantial increase in the cesarean section rate because of failed inductions. Pregnancies complicated by fetal macrosomia are best managed expectantly.”
Zamorski MA, Biggs WS. Management of suspected fetal macrosomia. Am Fam Physician. 2001 Jan 15;63(2):302-6. PMID: 11201695.
Birth is not ‘one size fits all’ and our approach to birth shouldn’t be either.
We all come in different sizes, shapes and a myriad of clinical situations and medical history. This is why informed choice is so important. Understanding routine procedures and evidence based practices is a key piece to informed decision making.
The management of macrosomia is not clearly defined and should be multifaceted. The ACOG recommends against delivery before 39 0/7 weeks of gestation unless it is medically indicated. At this time, and until additional studies are reported, suspected macrosomia or LGA fetus is not an indication for induction of labor before 39 0/7 weeks of gestation because there is insufficient evidence that benefits of reducing shoulder dystocia risk would outweigh the harms of early delivery. (Akanmode AM, Mahdy H. Macrosomia)
I would love to hear from you! Were you given a third trimester ultrasound to estimate fetal weight? Were you diagnosed with macrosomia and offered induction or Cesarean? What was your babies weight at birth?
Below you will find some fantastic evidence based resources on 'Suspected Big Babies";
Bothou A, Apostolidi D, Tsikouras P, Iatrakis G, Sarella A, Iatrakis D et al. Overview of techniques to manage shoulder dystocia during vaginal birth. European Journal of Midwifery. 2021;5(October):1-6. https://doi.org/10.18332/ejm/142097
Moraitis AA, Shreeve N, Sovio U, Brocklehurst P, Heazell AEP, Thornton JG, Robson SC, Papageorghiou A, Smith GC. Universal third-trimester ultrasonic screening using fetal macrosomia in the prediction of adverse perinatal outcome: A systematic review and meta-analysis of diagnostic test accuracy. PLoS Med. 2020 Oct 13;17(10):e1003190. doi: 10.1371/journal.pmed.1003190. PMID: 33048935; PMCID: PMC7553291.
Chatfield J. ACOG issues guidelines on fetal macrosomia. American College of Obstetricians and Gynecologists. Am Fam Physician. 2001 Jul 1;64(1):169-70. PMID: 11456432.
Boulvain M, Thornton JG. Induction of labour at or near term for suspected fetal macrosomia. Cochrane Database Syst Rev. 2023 Mar 8;3(3):CD000938. doi: 10.1002/14651858.CD000938.pub3. PMID: 36884238; PMCID: PMC9995561
Akanmode AM, Mahdy H. Macrosomia. [Updated 2023 May 16]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2024 Jan-