I am often asked early in the pregnancy about when to have gestational diabetes mellitus (GDM) screening, with the patient under the impression that is should be done as soon as possible. This is not the case.
GDM screening is usually done at 26 to 28 weeks pregnancy.
Sometimes, because of indications that a pregnant woman is at higher risk of gestational diabetes, GDM screening is done in the first trimester of pregnancy. If the early GDM result is normal the screening will need to be repeated at 26 to 28 weeks pregnancy.
Mechanism of GDM
Pregnancy hormones, made by the placenta, work to help prevent the mother from developing low blood sugar, which is dangerous for her unborn baby. They do this by resisting the actions of insulin, the hormone responsible for getting sugar from the blood into the tissues. Consequently, there is a progressive increase in blood sugar levels (impaired glucose intolerance) during pregnancy. To counteract this and try to decrease blood sugar levels, the mother’s pancreas makes more insulin. Usually, the mother’s pancreas can produce enough insulin to overcome the effect of the pregnancy hormones. A pregnant woman needs two or three times more insulin than normal. If her pancreas gland is unable to produce this much insulin, her blood sugar level rises and GDM develops.
As GDM is an increasing risk as pregnancy advances screening is usually done at 26 to 28 weeks pregnancy.
If is done earlier and the result is normal it does not negate the possibility of GDM developing a later in the pregnancy. Consequently, the GDM screening will need to be repeated at 26 to 28 weeks pregnancy.
A normal 26 to 28-week screening result does not mean there is no risk of GDM in the third trimester of pregnancy
Sometimes when the GDM screening at 26 to 28 weeks has a normal result, there are clinical signs associated with diabetes that warrant screening needs to be repeated after 28 weeks. I have had many patients over the years when there have been a normal screening result at 28 weeks and a subsequent abnormal screening result consisted with developing gestational diabetes in the third trimester of pregnancy.
Because of the increasing demand for the pancreas to produce more insulin as pregnancy advances a pregnant woman who develops mild GDM, where diet and exercise adequately control her blood sugar level, may need insulin as the pregnancy advances. It is also why a diabetic woman on insulin when she conceives or who is a gestational diabetic who needs insulin will need more insulin as the pregnancy advances.
The excess demand for insulin production ends with delivery of the placenta. That means blood sugars usually revert to normal immediately after the placenta is delivered and the GDM condition ends.
What is the danger of leaving screening until after 28 weeks?
The risk of developing GDM gradually increases as pregnancy advances. 26 to 28 weeks is not a magical gestation when screening must be done. It has been chosen as it is an optimal time to identify most gestational diabetes.
If it is not done until say 30 or even 32 weeks there will be a small number of GDM patients where there a delay in diagnosis but is it unlikely that this will have any impact on baby’s well-being.
Management of diabetes in pregnancy
I always refer a patient to an endocrinologist doctor for diabetes management. An endocrinologist is a doctor who is specialist in diabetes management, just as I am a doctor who is specialist in pregnancy care management. The endocrinologist is the most experienced and best person to optimise care and give the best diabetes safety for you and your baby.
The endocrinologist will arrange a dietician to see you and review your diet. The endocrinologist will also start monitoring your blood sugar level. For most patients, diet modification and exercise are sufficient to keep the blood sugar level normal. Sometimes supplementary insulin is needed. The endocrinologist will advise you of this if relevant for you.
Sometimes there can be short delay in getting an appointment to see the endocrinologist. This is no consequence for you or your baby’s well-being, so don’t be alarmed. If there is a more significant delay, then give my office a call and my secretary will arrange the appointment for you.
Often patients ask what management steps can they take before seeing the endocrinologist. I advise simply minimising your sugar intake will be sufficient.
Urine checking for sugar
Urine checking for sugar and protein at each antenatal visit has always been a part of routine antenatal care.
Significant sugar in the urine (called ‘glycosuria’) is suggestive of GDM and warrants GDM screening, irrespective of the gestation of pregnancy and irrespective of the GDM screening at 26 to 28 weeks having a normal result.
If repeat GDM screening because of sugar in the urine has a normal result, then it is likely that the sugar in the urine is not due to GDM but to a low renal threshold for passing sugar into the urine with a normal blood sugar level. This is of no concern to you or baby’s wellbeing.
Also see in the Question and Answer section Diabetes Screening In Pregnancy