Gestational Diabetes Causes and Risk Factors

Gestational diabetes is extremely common, occurring in 3 to 5 percent of pregnant women.  It is defined as having high blood glucose levels (blood sugar levels) that only occur when a woman in pregnant.  No one knows exactly why a woman might develop the disorder but it appears to occur only in women even if they never had the diagnosis of type 2 diabetes prior to the pregnancy.

It is important for both the pregnant woman and the newborn to have normal blood sugar levels throughout the pregnancy.  This is why women are generally screened for gestational diabetes between 24 and 28 weeks’ gestation.   Those women at high risk for the disorder or who show evidence of gestational diabetes (such as having glucose show up in the urine) are screened earlier in the pregnancy so that treatment can begin as soon as possible.

As mentioned, even with research, no one knows why some women come down with gestational diabetes and why some women don’t.  In order to understand why this disorder is present in pregnancy, it helps to understand the ways in which pregnancy affects the way the body processes glucose when in the pregnant state.

Blood Sugar and Pregnancy

During pregnancy, the woman’s body digests carbohydrates in the diet by allowing the carbohydrates taken in as part of a healthy diet to turn into glucose.  After digestion in the gut, the glucose in the GI tract is absorbed into the bloodstream.

When blood sugars are elevated, the pancreas, which is a gland located slightly below and behind the stomach, is activated, causing the islet cells of the pancreas to produce insulin.  Insulin is a protein-based hormone that helps the blood glucose into the cells of the body that use glucose as part of cellular metabolism.

When a woman is pregnant, the placenta (which is the part of the fetus that connects the fetal blood supply to the maternal blood supply) makes elevated levels of other types of hormones.  Most of these hormones counteract the effect of insulin, allowing blood sugar levels to become elevated. Small increases in blood sugar levels after eating a meal is normal in pregnancy but some women have insulin resistance, which causes spikes of blood glucose in the bloodstream after eating.

As the fetus grows and develops, the placenta produces more and more hormones that block the function of insulin.  In gestational diabetes, the hormones in pregnancy made by the placenta cause an increase in blood glucose levels to a high enough degree that the health and growth of the fetus can be compromised.

Because the insulin resistance in pregnancy worsens with increasing size of the placenta, this is why gestational diabetes is more common in the latter part of the pregnancy, when both the baby and the placenta are growing. Gestational diabetes is the most common after 20 weeks into the pregnancy; however, some women have risk factors for the disease and will develop gestational diabetes earlier in the pregnancy.

The main cause of gestational diabetes is insulin resistance.  While type 1 diabetes involves a lack of insulin, women who have gestational diabetes have normal or elevated levels of blood insulin but there are hormones made by the placenta that counteract the insulin so the blood sugar levels become elevated.

During pregnancy, the pancreas has to work harder and puts out as much as 3 times more insulin than is produced prior to the pregnancy.  If the pancreas in pregnancy cannot keep up with the demand for glucose-lowering insulin, the blood glucose level rises and the woman develops gestational diabetes.

Why a Woman gets Gestational Diabetes

Not every woman gets gestational diabetes.  Women at the greatest risk of gestational diabetes are women who have the following problems:

• The woman is American Indian, Hispanic, African American or Pacific islander.
• The woman has polycystic ovarian syndrome (PCOS) prior to the pregnancy.
• The woman has had at least one baby who was born stillborn.
• The woman had a previous pregnancy resulting in an infant greater than 9 pounds at birth.
• The woman is 25 years of age or older when she becomes pregnant.
• The woman had gestational diabetes during other pregnancies.
• The blood sugars indicate that the woman had prediabetes prior to becoming pregnant.
• The woman has a first degree relative (parent, sibling, or child) who have type 2 diabetes.
• The woman has gained excessive amounts of weight, especially in the first half of pregnancy.
• The woman is overweight prior to becoming pregnancy (being overweight is a risk factor for insulin resistance so the body can’t use insulin properly).

What can be done?

Gestational diabetes doesn’t usually show up as soon as the woman becomes pregnant.  This is because the placenta is very small in early pregnancy and doesn’t put out insulin-resistant hormones to a high enough degree.  Instead, gestational diabetes generally starts when the placenta becomes big enough to produce insulin-resistant hormones, which doesn’t happen until the woman reaches her third trimester.

Every woman is screened for gestational diabetes at around 24 to 28 weeks’ gestation.  Screening is done by drinking a 50-gram load of glucose in a sugary beverage and then checking the blood glucose level one hour after drinking the beverage.  If the blood sugar is elevated, the diagnosis of gestational diabetes is confirmed by doing a 3-hour glucose tolerance test, in which a 100-gram load of glucose is given and blood sugar levels are checked fasting and up to three hours after drinking the beverage.

If the blood tests indicate gestational diabetes, steps are made to decrease the blood sugar levels on a fasting basis and after eating meals.  This is usually achieved by changing the diet to include fewer carbohydrates and eating less so that the weight gain is as low as possible.  Exercise is stepped up in order to bring the blood sugars down.

If lifestyle changes are unable to bring the blood sugars in the normal range, medications (such as insulin or metformin) are used to keep the blood sugars as normal as possible.  This involves checking the blood sugars several times per day and using insulin or an oral medication to control the blood sugars along with doing the lifestyle changes already mentioned.

References:

• What causes gestational diabetes? http://www.webmd.com/diabetes/gestational-diabetes-guide/what-causes-gestational-diabetes
• Risk Factors. http://www.mayoclinic.org/diseases-conditions/gestational-diabetes/basics/risk-factors/con-20014854
• Causes of gestational diabetes. http://www.mayoclinic.org/diseases-conditions/gestational-diabetes/basics/causes/con-20014854