Macrosomia, a larger than average baby

What is it?

Babies vary in size. Macrosomia describes the upper end of that range, generally above 4,000 grams, with 4,500 grams used as a higher threshold associated with greater risk. ¹

A related term is large for gestational age, which means above the 90th percentile of weight for that number of weeks. A baby can be large for gestational age without being macrosomic, and the reverse.

Why it matters at birth

A larger baby has a larger trunk and shoulders relative to the pelvis. That increases the chance of shoulder dystocia, where the shoulder is caught behind the pubic bone after the head is born. ³

Maternal diabetes adds a specific twist. Insulin drives growth in tissue that is rich in insulin receptors, which means the trunk and shoulders grow more than the head does. A baby of the same weight born to a mother with diabetes therefore carries a higher risk than one born to a mother without. ²

The prediction problem

This is the part families find most surprising. A baby's weight cannot be measured before birth. Ultrasound estimates it from measurements of the head, abdomen and thigh, and the error range is wide, particularly at the high end where the decision would matter most. Clinical estimation by feel is no better. ¹

That is why guidance is cautious about acting on suspected macrosomia. Acting on an imprecise estimate leads to a large number of unnecessary cesareans to prevent a small number of injuries.

What does standard care look like?

Growth is assessed during pregnancy, clinically and with ultrasound where indicated.

The limits of estimation are acknowledged. Guidance states plainly that fetal weight estimation is imprecise. ¹

Diabetes is screened for and managed, because it changes both the size and the shape of growth. ²

Cesarean is discussed at defined thresholds. Guidance addresses when cesarean may be considered for suspected macrosomia, with different estimated weight thresholds for people with and without diabetes. ¹

Induction is not routine for suspected macrosomia. Guidance addresses the evidence and does not support it as a routine intervention. ¹

Shoulder dystocia is anticipated but not predicted. Guidance states that shoulder dystocia cannot be accurately predicted or prevented, and that most cases occur in births with no risk factors. ³

The team is prepared. Where macrosomia is suspected, the delivery is attended by people trained in the shoulder dystocia maneuvers. ³

What can go wrong?

Diabetes not screened for or not managed, where guidance sets out screening in pregnancy. ²

Growth not assessed where there was a clinical reason to.

A documented estimated weight above the discussion threshold not discussed with the patient. ¹

The prediction limits not explained, so a family was either falsely reassured or told a cesarean was unnecessary on the basis of a number that could be wrong by a large margin. ¹

A shoulder dystocia managed outside the recognized sequence. ³

The baby not screened for low blood sugar afterward, which is a specific risk for large babies and for babies of mothers with diabetes. ⁴

Whether any of this amounts to a departure from the standard takes a physician expert reading the whole record. Most large babies are born without complication, and most shoulder dystocia occurs in babies who are not macrosomic. ³

Which injuries can follow?

For the baby

• Shoulder dystocia and brachial plexus injury. ^{3 5}
• Clavicle fracture.
• Low blood sugar after birth, particularly where the mother had diabetes. ⁴
• HIE, where a prolonged shoulder dystocia interrupts oxygen supply. ⁶
• Facial nerve palsy and skull injury where instruments are used. ⁷

For the mother

• Severe perineal tearing. ⁸
• Postpartum hemorrhage, from a uterus that has been overstretched. ⁹

What does this look like in the records?

From the prenatal record

• Every growth scan, with the estimated fetal weight and the date.
• Diabetes screening results. ²
• Any documented discussion about mode of delivery. ¹

From the labor and delivery record

• The delivery note, including any shoulder dystocia note with maneuvers and times. ³
• The fetal monitoring strip. ¹⁰
• The birth weight, which is the number to compare with the estimates.
• The cord blood gas and Apgar score scores. ¹¹

From the newborn record

• Arm movement and the startle response on each side. ⁵
• Blood glucose values with times. ⁴
• Any X-ray of the collarbone or arm.

Comparing the last documented estimate with the actual birth weight is often instructive, in both directions.

What can you do this week?

1. Request the complete labor and delivery record for you and your baby. The records request builder writes the letters for you.
2. Write down what you remember while it is fresh, using the delivery timeline builder.
3. Read how to read your labor and delivery records before the records arrive.
4. Check the deadline to file in your state with the statute of limitations lookup.