Fetal Macrosomia (Large Baby)

What counts as macrosomia

ACOG uses birth weight thresholds:

Large for gestational age (LGA) is a different measure: weight above the 90th percentile for the baby's age in weeks. A baby can be LGA at 36 weeks without meeting the macrosomia threshold.

About 8% of US babies weigh more than 4,000 g at birth, and about 1% weigh more than 4,500 g.

Causes and risk factors

• diabetes: high maternal blood sugar crosses the placenta, and the baby makes extra insulin, which acts as a growth hormone, especially for fat around the shoulders and belly. This applies to gestational diabetes and pre-existing type 1 or type 2 diabetes. See pregnancy in diabetes
• pre-pregnancy obesity
• excess weight gain in pregnancy
• post-term pregnancy: 41 weeks or more
• previous macrosomic baby
• male baby
• parental height and genetics
• higher number of previous births
• rare genetic overgrowth syndromes, such as Beckwith-Wiedemann syndrome

How macrosomia is detected

Macrosomia can only be confirmed after birth. Before birth, doctors estimate size by:

• fundal height: measuring from the pubic bone to the top of the uterus
• Leopold maneuvers: feeling the baby through the belly
• ultrasound: estimating weight from head, abdomen and thigh bone measurements

Ultrasound estimates of fetal weight in late pregnancy can be off by 10 to 15% or more, and accuracy is lower in larger babies. Many babies predicted to be large are not, and vice versa.

Risks

For the baby

• shoulder dystocia: after the head is born, a shoulder gets stuck behind the mother's pubic bone. It is an obstetric emergency
• brachial plexus injury: stretching of nerves in the neck and shoulder, causing arm weakness (Erb palsy). Most recover within the first year
• fractured collarbone or arm
• low blood sugar after birth, from high insulin levels
• breathing problems, jaundice and admission to the newborn unit
• later childhood obesity and metabolic problems

For the parent

• cesarean delivery and longer labor
• severe perineal tears in vaginal birth
• postpartum hemorrhage from an overstretched uterus
• uterine rupture in a trial of labor after previous cesarean, rarely

Management in pregnancy

• screen for gestational diabetes at 24 to 28 weeks, and earlier in people at high risk
• control blood sugar with diet, exercise and, if needed, insulin or other medicines. Good control reduces macrosomia
• follow weight gain guidelines based on pre-pregnancy BMI
• stay active with regular moderate exercise, if your clinician approves

Delivery options

ACOG guidance:

• Suspected macrosomia alone is not a reason to induce labor early before 39 weeks. Studies show mixed results on whether early induction reduces shoulder dystocia, and early birth has its own risks.
• Planned cesarean may be offered when the estimated fetal weight is at least 5,000 g in people without diabetes, or at least 4,500 g in people with diabetes.
• Shoulder dystocia history: previous shoulder dystocia raises the risk of recurrence, so delivery is planned individually.
• During labor, the team prepares for shoulder dystocia, using specific maneuvers to free the shoulder if it occurs.

After birth

Large babies of parents with diabetes have blood sugar checks in the first hours and frequent early feeding. They are examined for birth injuries such as clavicle fractures and arm weakness. Breastfeeding is encouraged.

Frequently asked questions

What is macrosomia?

Macrosomia means a baby who is much larger than average at birth, usually weighing more than 4,000 g (8 lb 13 oz) or 4,500 g (9 lb 15 oz). It increases risks during birth for both the baby and the parent.

What causes a big baby?

The main causes are diabetes in pregnancy, obesity before pregnancy, excess weight gain, going past the due date, having had a large baby before, a male baby and genetics. High blood sugar in pregnancy is the most important modifiable cause.

Can ultrasound tell if my baby is too big?

Ultrasound can estimate fetal weight, but in late pregnancy estimates can be off by 10 to 15% or more and are less accurate for large babies. Doctors combine ultrasound with other findings and avoid making decisions on ultrasound alone.

Will I need a C-section for a big baby?

Not necessarily. Most suspected large babies are born vaginally. ACOG says a planned cesarean may be offered when the estimated weight is at least 5,000 g without diabetes, or 4,500 g with diabetes. Your history and preferences also matter.

What is shoulder dystocia?

Shoulder dystocia happens when, after the baby's head is born, a shoulder becomes stuck behind the pubic bone. It is an emergency that the birth team resolves with specific maneuvers. Large babies, diabetes and a previous shoulder dystocia increase the risk.