Page created on January 4, 2022. Not updated since.
Definition and epidemiology
Gestational diabetes mellitus (GDM) refers to the development of new-onset diabetes in pregnancy. It usually develops in the second or third trimester.
Pregestational diabetes refers to pregnancies in women who already had diabetes mellitus prior to the pregnancy, most frequently type 2.
Diabetes mellitus in the pregnancy, whether pregestational or gestational, increases the risk for complications in both the mother and the foetus. Both are relatively common conditions. GDM occurs in 5 – 7% of pregnancies, while pregestational diabetes is seen in 1%.
- Previous gestational diabetes mellitus
- Family history of diabetes (gestational or otherwise)
- Maternal age > 30
Pregnancy is characterised by hormonal changes which have a diabetogenic effect. Hormones like progesterone, hPL, and oestrogens cause insulin resistance. As such, pregnancy is a high-risk state for development of diabetes. However, in physiological cases, the beta cells of the pancreas can compensate for the insulin resistance by increasing insulin secretion. As such, the blood glucose level remains in the normal range. Gestational diabetes develops if the beta cells cannot compensate for the insulin resistance. These physiological changes may also worsen a pregestational diabetes.
Diabetes mellitus is usually asymptomatic. However, ultrasound examination of the foetus may show a foetus which is large for gestational age (LGA) with an increased abdominal circumference, and polyhydramnios.
Diagnosis and evaluation
In gestational diabetes, postprandial glucose levels rise above normal before the fasting glucose level. For this reason, we use oral glucose tolerance test (OGTT) to screen for gestational diabetes.
OGTT is routinely performed in weeks 24 – 28. A fasting glucose is taken, after which the woman consumes 75 g of glucose orally. After 2 hours, another blood glucose measurement is made. Gestational diabetes is diagnosed if:
- Fasting glucose level is > 5,5 mM OR:
- Glucose level after 2 hours is > 7,8 mM
Early screening (week 16) is indicated in case of high-risk patients, including those with obesity, previous GDM, age > 40, etc.
Individuals at high risk for diabetes mellitus (obesity, family history) should be screened for pregestational diabetes by measuring HbA1c early (< week 16) in the pregnancy. However, HbA1c is not suitable for diagnosis of gestational diabetes.
Oral antidiabetic drugs are contraindicated in pregnancy (except metformin, see below). As such, gestational diabetes must be managed with lifestyle changes and, if necessary, insulin therapy.
Lifestyle changes include regular exercise and limiting carbohydrate intake to 180 g/day, preferably mostly carbohydrates with a low glycaemic index. The mother should also monitor her own blood glucose regularly.
Women with diabetes in pregnancy should undergo more frequent ultrasound examinations to monitor the foetal health.
Because foetuses of mothers with diabetes may grow large, C-section is usually indicated above a certain foetal weight (> 4500 g) to avoid possible trauma from shoulder dystocia during vaginal birth.
It’s known that metformin passes the placenta, but studies so far have not shown evidence of teratogenicity or long-term complications of the neonate. As such, metformin is not strictly contraindicated, but the risk/reward versus insulin therapy should be weighted as it’s impossible to state with certainty that it’s safe. Insulin does not cross the placenta and is known to be safe.
- Foetal complications
- Neonatal hypoglycaemia
- Spontaneous abortion
- Congenital malformations
- Maternal complications
- Placental insufficiency
- Later development of type 2 diabetes
- Urinary tract infection
Macrosomia due to diabetes is due to excessive abdominal growth, while the rest of the body develops more or less the same. This mostly-abdominal growth can be visualised on ultrasound (as increased abdominal circumference).
Gestational diabetes often resolves after delivery. However, women who’ve had GDM have a high (50%) risk for subsequent development of type 2 diabetes, as well as a high (50%) risk for repeat GDM in later pregnancies. These women should be screened regularly (every year/every three years) for development of T2DM.