Gestational Diabetes Mellitus-1

Clinical Features and Diagnosis


Diabetes mellitus (DM) is a group of disorders in which blood glucose (BG) levels are higher than normal. Glucose is a simple sugar and is the final single-unit molecule produced as the body digests and breaks down all the carbohydrates. Glucose is then absorbed from small intestines and travels in the blood to organs such as the heart, brain, muscles, etc. At the same time, our pancreas releases a hormone called insulin into our blood. Insulin is transported via blood and reaches various tissues and organs. Insulin interacts with our cells and tissues and this allows glucose to enter the cells. 

Once the glucose is in the cells:

  • It is broken down by chemical reactions which generate energy. This energy is used by the body to make ATP (adenosine triphosphate). ATP is a high-energy molecule that stores the energy we need to do most of the work our body performs.
  • Glucose, which is not used immediately, is stored in the liver as Glycogen. Glycogen is the storage form of glucose; it is made up of many connected glucose molecules. Glycogen is broken down to glucose when we have not eaten for 3-4 hours. It can provide energy for about 12 to 14 hours.
  • Remaining glucose is converted to glycerol which is used to make fat (Triglycerides). Triglycerides are stored in our adipose (fatty tissue).

In DM, insulin is either not produced by the pancreas (type 1 DM) or insulin is produced but it does not work (type 2 DM). Gestational diabetes (GDM) is another type of DM that occurs in some women in the second half of pregnancy. Usually, these women are overweight and have a genetic predisposition to DM. 

How does pregnancy affect glucose metabolism?

Placenta is a temporary organ that connects the mother’s uterus to the baby during pregnancy. Placenta not only provides nourishment and oxygen to the fetus, but it also makes various hormones that are essential to sustain a pregnancy. Unfortunately, some of the hormones produced by the placenta cause insulin resistance (IR). IR is a condition in which our cells do not respond to insulin properly. The body still manages to maintain normal blood glucose level for a while by increasing insulin production. While this can compensate for the IR initially, over time the pancreas starts to run out of insulin. Eventually, insulin production begins to decrease and BG levels start going up.

So how and why does Gestational Diabetes occur in some women?  

Gestational diabetes (GDM) is diabetes that is first diagnosed during pregnancy and resolves after the patient delivers. Typically, GDM occurs after about 6 months of pregnancy. As mentioned above, the placenta makes certain hormones to help sustain a pregnancy, but some of these hormones cause IR which may lead to GDM.

Not all patients with IR develop GDM. Only those who have a genetic tendency of DM. GDM occurs in about 10% of pregnancies and is more common in ethnic minorities, such as Native American Indians, Black Africans, Latino Americans, and Pacific Islanders. 

Other risk factors for GDM are outlined in table 1.

Table 1: Gestational Diabetes Mellitus Risk Factors
• Older age • Ethnicity (most ethnic minorities in the US) • GDM during previous pregnancies (40% chance) • Having PCOS • Having pre-diabetes • Having ↑ blood triglyceride and ↓ HDL-C • Obesity & inactivity • FH of type 2 diabetes (in a 1st degree relative) • Previous delivery of a large baby (>9lb) • High blood pressure

GDM usually develops in the second half of pregnancy. If diabetes occurs in the earlier part of pregnancy, particularly in the first trimester, it is often related to previously undiagnosed type 2 or even type 1 diabetes. This form of DM may notgo away after delivery.

How is Gestational Diabetes Mellitus diagnosed?

In the majority of cases, GDM does not cause symptoms, unless a complication occurs (see below). Therefore, special tests are done after the 20th week of pregnancy (most commonly between weeks # 24 and 28). These tests assess your body’s ability to keep BG within range after you drink a sugary drink.

There are two protocols for GDM testing:

1. Two-step method:

A screening test called Glucose Challenge Test (GCT) is done first. The patient is given a sugary liquid (50 grams of sugar) and BG is checked 1 hour later. If BG is higher than 140 mg/dL, it is considered abnormal. These patients will need to do an oral glucose tolerance test (OGTT). In this test, the patient should be fasting (nothing but water after midnight). A sugary liquid (containing 100 grams of sugar is given to the patient and BG is checked hourly for three hours). If the glucose level is higher than the limits (see Table 2), the patient has GDM.

The 2 step method is used for most average-risk patients

2. One-step method: 

An oral glucose tolerance test (OGTT) is done without a screening test.

Done in patients who have a high risk of developing GDM (see Table 1)

Table 2: GDM cut-off BG levels with 100 g OGTT.

60 min or 1 hour180
120 min or 2 hours155
180 min or 3 hours140

Note: HbA1c is not a good test to diagnose GDM

Why is it important to test for GDM?
Most women who develop GDM have no unusual symptoms. At the same time, if GDM is not recognized or managed well, both the mother and the baby may develop serious complications. With good treatment, these complications can be avoided. 

Potential complications of GDM are listed in Table 3.

Table 3: Complications of Gestational Diabetes Mellitus (GDM):

Pre-term laborFetal death
↑ risk of preeclampsiaMacrosomia (large baby)
Pelvic injuryJaundice
Need for C-sectionPremature baby (respiratory problems)
Future risk of type 2 diabetesLater risk of DM and obesity

….continue to part 2 Gestational Diabetes Mellitus-2 Management (Diet & Medications)

Untreated GDM increases the risk of complications for both mother and baby. 

Complications that may affect the baby include:

  • risk of early birth 
  • increased birth weight (which may increase the risk of birth injuries)
  • low BG after birth
  • obesity and type 2 diabetes later in life
  • stillbirth 
  • low blood calcium level

Complications of GDM which affects the mother include:

  • high blood pressure and preeclampsia
  • need for C-section
  • type 2 diabetes mellitus later in life

These complications are preventable if BG readings are kept near normal levels, especially after the meals.

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