Foetal macrosomia and shoulder dystocia in gestational diabetes
Poorly controlled gestational diabetes (diabetes that first appears in pregnancy) can produce an unusually large baby, called macrosomia, with shoulder dystocia (shoulders stuck at birth) as the delivery danger.
Remember
In poorly controlled gestational diabetes the baby receives extra glucose and makes extra insulin, which drives growth, so think macrosomia first and shoulder dystocia as its main danger.
Memory aid
more sugar in, more insulin, bigger baby, stuck shoulders
Say the chain in order, from the glucose reaching the baby to the shoulders stuck at delivery.
Gestational diabetes is diabetes that first appears in pregnancy, because the mother's tissues respond poorly to insulin (insulin resistance), so her blood glucose rises. When control is poor, the extra glucose crosses the placenta (the organ that links mother and foetus) into the baby's blood.
The baby's pancreas answers by making more insulin, called foetal hyperinsulinaemia (high foetal insulin levels). Insulin promotes growth, and the surplus glucose is stored as body fat. The result is an unusually large baby, known as macrosomia, usually a birth weight above 4 to 4.5 kg.
A larger baby is more likely to have its shoulders stick during vaginal delivery. This is shoulder dystocia: the head is born, but the anterior shoulder is wedged behind the pubic symphysis (the joint at the front of the pelvis). It is an obstetric emergency, meaning a situation that needs immediate coordinated action. The baby can suffer brachial plexus injury (damage to the network of nerves that runs from the neck into the arm) or bone fractures, and the mother can have a postpartum haemorrhage (heavy bleeding after the birth).
A large baby raises the risk rather than settling the outcome, because most cases of shoulder dystocia are unpredictable. Good glucose control lowers the chance of these problems, and the baby's growth is monitored regularly so that the team can prepare for macrosomia.
Key facts
- Chain of events
- in poorly controlled gestational diabetes, extra glucose crosses the placenta, the baby's pancreas makes more insulin, and the surplus glucose is stored as body fat
- Macrosomia
- an unusually large baby, usually a birth weight above 4 to 4.5 kg
- Delivery danger
- shoulder dystocia, where the head is born but delivery then stops because a shoulder is wedged behind the pubic symphysis
- Harm from shoulder dystocia
- brachial plexus injury or bone fractures in the baby, and postpartum haemorrhage in the mother
- Risk and outcome
- macrosomia raises the risk of shoulder dystocia rather than settling the outcome, and most cases of shoulder dystocia are unpredictable
- Amniotic fluid
- gestational diabetes can cause too much amniotic fluid (the fluid around the baby), called polyhydramnios, not too little
Exam traps
- Neural tube defects (defects of the neural tube that affect the brain or spinal cord) are linked to too little folic acid or vitamin B12, and they do not arise from the glucose-driven overgrowth of gestational diabetes, although high glucose early in pregnancy can raise the general risk of malformations (problems in how the organs form).
- Oligohydramnios (too little amniotic fluid) is the opposite problem, because gestational diabetes is linked to polyhydramnios (too much fluid), and neither fluid problem causes the shoulders to stick during delivery.
- A baby with growth restriction grows too slowly, usually because of placental insufficiency (poor blood supply through the placenta), which is the opposite of the overgrowth that is the typical pattern when maternal glucose is high.
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