Large-for-Gestational-Age Fetus

A large for gestational age fetus has an estimated fetal weight or abdominal circumference above the 90th centile for gestation, and macrosomia is a birth weight above 4000 g. Causes include maternal diabetes, obesity, excess weight gain, post term pregnancy, previous large baby and constitutional size. It matters because it raises the risk of shoulder dystocia, birth injury, postpartum haemorrhage and neonatal hypoglycaemia.

• Maternal diabetes, including undiagnosed gestational diabetes, is the main treatable cause.
• Risk factors: body mass index above 30 kg/m2, previous macrosomic baby, age above 35 years, male fetus, gestation above 41 weeks.
• Ultrasound weight estimates can be wrong by up to 15 percent.

Fundal height is 3 cm or more greater than dates, with a large palpable fetus, and women may report breathlessness and discomfort. Some have symptoms of diabetes such as thirst, polyuria and recurrent candidiasis.

• Fundal height plotted above the 90th centile.
• Glycosuria on urine dipstick.
• Differential diagnosis: wrong dates, polyhydramnios, multiple pregnancy, uterine fibroid, constitutionally large fetus, fetal tumour or hydrops.
• Complications: shoulder dystocia, brachial plexus injury, clavicle fracture, perineal tear, postpartum haemorrhage, caesarean birth, neonatal hypoglycaemia, stillbirth in untreated diabetes.

The approach is ultrasound measurement with a search for diabetes.

• Ultrasound: estimated fetal weight above the 90th centile, abdominal circumference above the 95th centile, liquor volume (polyhydramnios when deepest pocket is above 8 cm).
• 75 g oral glucose tolerance test: fasting 5.1 mmol/L or more, 1 hour 10.0 mmol/L or more, 2 hours 8.5 mmol/L or more; HbA1c 6.5 percent or more suggests existing diabetes.
• Capillary glucose profile if diabetes is confirmed; confirm dates.
• Where ultrasound is unavailable, use fundal height and a glucose meter, and refer for delivery planning.

The principle is tight glucose control when diabetes is present and planned delivery with preparation for shoulder dystocia.

• Supportive: dietitian review with 40 to 45 percent carbohydrate; walking 30 minutes after meals; limit weight gain; counsel on birth options; deliver in a facility able to do caesarean section.
• Pharmacological: targets fasting below 5.3, 1 hour below 7.8 and 2 hours below 6.4 mmol/L; metformin 500 mg oral with meals increased to 1 g 12-hourly (maximum 2.5 g daily); isophane insulin 0.2 units/kg subcutaneous at bedtime plus soluble insulin 4 to 6 units subcutaneous before meals, adjusted by 2 units every 3 days; oxytocin 10 IU IM at birth; tranexamic acid 1 g IV over 10 minutes for blood loss above 500 mL.
• Procedures: induction at 38 to 39 weeks; elective caesarean section when estimated fetal weight is above 4500 g in diabetes or above 5000 g without diabetes; shoulder dystocia drill: call for help, McRoberts position, suprapubic pressure, then delivery of the posterior arm or rotation.
• Special groups: diabetes in pregnancy needs insulin infusion in labour of 1 to 2 units per hour with hourly glucose (target 4 to 7 mmol/L); women with previous shoulder dystocia are offered caesarean section; newborns feed within 30 minutes and have glucose checked at 2 hours, with glucose below 2.6 mmol/L treated by dextrose 10 percent 2 mL/kg IV over 10 minutes then 80 mL/kg per day.
• Red flags: glucose above 15 mmol/L, ketones, reduced fetal movements, prolonged second stage, turtle sign at birth, bleeding above 500 mL.
• Follow-up: growth scan every 2 to 4 weeks from 28 weeks; glucose profile weekly; 75 g oral glucose tolerance test at 6 to 13 weeks postpartum; yearly HbA1c.