Gestational Diabetes Mellitus

Introduction

Historically, the term "gestational diabetes mellitus (GDM)" has been defined as onset or first recognition of abnormal glucose tolerance during pregnancy.

Women with type 1 or 2 diabetes diagnosed prior to pregnancy are classified as having pre-existing diabetes.

  • NICE guidelines recommend that women with pre-existing diabetes who plan for pregnancy to aim for HbA1c <6.5% (48 mmol/mol) if this is achievable without causing hypoglycaemia. Any reduction in HbA1c level towards the target is likely to reduce the risk of congenital malformations in the baby. Those with HbA1c >10% (86 mmol/mol) are strongly advised not to get pregnant because of the associated risks.

NOTE: Low-dose aspirin is recommended for preeclampsia prevention in both pre-existing type 1 and 2 diabetes.



Complications of GDM

Complications of pregnancy which is more common in GDM include:

Risks associated with GDM extend beyond the pregnancy and neonatal period.
  • GDM is also a strong marker for maternal development of type 2 diabetes, including diabetes-related vascular disease.
  • GDM increases the offspring's risk for developing obesity, impaired glucose tolerance and metabolic syndrome.

Hence, lifestyle modification strategies such as exercise, diet and weight management are important to prevent GDM.



Screening

Ideally, universal screening should be adhered to. However, if resources are limited, selective screening is acceptable focusing on individuals at risk of developing GDM.

  • Screening for gestational diabetes mellitus based on risk factors using 75 gram glucose tolerance test (OGTT) should be done at booking. If the test is negative, it should be repeated at 24-28 week of gestation.
  • For women at the age of 25 or more with no other risk factors, OGTT should be done at 24-28 weeks of gestation.
Screening and Diagnosis of Diabetes in Pregnancy
NOTE:
  • Overt diabetes in pregnancy should be managed as pre-existing diabetes
  • There is insufficient evidence to support HbA1c alone as a useful diagnostic test for GDM and thus, it is not a useful alternative to OGTT.



Self-Monitoring of Blood Glucose

Glycemic control is the cornerstone of management of any diabetic pregnancy. The frequency of self-monitoring of blood glucose (SMBG) should be individualized base on mode of treatment and glycemic control.

The blood glucose targets should be as the following:

  • fasting or preprandial ⩽5.3 mmol/L
  • 1-hour postprandial ⩽7.8 mmol/L
  • 2-hour postprandial ⩽6.7 mmol/L

Pregnant women who are on insulin or oral antidiabetic agents (OAD) should maintain their capillary blood glucose level >4.0 mmol/L.



Metformin

In gestational diabetes mellitus, metformin should be offered when blood glucose targets are not met using changes in diet and exercise within 1-2 weeks.

  • Metformin should be continued in women who are already on the treatment before pregnancy.

NOTE: When I started my provisional registered pharmacist journey, my senior used to tell me that insulin is the only option for pregnant patients with pre-existing DM or GDM. Today, metformin and glibenclamide are two OADs that have been used in GDM. However, glibenclamide has limited human data.



Insulin

Insulin therapy should be initiated when

  • Blood glucose targets are not met after medical nutrition therapy and metformin therapy
  • Metformin is contraindicated or unacceptable
  • FPG ⩾7.0 mmol/L at diagnosis (with or without metformin)
  • FPG of 6.0-6.9 mmol/L with complications such as macrosomia or polyhydramnios (start insulin immediately, with or without metformin)

The preferred choice of insulin regime in diabetes in pregnancy is multiple daily injections.



Postpartum Management of Diabetes in Pregnancy

Women with GDM should be able to resume a normal diet postpartum. After delivery, the hyperglycemic effects of placental hormones dissipate rapidly. Thus, most women revert back to their prepregnancy glycemic status shortly after delivery, ranging from almost immediately to a week postpartum.

Metformin and intensive lifestyle intervention (addition of moderate physical activity, 50-60 minutes, 4 days per week) during postpartum period should be considered to prevent diabetes.

In women with history of gestational diabetes mellitus, oral glucose tolerance test should be performed at 6 weeks after delivery to detect diabetes and prediabetes.

  • If negative, annual screening should be performed.



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